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What Do Dissolution Experiments Tell Us About Natural Weathering
Abstract
Much information about the earliest stages of weathering comes from laboratory experiments where the mineral surface chemistry is rigorously controlled and the solution composition is maintained far from equilibrium with the solid. The experiments show that the pathways for dissolution are similar to those for ligand exchange around dissolved metal complexes. One difference is that the rates of mineral dissolution are controlled by the concentrations of surface species while rates of ligand exchange are proportional to bulk concentration. Nevertheless, the correlation is so strong that rates of olivine dissolution are predictable from the rates of solvent exchange around the corresponding divalent metal in solution. However, this level of resolution is achieved by maintaining unnaturally high fluid/mineral ratios and by taking great care to avoid precipitation of secondary minerals.
... Finally, ligand exchange rates of cations correlate well with the reactivity of both divalent metal oxides (Casey et al., 1993) and sulphides (Morse and Luther, 1999) at the solidwater interface. Casey et al. (1993) reported that the water exchange rate of the hydrated ion is particularly low for Ni 2+ (10 4.4 s -1 ) compared to that for Fe 2+ (10 6.5 s -1 ) and that this difference may explain the lower dissolution rate of Ni-oxides and orthosilicates compared to their Fe analogues. ...
... Finally, ligand exchange rates of cations correlate well with the reactivity of both divalent metal oxides (Casey et al., 1993) and sulphides (Morse and Luther, 1999) at the solidwater interface. Casey et al. (1993) reported that the water exchange rate of the hydrated ion is particularly low for Ni 2+ (10 4.4 s -1 ) compared to that for Fe 2+ (10 6.5 s -1 ) and that this difference may explain the lower dissolution rate of Ni-oxides and orthosilicates compared to their Fe analogues. In addition, Morse and Luther (1999) proposed that, since Ni 2+ has lower kinetics of water exchange than Fe 2+ , incorporation of Ni 2+ into FeS 2 should be kinetically favoured over precipitation of pure NiS 2 , as we demonstrate here. ...
Chemical and isotopic compositions of pyrites are used as biogeochemical tracers in Archean to modern sediments. Moreover, pyrite formation from monosulphide precursors has been proposed to be involved in prebiotic chemistry. However, the factors controlling pyrite formation and distribution in the sedimentary record are incompletely understood. Here, we show that Ni2+ ions accelerate similar to 5 times the nucleation of pyrite at ambient temperature. Using Fe and Ni K-edge EXAFS and TEM-EDXS we demonstrate that Ni(II) is directly involved in the nucleation of pyrite synthesised by reacting Fe(III) with Na2S in the presence of aqueous Ni(II) impurity. Initial formation of a Ni-enriched pyrite core is followed by overgrowth of a Ni-depleted pyrite shell, leading to compositional zoning of the Fe1-xNixS2 nanocrystals (x = 0.05 to 0.0004). The molar Ni/Fe ratio in the final aqueous solution was then 2000 times lower than the starting ratio of 0.01. This enhanced and accelerated trapping of Ni by pyrite could be of prime importance in controlling Ni concentration in the ocean during early diagenesis of marine sediments, and could thus have important implications for interpreting abundances of Ni and pyrite in the sedimentary record. In addition, acceleration of pyrite nucleation in the presence of nickel could help understanding the role of Fe-Ni sulphides in catalysing potential prebiotic reactions.
... Quantification and understanding of mineral weathering have important implications for many environmental problems, such as the relationship between silicate weathering and global climate over geological timescales (Berner, 1992), the availability of inorganic nutrients in soils (Federer et al., 1989;Likens et al., 1998;Huntington et al., 2000), geological carbon sequestration , global geochemical cycles (Lasaga et al., 1994), safety of radioactive waste repositories (Spycher et al., 2003), impacts of acid mines drainage and neutralization of acid precipitation in watersheds (Drever and Clow, 1995), release of toxic elements to soils and to the hydrologic cycle, and the distribution of porosity and permeability in hydrocarbon reservoir rocks (Morad et al., 2010). Weathering rates of silicate minerals observed in the laboratory are in general up to five orders of magnitude higher than those inferred from field studies (Schnoor, 1990;Stumm, 1992; van Grinsven and van Riemsdijk, 1992;Anbeek, 1993;Casey et al., 1993;Velbel, 1993;Blum and Stillings, 1995;Brantley, 1995, 2003;White et al., 1996White et al., , 2005White et al., , 2008Drever, 2003;Zhu et al., 2004;Zhu, 2005;Ganor et al., 2007;Moore et al., 2012). The many differences between experimental conditions in the laboratory and natural conditions in the field have been thoroughly discussed in previous studies (e.g., White and Brantley, 2003;Reeves and Rothman, 2013, and references therein), but the discrepancy was never fully reconciled. ...
... Many studies on dissolution kinetics of feldspars, the most common silicates minerals in earth's crust, were conducted under conditions (i.e., temperature, pH, and deviation from equilibrium) in which dissolution is relatively fast (Knauss and Wolery, 1986;Casey et al., 1989Casey et al., , 1993Lasaga, 1990;Burch et al., 1993;Blum and Stillings, 1995;Alekseyev et al., 1997;Chen and Brantley, 1997;Hellmann and Tisserand, 2006). The effect of each environmental variable, derived from such studies, is typically described using a simple rate law. ...
Weathering rates of silicate minerals observed in the laboratory are in general up to five orders of magnitude higher than those inferred from field studies. The differences between experimental conditions in the laboratory and natural conditions in the field have been thoroughly discussed in previous studies, however, the discrepancy was never fully resolved. It has been shown in past work that if the field conditions are fully simulated in standard laboratory experiments, it is not possible to measure the slow rates of mineral dissolution that are observed in the field using standard laboratory experiments. Therefore, a novel method that uses the change of Si isotopes ratio in spiked solutions is used in the present study to measure weathering rates of feldspar under close-to-natural conditions.
... A decrease in element content does not mean that the weathering process was slowed down or stopped. It rather proves that precipitating mechanisms have been involved [106][107][108]. Therefore, an essential step in demonstrating sandstone dissolution rates under certain conditions is a critical appraisal of the amounts of elements released, indicating potential donor phases and validating this appraisal with microscopic observations [72,109]. ...
Sandstones are common rocks mainly composed of a framework of grains dominated by relatively weathering-resistant silicates and aluminosilicates, whereas matrix and cement minerals in general are less resistant to weathering. Sandstone in both natural and anthropogenic environments are exposed to (bio)weathering processes, causing the release of elements from minor constituents (matrix and cement minerals) and major framework minerals and resulting in the presence of an irreversible fingerprint on the mineral/rock surface. As the result of bioweathering some adverse features such as corrosion, cracking and rock fatigue occur which arises a strong concern regarding durability of sandstones in a long-term perspective. This review paper describes analytical methods applied for determining the mineralogical composition of sandstones and their alteration features, defines bioweathering process and factors attributed to it, analyzes susceptibility of sandstone minerals to dissolution, presents experimental simulations dedicated to stability assessment. This review highlights that bioweathering processes may affect durability of construction and building materials. The gaps in the experimental research are indicated and recommendations on how to fill these gaps are provided.
... Previous attempts to densify NiO with various aqueous and non-aqueous transport phase have shown minimal success [24,26]. The slow dissolution kinetics and slow ligand exchange rate of Ni 2? ions compared to other transition metal cations have been cited as potential reasons for the increased difficulty to cold sinter this material [27][28][29]. ...
Densifying ZnFe2O4 and BaFe12O19 at 300 °C to values greater than 90% of theoretical is demonstrated via hydroflux-assisted densification (HAD), a derivative of the cold sintering process employing non-aqueous, flux-based mass transport phases to facilitate particle consolidation. Previous attempts to cold sinter these materials with aqueous-based mass transport phases were not as successful with final densities < 90%. Attempts to densify NiFe2O4 and (Ni0.5Zn0.5)Fe2O4 only achieved densities around 80%, indicating an alternative transport phase may be needed to achieve high densities in Ni-containing materials. Magnetic hysteresis measurements of the low-temperature densified BaFe12O19 samples produced magnetic saturation values as high as 93 emu/g and coercive fields as high as 1789 Oe, which are comparable to values reported in the literature for this material produced via other processing techniques. Additional techniques are suggested to further optimize the magnetic properties of BaFe12O19 densified following the HAD approach.
... Basalt application to acid soils presents an interesting scenario for effective CO2 mitigation combined with pH neutralisation. Basalts can increase soil pH through hydroxyl ion production from silicate dissolution, development of secondary high cation exchange clay minerals from chemical weathering [128], removal of protons from solution by exchange during mineral weathering [129], and the development of carbonate mineral species. Moreover, under strongly acidic soil conditions, the kinetics of silicate mineral weathering could be significantly enhanced. ...
The agricultural sector faces looming challenges including dwindling fertiliser reserves, environmental impacts of conventional soil inputs, and increasingly difficult growing conditions wrought by climate change. Naturally-occurring rocks and minerals may help address these challenges. In this case, we explore opportunities through which the geosphere could support viable agricultural systems, primarily via a literature review supplemented by data analysis and preliminary-scale experimentation. Our objective is to focus on opportunities specifically relating to emerging agricultural challenges. Our findings reveal that a spectrum of common geological materials can assist across four key agricultural challenges: 1. Providing environmentally-sustainable fertiliser deposits especially for the two key elements in food production, nitrogen (via use of slow release N-rich clays), and phosphorus (via recovery of the biomineral struvite) as well as through development of formulations to tap into mineral nutrient reserves underlying croplands. 2. Reducing contamination from farms—using clays, zeolites, and hydroxides to intercept, and potentially recycle nutrients discharged from paddocks. 3. Embedding drought resilience into agricultural landscapes by increasing soil moisture retention (using high surface area minerals including zeolite and smectite), boosting plant availability of drought protective elements (using basalts, smectites, and zeolites), and decreasing soil surface temperature (using reflective smectites, zeolites, and pumices), and 4. mitigating emissions of all three major greenhouse gases—carbon dioxide (using fast-weathering basalts), methane (using iron oxides), and nitrous oxide (using nitrogen-sorbing clays). Drawbacks of increased geological inputs into agricultural systems include an increased mining footprint, potential increased loads of suspended sediments in high-rainfall catchments, changes to geo-ecological balances, and possible harmful health effects to practitioners extracting and land-applying the geological materials. Our review highlights potential for ‘geo-agriculture’ approaches to not only help meet several key emerging challenges that threaten sustainable food and fiber production, but also to contribute to achieving some of the United Nations Sustainable Development Goals—‘Zero Hunger,’ ‘Life on Land,’ and ‘Climate Action.’
... In nature, mineral carbonation is technically a three-phase reaction, which involves the dissolution of CO 2 (a gas) and alkaline-earth metal cations from minerals (a solid) into water (a liquid) where they react to form the solid carbonate product [28,29]. The rate limiting step is usually the CO 2 dissolution step, since CO 2 is only very slightly soluble in pure water (around 1.5 grams per liter at 1 bar CO 2 pressure) [30]. ...
The appeal of mineral carbonation (MC) as a process technology for scalable and long-term CO2 reduction, is that it is a solution that has the sequestration capacity to match the amount of CO2 emitted from energy generation and industrial activities [1, 2, 3]. Many inorganic materials such as minerals [4, 5], incineration ash [6, 7], concrete [8, 9] and industrial residues [10, 11] are potentially huge sinks for anthropogenic CO2 emissions. These materials are typically abundant sources of alkaline and alkaline-earth metal oxides, which can react naturally with CO2 to form inorganic carbonates and bicarbonates. In addition, their products are thermodynamically stable and relatively inert at ambient conditions. On paper, MC should be able to fully sequester all anthropogenic CO2 emissions, since the abundance of magnesium and calcium atoms on Earth far exceeds the total amount of carbon atoms [12, 13]. However, despite the apparently favorable pre-conditions, we still observe a net accumulation of CO2 in the atmosphere because the rates of reaction to form (bi)carbonates in nature are too slow compared to the current rate at which CO2 is being emitted [14, 15]. If left to their own devices, thousands of years are needed to achieve any substantial sequestration of CO2 [16]. This is clearly not rapid enough to solve the pressing problem of climate change that is already affecting us now. Therefore there is a need to employ mineral carbonation as an artificial method to accelerate the rates of CO2 sequestration. In this chapter, we will take a look into the chemistry and thermodynamics of mineral carbonation and discuss some of the main obstacles to large scale MC implementation. Additionally, we highlight the types of starting materials from which basic alkaline-earth metal oxides can be obtained and discuss how their abundance and properties affect MC performance. We will also give a short review of current research in the area to develop MC into viable and economic processes, with some focus on the main categories of process designs and their working principles. We will then look at MC from a techno-economic standpoint and assess the opportunities to integrate MC into the existing industrial and environmental landscape. Lastly, we conclude the chapter with a hypothetical scenario of MC deployment in Singapore, an economically developed but land-scarce country under threat by rising sea levels.
... More complex dissolution experiments at different pH ranges and using different experimental conditions are described by Guidry and McKenzie (2003), Tang et al. (2003), Valsami-Jones et al. (1998), andJahnke (1984). Although these experiments tend to provide quantitative information about the dissolution rates of different minerals, Casey et al. (1993) point out that the appropriate level of comparison of field and laboratory weathering is at the scale of reactivity trends and not at that of quantitative similarity in reaction rates. ...
From a biogeochemical perspective, catchments can be regarded as reactors that transform the input of various substances via precipitation, deposition, or human activities as they pass through soils and aquifers towards receiving streams. Understanding and modeling the variability of solute concentration in catchment waters require the identification of prevailing processes, determining their respective contributions to the observed transformation of substances, their interplay with hydrological processes, and the determination of anthropogenic impacts. However, numerous biogeochemical processes often interact in a highly non-linear way and vary on temporal and spatial scales, resulting in temporally and spatially varying water chemistry in catchments. This is particularly true for riparian wetlands. Processes in this catchment area often superimpose the influence of the hill slope (and largest) area of the catchment on surface water quality. Accordingly, the first part of this thesis (Study 1 and 2), focuses on the temporal and spatial variability of biogeochemical processes at the catchment scale. Therefore, the first aim was to identify the prevailing biogeochemical processes which affect the quality of catchment waters in two forested granitic catchments. Based on these results, (i) the long-term behavior of these processes was determined (Study 1) and (ii) hot spots of these processes at the catchment scale along different flow paths were identified (Study 2). The second part (Study 3) focuses on the interplay between hydrological and biogeochemical processes in a riparian wetland, with the aim of systematically tracing back the temporal patterns of stream water chemistry to different biogeochemical processes and antecedent hydrological boundary conditions in the wetland. The third part (Study 4 and 5) focuses on weathering processes with the goal (i) of identifying the mineralogical sources of the groundwater’s buffer capacity against acid atmospheric deposition in a forested granitic catchment and (ii) determining the mineralogical sources of the high cation loads in surface water, induced by intensive agricultural activities in two agricultural granitic catchments. To reach these aims, multivariate statistical methods of dimensionality reduction (linear Principal Component Analysis, non-linear Isometric Feature Mapping), a low-pass filtering of time-series, a Cluster analysis, and major and trace element ratios and strontium isotopes were used. A small number of biogeochemical process bundles explained 94% and 89% of the variance of the data set in Study 1 and 2, respectively. In Study 1, redox and topsoil processes, road salt and sulfate contamination were identified as predominating processes influencing water chemistry in the respective catchments. Low-pass filtered time series of component scores revealed a different long-term behavior at different sampling sites in both catchments, which could be traced back to the fraction of wetland area in the respective subcatchments as well as by the varying thickness of the regolith. Study 2 revealed that the upper 1 m topsoil layer could be considered as a biogeochemical hot spot for redox processes, acid-induced podsolization, and weathering processes along different flow paths. Up to 97% of the biogeochemical transformation of the chemical composition of soil solution, groundwater and stream water in the Lehstenbach catchment was restricted to this soil layer representing less than 2% of the catchment’s regolith. Wetland stream water, mobilized in the topsoil layer being considered a biogeochemical hot spot, showed a highly dynamic temporal pattern of component scores. Study 3 revealed four different types of wetland stream water chemical status, depending on the interplay between discharge dynamics, biological activity, and the water table position in the wetland. The sequence of different stream water types roughly followed a seasonal pattern, albeit being heavily modified by the respective hydrological boundary conditions for different years. Extended periods of low groundwater level in the second half of the growing season drastically changed the chemical boundary conditions, becoming evident in a drastic reoxidation of reduced species like sulfides and corresponding effects. Weathering processes are one of the predominating biogeochemical process bundles influencing water chemistry in forested catchments. Study 4 showed that the mineralogical sources of the groundwater’s buffer capacity against acid atmospheric deposition were dominated by the release of base cations from apatite dissolution, preferential cation release from feldspars and biotite, and feldspars weathering. In Study 5, determining the mineralogical sources of the high cation loads in surface water induced by intensive agricultural activities revealed a dominant manure contribution in the topsoil, and enhanced mineral dissolution (plagioclase and biotite) by fertilizer application in subsoils, becoming the unique source of base cations in the saprolite. Stream water chemistry differed from that of soil water, suggesting that stream water chemistry was dominated by elements issued from enhanced mineral and rock weathering. Soil acidification induced by agriculture allows the mobilization of cations stored in soil layers, enhances the rock weathering and accelerates plagioclase dissolution, which can highly influence stream water quality. Numerous biogeochemical, hydrological, and anthropogenic processes were found to interact with each other, mostly with non-linear patterns, influencing catchment water chemistry. The integral approach used in this thesis would be a useful prerequisite to develop accurate and parsimonious models commonly used for water management purposes by distinguishing between short- term and long-term shifts, reducing the number of processes to the predominating ones ultimately to be included in the model, focusing on hot spots and including spatial patterns where necessary and appropriate.
... The role of precipitation mechanisms on mineral dissolution kinetics is increasingly reported. 5,7,[9][10][11] In the field of glass alteration, condensation mechanisms forming amorphous layers have also long been a subject of interest. [12][13][14][15] Today it is considered that a saturation state can only be expressed with respect to a hydrated layer. ...
Computational codes are necessary tools for geochemical modeling of the alteration of minerals due to their ability to handle key mechanisms, such as dissolution, precipitation, diffusion, and convection at many temporal and spatial resolutions. Modeling glass corrosion specifically requires a description of the amorphous layer that forms on the surface of the glass and its effect on glass alteration kinetics. The objective of the GRAAL model (glass reactivity in allowance of the alteration layer) is both to provide a simple implementation of the passivation process in a reactive transport code and to provide data relative to the composition and the solubility of the amorphous layer. The size and properties of the protective amorphous layer drives the glass alteration rate, with regard to passivation; the greater the quantity of the protective amorphous layer, the lower the dissolution rate of the primary mineral. Here, concepts, equations, and implementation of GRAAL are reported. Simple glass alteration experiments are used to apply the model and measure parameters. The International Simple Glass used for nuclear glass long-term behavior studies is at the center of the glass compositions studied.
... However, popular models of stress corrosion rates focus on dissolution kinetics-based theories (i.e., Eppes & Keanini, 2017). The main issue is that dissolution rates of carbonate rocks in laboratory studies exceeds those in natural environments by 1-3 orders of magnitudes (Casey et al., 1993;Velbel, 1993;White & Brantley, 2003). Applying these dissolution rates to stress corrosion cracking (i.e., Atkinson, 1984;Eppes & Keanini, 2017) likely overestimates the chemical effects. ...
Our concept of progressive rock slope failures is on the one hand embedded in aggregated subcritical crack growth mechanisms and on the other sensitive to environmental conditions, especially water. To anticipate failure dynamics in rock slopes, it is a key requirement to reveal the influence of water on subcritical crack growth mechanisms and material properties. We present experimental data on the time-dependent deformation of an exemplary rock, Carrara marble. We employed inverted single-edge notch bending creep tests on large Carrara marble samples to mimic an open joint system with controlled water supply. Constant stress was applied in two steps approaching 22-85% of a previously determined critical baseline stress. Introducing calcite-saturated water to subcritical stressed samples caused an immediate increase in strain by up to an order of magnitude. Time-dependent accumulation of inelastic damage at the notch tip occurred in wet and dry samples at all load levels. Subcritical crack growth and the evolution of localized intergranular fractures are enhanced if water is present and readily approach tertiary creep when loaded above 80%. The immediate strain response is attributed to the reduction of surface energy and diffusion of the water into the rock. The resultant more compliant and weaker rheology can even turn the subcritical stress into a critical state. Over time, subcritical and chemically enhanced mechanisms progressively alter especially grain boundaries, which become the key controls of progressive failure in Carrara marble.
... Due to unavoidable differences in extrinsic (scale, timeframe) and intrinsic factors (grain size, composition), laboratory experiments never exactly mimic natural processes (e.g., Casey et al., 1993). For example, pedogenic processes -i.e., vertical mobility of dissolved elements and pH variations within weathering profiles -are not reproduced in our experimental setting, although they have a primary influence on the nature and abundance of secondary phases (e.g., Gaudin et al., 2011). ...
Laboratory experiments are useful to constrain the environmental parameters that have allowed the formation of the ancient hydrous mineralogical assemblages observed at the surface of Mars, which are dominated by ferric smectites. Weathering under a dense CO2 atmosphere on early Mars is a process frequently invoked to explain their formation, but has proven difficult to test in the laboratory due to low reaction rates. Here, we present a long-term weathering experiment (470 days, at 45 °C) of forsteritic olivine specially designed to increase as much as possible the amount of reaction products and thus allow their detailed mineralogical, petrological and chemical characterization by FTIR, SEM and TEM. Our results show the formation of crystalline smectites both under 1 bar of CO2 and under ambient air. However, important differences are observed between the two types of conditions. The smectite formed under CO2 has an average chemical formula per half unit-cell of Si3.92Al0.16Fe³⁺0.78Mg1.66 Cr0.01Ni0.06K0.04Ca0.04.O10(OH)2. It is thus intermediate between a trioctahedral Mg-rich saponite and a dioctahedral ferric smectite. It is also clearly enriched in Fe compared its counterpart formed under ambient air, which has an average chemical formula per half unit-cell of Si3.68Al0.12Fe³⁺0.37Mg2.61Cr0.01Ni0.02K0.04Ca0.25.O10(OH)2. This result demonstrates that the enrichment in Fe observed for Martian smectites is to be expected if they were formed by low-temperature weathering under a dense CO2 atmosphere. Another difference is the nature of the accompanying phases, which includes amorphous silica (in the form of opal spheres 10 to 100 nm in diameter) and Mg-carbonates under CO2, but is limited to rare kaolinite under ambient air. The observation of kaolinite particles under air and the significant amount of Al measured in smectites under both atmospheres, despite the Al-poor nature of the initial material, shows that this element is easily concentrated by low-temperature weathering processes. At a larger scale, this concentration mechanism could be responsible for the formation of Al-rich upper horizons, as frequently observed on Mars.
... This is certainly a limitation of this study and work is on-going to incorporate this mechanism into 25 the model. A number of studies (Casey et al., 1993; Goddéris et al., 2006; Maher et Tables Figures Back Close Full Screen / Esc Printer-friendly Version ...
Silicate mineral dissolution rates depend on the interaction of a number of
factors categorized either as intrinsic (e.g. mineral surface area, mineral
composition) or extrinsic (e.g. climate, hydrology, biological factors,
physical weathering). Estimating the integrated effect of these factors on
the silicate mineral dissolution rates therefore necessitates the use of
fully mechanistic soil evolution models. This study applies a mechanistic
soil evolution model (SoilGen) to explore the sensitivity of silicate mineral
dissolution rates to the integrated effect of other soil forming processes
and factors. The SoilGen soil evolution model is a 1-D model developed to
simulate the time-depth evolution of soil properties as a function of various
soil forming processes (e.g. water, heat and solute transport, chemical and
physical weathering, clay migration, nutrient cycling and bioturbation)
driven by soil forming factors (i.e., climate, organisms, relief, parent
material). Results from this study show that although soil solution chemistry
(pH) plays a dominant role in determining the silicate mineral dissolution
rates, all processes that directly or indirectly influence the soil solution
composition equally play an important role in driving silicate mineral
dissolution rates. Model results demonstrated a decrease of silicate mineral
dissolution rates with time, an obvious effect of texture and an indirect but
substantial effect of physical weathering on silicate mineral dissolution
rates. Results further indicated that clay migration and plant nutrient
recycling processes influence the pH and thus the silicate mineral
dissolution rates. Our silicate mineral dissolution rates results fall
between field and laboratory rates but were rather high and more close to the
laboratory rates owing to the assumption of far from equilibrium reaction
used in our dissolution rate mechanism. There is therefore need to include
secondary mineral precipitation mechanism in our formulation. In addition,
there is need for a more detailed study that is specific to field sites with
detailed measurements of silicate mineral dissolution rates, climate,
hydrology and mineralogy to enable the calibration and validation of the
model. Nevertheless, this study is another important step to demonstrate the
critical need to couple different soil forming processes with chemical
weathering in order to explain differences observed between laboratory and
field measured silicate mineral dissolution rates.
... This is certainly a limitation of this study and work is on-going to incorporate this mechanism into the model. A number of studies (Casey et al., 1993;Goddéris et al., 2006;Moore et al., 2012;Zhu, 2005;Zhu et al., 2010) have already demonstrated that solute composition and secondary mineral precipitation controls the reaction affinity of primary minerals. ...
Soil is a cornerstone to many ecosystem services such as water purification, food, wood and fibre production, nutrient cycling, climate regulation and physical support to human infrastructures. Unfortunately, the sustainability of this important resource is under many threats arising from human interventions and climate change. The soil system processes are therefore significantly changing soil properties even at shorter time scales. Tools capable of assessing such changes are currently needed to be able to guide the sustainable use of the soil resource. Mechanistic soil evolution models are increasingly becoming such invaluable tools to facilitate an improved understanding and prediction of soil evolution. Such information is needed to provide answers to many environmental questions including soil sustainability, soils and climate change interactions, food security and provision of ecosystem services. However modelling of the soil system as a whole, both at profile and landscape scales remains a big challenge. Although modelling of soil and landscape evolution has progressed rapidly over the last decade, emphasis has been on parallel scales rather than integral scales. Therefore, there are models capable of modelling landscape evolution but lack detailed vertical soil forming processes defined in profile scale models and viceversa. It is becoming inevitable to integrate these two approaches in order to have models capable of simulating soil development and global change feedbacks. However such integration is only feasible after addressing challenges such as process coverage, model calibration and verification, and model result quality. The objectives of this study were to contribute to the state of art overview of soil modelling especially at pedon scale, adapt an existing soil evolution model (the SoilGen model) and extend its chemical weathering and biogeochemical modules such that the fate of a wide range of minerals and elements can be simulated, and to apply the SoilGen model to assess the sensitivity of soil processes and properties to global change (i.e., change in model boundary conditions such as climate, vegetation and soil use). From an overview study, we could conclude that the strengths of the SoilGen soil evolution model included its ability to simulate the integrated effect of biological, geochemical and physical soil processes moreover at a millennium time scale. However some of the processes such as chemical weathering were simplified and needed to be extended. In this study therefore, we formulated an extended chemical weathering mechanism that includes the weathering of primary and secondary minerals and the precipitation of secondary minerals. The approach also takes into account the effect of physical weathering on chemical weathering through its effect on mineral specific surface areas over time. The chemical module of the SoilGen model was also reformulated to include the cycling of Fe and Si in addition to the already simulated elements (i.e., Na, K, H, Mg, CA and Al). Furthermore, the SoilGen model was applied in two case studies. In the first case study, the model was applied to test the hypothesis that the relationships between soil properties (e.g., soil horizon depth) and landscape position that were surprisingly absent (based on field measurements) could be explained by the variation in local factors such as tree falls. Two model scenarios of soil development (i.e., with and without tree uprooting events) were defined. The same boundary conditions (climate, vegetation) were used in both scenarios and the SoilGen model simulations (in terms of profiles of a.o. organic carbon, clay percentage) were first converted to horizon depth before being confronted with measured horizon depth. In total, 108 soil profiles (from Meerdaal forest, Belgium) were simulated and the results from the model with treefall events exhibited the same trends as the observed data. The conclusion was drawn that bioturbation due to treefalls could be an explanation for the lack of correlation between landscape properties and soil horizon depths as observed in the field. In the second case study, the extended SoilGen model was applied to explain the sensitivity of silicate mineral dissolution rates to physical weathering. Our working hypothesis was that physical weathering affects the magnitude of chemical weathering and this could partly explain the systematic deviations between laboratory and field silicate mineral dissolution rates. We tested the hypothesis on the forested loess soil, in the Zinnia forest, Belgium (50°46’31”N, 4°24’9”E). Climate and vegetation evolution over the last 15000 years of model simulation period were reconstructed and were readily available for this site. Our results demonstrated a dominant role of pH and an indirect but substantial effect of physical weathering on silicate mineral dissolution rates. Furthermore, clay migration and plant nutrient recycling processes influenced the pH and thus the dissolution rates. SoilGen simulated silicate dissolution rates were between laboratory and field measured rates. Results from this study demonstrated the necessity to couple different soil-forming processes in mechanistic soil models in order to explain the differences between lab and field dissolution rates. In general, we could conclude that the SoilGen model is capable of modelling soil processes under different scenarios of climate and land use change. Given field measurements, it is also possible to verify most of the processes defined in the model. However, a number of challenges need to be addressed (e.g., computation time, process coverage) and more model tests in other environments are required before the model can be integrated into the landscape evolution and other interdisciplinary models.
... − 0.43 · pH) for the non-reductive, proton-promoted dissolution rate of goethite and hematite (marked with × and +, respectively) as shown in Fig. 5.9a, and the solid line is the predicted dissolution rate of Fe(II)O. This rate was predicted based on the proportionality observed for divalent metal oxides (see for example Fig. 5.10a) between the log (dissolution rate) and the log (first-order rate constant, k solv ) for water exchange within the inner-coordination sphere of the hydrated metal ion (Casey et al., 1993a). These authors report dissolution rates for BeO, NiO, CoO, MnO, ZnO, and CaO that can be modeled as log (rate at pH2) (mol m −2 s −1 ) = −17.20(±0.69) ...
... The possible relevance of transport to the kinetics of silicate weathering has been discussed for decades (e.g., White and Brantley 2003;Maher 2010;Brantley et al. 1986;Mikesell et al. 2004;Velbel 2011;Muir et al. 1989;Knapp 1989;Schnoor 1990;Casey et al. 1993;Kump et al. 2000;Li et al. 2008;Raoof and Hassanizadeh 2010;Dentz et al. 2011). Traditionally, the transport process considered was diffusion (e.g., White and Brantley 2003). ...
... In addition to shedding light on fault motion, our results could also have important implications for the reactivity of rocks and minerals in other environments. Relative to the rates measured in laboratory experiments, minerals in natural settings often weather much more slowly [e.g., Anbeek, 1993;Casey et al., 1993; White and Figure 4. High-angle annular dark field STEM image of a dolostone fault mirror. The nanoparticle layer making up the fault mirror surface appears to have a denser, less porous, upper layer and a lower density, more porous lower layer. ...
Fractured rocks in fault zones regain their mechanical strength through a process called healing. A central pathway for healing involves the dissolution and reprecipitation of minerals in the fault zone which cements the fractured rocks during interseismic periods. However, some faults contain highly polished surfaces - coated in a thin nano-particle layer - along which slip is localized. Crucially, these surfaces show little evidence of post-seismic mineralization and healing. Here, we use atomic force microscopy to show that naturally polished rocks from carbonate fault zones are resistant to dissolution, in stark contrast to the reactive minerals that make up the fault breccia. Our results suggest that the low reactivity of the nano-particle layer could retard healing, helping to maintain the localization of the fault zone between seismic slip events. As fault localization affects seismic motion, the geochemical reactivity of fault mirrors could be an important control on seismicity along faults.
... This inference is based on a flow rate of 6.5 ¥ 10 -5 m 3 /m 2 ·s 1 (this experiment), whereas fluxes during natural serpentinization are estimated to range between 10 -10 and 10 -12 m 3 /m 2 ·s 1 (Macdonald 1984). Other parameters that may significantly affect the above inference when applied to natural systems include P H2O (i.e., P H2O < P Fluid and P Fluid < P Total ), and the electrostatic effects produced by phyllosilicates on the fluid as it circulates along extremely small channels (Casey et al. 1993). ...
A single-pass flow-through experiment was conducted to induce serpentinization, and to relate serpentine mineralogy to reaction progress during open-system alteration of San Carlos olivine by air-saturated water. The fluid flowed at 300 °C and 300 bars for 1368 hours through crushed olivine contained in a tubular titanium reaction cell. Chemical analysis of the fluids indicated a steady-state composition with respect to Mg and pH after 550 hours, which was interpreted to reflect saturation with brucite and one or more serpentine-group minerals. The alteration products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). Nucleation and growth characteristics of serpentine phases on the fracture surfaces of olivine were studied by TEM using platinum/carbon replica techniques. After reaction, olivine fragments displayed clear signs of dissolution, and growth of neoformed phases. Brucite and magnetite were distributed uniformly through the reaction cell, although magnetite was more abundant. There is, however, a distinct zonal distribution of the serpentine phases from the inlet to the outlet of the reaction cell. Lizardite is the dominant serpentine phase at the inlet, whereas fibrous serpentines are predominant near the outlet. Lizardite crystals are more numerous (but smaller) in the middle of the cell, and are topoctactically aligned a single preferential crystallographic orientation of the olivine. Cylindrical chrysotile formed predominantly on the surfaces of lizardite, conical or polygonal serpentine. The morphology and sorting of the serpentine phases through the reaction cell suggest that crystallization of lizardite was controlled by heterogeneous nucleation on olivine under supersaturated conditions, and crystallization of chrysotile by nucleation on lizardite at higher levels of supersaturation.
... This is certainly a limitation of this study and work is on-going to incorporate this mechanism into the model. A number of studies (Casey et al., 1993; Goddéris et al., 2006; Maher et al., 2009; Moore et al., 2012; Zhu, 2005; Zhu et al., 2010) have already demonstrated that solute composition and secondary mineral precipitation controls the reaction affinity of primary minerals. The dissolution rates from this study are therefore expected to be faster than they would be if secondary mineral precipitation were to be taken into account. ...
Silicate mineral dissolution rates depend on the interaction of a number of
factors categorized either as intrinsic (e.g. mineral surface area, mineral
composition) or extrinsic (e.g. climate, hydrology, biological factors,
physical weathering). Estimating the integrated effect of these factors on
the silicate mineral dissolution rates therefore necessitates the use of
fully mechanistic soil evolution models. This study applies a mechanistic
soil evolution model (SoilGen) to explore the sensitivity of silicate
mineral dissolution rates to the integrated effect of other soil-forming
processes and factors. The SoilGen soil evolution model is a 1-D model
developed to simulate the time-depth evolution of soil properties as a
function of various soil-forming processes (e.g. water, heat and solute
transport, chemical and physical weathering, clay migration, nutrient
cycling, and bioturbation) driven by soil-forming factors (i.e., climate,
organisms, relief, parent material). Results from this study show that
although soil solution chemistry (pH) plays a dominant role in determining
the silicate mineral dissolution rates, all processes that directly or
indirectly influence the soil solution composition play an equally important
role in driving silicate mineral dissolution rates. Model results
demonstrated a decrease of silicate mineral dissolution rates with time, an
obvious effect of texture and an indirect but substantial effect of physical
weathering on silicate mineral dissolution rates. Results further indicated
that clay migration and plant nutrient recycling processes influence the pH
and thus the silicate mineral dissolution rates. Our silicate mineral
dissolution rates results fall between field and laboratory rates but were
rather high and more close to the laboratory rates possibly due to the assumption
of far from equilibrium reaction used in our dissolution rate mechanism.
There is therefore a need to include secondary mineral precipitation mechanism
in our formulation. In addition, there is a need for a more detailed study
that is specific to field sites with detailed measurements of silicate
mineral dissolution rates, climate, hydrology, and mineralogy to enable the
calibration and validation of the model. Nevertheless, this study is another
important step to demonstrate the critical need to couple different soil-forming processes with chemical weathering in order to explain differences
observed between laboratory and field measured silicate mineral dissolution
rates.
... Reversible transformations between chemically labile (but often non-reactive) and reactive 78 metal pools can occur, implying that the size of the reactive fraction in soil can change with the 79 aging time. The reactivity of a metal in soil can vary depending on the anthropogenic source from 80 which it is emitted (Degryse et al., 2004; Roberts et al., 2002; Vespa et al., 2010) and furthermore it 81 can increase with time due to weathering (resulting in destruction of chemical bonds, most often 82 those between metal and oxygen) (Casey et al., 1993) or it can decrease with time due to fixation in 83 the soil through short-and long-term reactions with soil constituents (Buekers et al., 2008b; Crout 84 et al., 2006; Ma et al., 2013 Ma et al., , 2006 Wendling et al., 2009). 85 To accommodate these potentially important influences on the metal ecotoxicity in LCA, some 86 modifications and further developments of the methodology are needed: (i) in the LCI phase, metal 87 emissions should be reported together with sufficient information to allow capturing the differences 88 that exist in reactivity between the various chemical forms that are present in the emissions from 89 different sources; (ii) in the LCIA phase, CTPs for the metal of interest have to be available that 90 match the information from the emission inventory and represent the ecotoxicity potential of the 91 different metal forms; and (iii) in the LCIA phase, such a set of CTPs for the metal of interest has to 92 consider metal aging in the soil, as influenced both by soil properties and by the aging pattern 93 exhibited by the different metal forms reported in the LCI phase. ...
Metal exposure to terrestrial organisms is influenced by the reactivity of the solid-phase metal pool. This reactivity is thought to depend on the type of emission source, on aging mechanisms that are active in the soil, and on ambient conditions. Our work shows, that when controlling for soil pH or soil organic carbon, emission source occasionally has an effect on reactivity of Cd, Co, Cu, Ni, Pb and Zn emitted from various anthropogenic sources followed by aging in the soil from a few years to two centuries. The uncertainties in estimating the age prevent definitive conclusions about the influence of aging time on the reactivity of metals from anthropogenic sources in soils. Thus, for calculating comparative toxicity potentials of man-made metal contaminations in soils, we recommend using time-horizon independent accessibility factors derived from source-specific reactive fractions.
Copyright © 2015 Elsevier Ltd. All rights reserved.
... The evaluation of the stability of nuclear waste forms requires our ability to predict dissolution rates of fluid-solid reactions with high reliability for long time periods of many thousands of years. The reaction rates of natural and technical materials, determined under similar laboratory conditions show broad variations, often several orders of magnitude [1][2][3][4][5] . Even though these effects were already attributed to properties of the material under investigation, e.g., single crystals versus polycrystalline ceramics, so far no systematic understanding of the different contributors to the overall rate has been provided. ...
The long-term stability of ceramic materials which are considered as potential nuclear waste forms is governed by heterogeneous surface reactivity. Thus, instead of a mean rate, the identification of one or more dominant contributors to the overall dissolution rate is the key to predict the stability of waste forms quantitatively. Direct surface measurements by vertical scanning interferometry (VSI) and their analysis via material flux maps and resulting dissolution rate spectra provide data about dominant rate contributors and their variability over time. Using pyrochlore (Nd2Zr2O7) pellet dissolution under acidic conditions as an example, we demonstrate the identification and quantification of dissolution rate contributors, based on VSI data and rate spectrum analysis. Heterogeneous surface alteration of pyrochlore varies by a factor of about 5 and additional material loss by chemo-mechanical grain pull-out within the uppermost grain layer. We identified four different rate contributors that are responsible for the observed dissolution rate range of single grains. Our new concept offers the opportunity to increase our mechanistic understanding and to predict quantitatively the alteration of ceramic waste forms.
... − 0.43 · pH) for the non-reductive, proton-promoted dissolution rate of goethite and hematite (marked with × and +, respectively) as shown in Fig. 5.9a, and the solid line is the predicted dissolution rate of Fe(II)O. This rate was predicted based on the proportionality observed for divalent metal oxides (see for example Fig. 5.10a) between the log (dissolution rate) and the log (first-order rate constant, k solv ) for water exchange within the inner-coordination sphere of the hydrated metal ion (Casey et al., 1993a). These authors report dissolution rates for BeO, NiO, CoO, MnO, ZnO, and CaO that can be modeled as log (rate at pH2) (mol m −2 s −1 ) = −17.20(±0.69) ...
Over the last several billion years, rocks formed at equilibrium within the mantle of the Earth have been exposed at the surface and have reacted to move towards a new equilibrium with the atmosphere and hydrosphere. At the same time that minerals, liquids, and gases react abiotically and progress toward chemical equilibrium at the Earth’s surface, biological processes harvest solar energy and use it to store electrons in reservoirs which are vastly out of equilibrium with the Earth’s other surface reservoirs. In addition to these processes, over the last several thousand years, humans have produced and disseminated non-equilibrated chemical phases into the Earth’s pedosphere, hydrosphere, and atmosphere. To safeguard these mineral and fluid reservoirs so that they may continue to nurture ecosystems, we must understand the rates of chemical reactions as driven by tectonic, climatic, and anthropogenic forcings.
... In this study, we utilized an in situ X-ray diffraction (XRD) technique to characterize the carbonation steps of synthetic forsterite (Mg 2 SiO 4 ) during exposure to water-saturated scCO 2 fluids that had been equilibrated with organic-bearing aqueous solutions of acetate (C2H3O2 -), oxalate (C2O4 2-), malonate (C3H2O4 2-), or citrate (C6H5O7 3-). Although aqueous-based forsterite dissolution has been extensively studied 15 , it is also important to investigate the coupled dissolution-precipitation carbonation reactions because forsterite is often in intimate contact with secondary phases in water-poor weathering environments [16][17][18] . Acetate, malonate, and oxalate were chosen as they are the most abundant organic ligands in the subsurface, with concentrations exceeding 20 mM in hydrocarbon-associated waters, in which they can survive in significant concentrations at temperatures up to 200 °C for geologic timescales [19][20][21][22][23] . ...
... All rights reserved. and a solid content below 50 g/L) (Awad et al., 2000;Blum and Lasaga, 1988;Casey et al., 1993;Chen and Brantley, 2000;Daval et al., 2010Daval et al., , 2013Grandstaff, 1986;Hänchen et al., 2006;Oelkers, 2001;Olsen and Donald Rimstidt, 2008;Pokrovsky and Schott, 2000b;Rosso and Rimstidt, 2000;Wogelius and Walther, 1992). These researchers agree that the dissolution mechanism is controlled by surface reactions, and the kinetics depends on the hydrogen ion activity, surface area of olivine and temperature: ...
This article addresses the kinetics of the dissolution of olivine for nano-silica production at extreme conditions. The extreme conditions are pH values between -0.7 and 1, temperature between 50 and 90 C, solid content around 250 g/l and percentage dissolved between 80 and 99%. This work is structured in 3 parts: 1) chemical and mineralogical characterization of the dunites employed; 2) mechanism of the olivine dissolution focusing on the possible resistances to the transport; and 3) determination of the kinetic parameters kT and n.
The results shown here demonstrate that: 1) the limiting step of this process is not the diffusion through a silica layer but the surface reaction; and 2) the dissolution of olivine under the olivine nano-silica production conditions is well described by:
The average error of the reaction rate calculated using these parameters is 5.5% for dunite CRS-US. In addition, this model is successfully applied to the dissolution of other commercial dunites and for bigger reactor volumes. Therefore, this model can be considered to be robust, and it can be used in the industrial production of olivine nano-silica.
... It is well known that dissolution rates produced in different laboratories differ in the order of up to two orders of magnitude (Kump et al. 2000; Arvidson et al. 2003). In addition to the problematic role of surface areas of minerals (Klotz 1991; Gaus et al. 2008; Fischer et al. 2012) and mineral-fluid ratio (Casey et al. 1993) in rate determinations, a major influencing factor is the hydrodynamic condition chosen for the laboratory experiment. In zerodimensional batch experiments the stirring rate and in rotating disk experiments the rotating rate, respectively , influence the dissolution rate, as it has already been intensely discussed by Morse and Arvidson (2002), Plummer et al. (1978), Pokrovsky et al. (2005 and Sjöberg and Rickard (1984). ...
Among the risks of CO2 storage is the potential of CO2 leakage into overlaying formations and near-surface potable aquifers. Through a leakage, the CO2 can intrude into protected groundwater resources, which can lead to groundwater acidification followed by potential mobilisation of heavy metals and other trace metals through mineral dissolution or ion exchange processes. The prediction of pH buffer reactions in the formations overlaying a CO2 storage site is essential for assessing the impact of CO2 leakages in terms of trace metal mobilisation. For buffering the pH-value, calcite dissolution is one of the most important mechanisms. Although calcite dissolution has been studied for decades, experiments conducted under elevated CO2 partial pressures are rare. Here, the first study for column experiments is presented applying CO2 partial pressures from 6 to 43 bars and realising a near-natural flow regime. Geochemical calculations of calcite dissolution kinetics were conducted using PHREEQC together with different thermodynamic databases. Applying calcite surface areas, which were previously acquired by N2-BET or calculated based on grain diameters, respectively, to the rate laws according to Plummer et al. (Am J Sci 278:179–216, doi:10.2475/ajs.278.2.179, 1978) or Palandri and Kharaka (US Geol Surv Open file Rep 2004–1068:71, 2004) in the numerical simulations led to an overestimation of the calcite dissolution rate by up to three orders of magnitude compared to the results of the column experiments. Only reduction of the calcite surface area in the simulations as a fitting procedure allowed reproducing the experimental results. A reason may be that the diffusion boundary layer (DBL), which depends on the groundwater flow velocity and develops at the calcite grain surface separating it from the bulk of the solution, has to be regarded: The DBL leads to a decrease in the calcite dissolution rate under natural laminar flow conditions compared to turbulent mixing in traditional batch experiments. However, varying the rate constants by three orders of magnitudes in a field scale PHREEQC model simulating a CO2 leakage produced minor variations in the pH buffering through calcite dissolution. This justifies the use of equilibrium models when calculating the calcite dissolution in CO2 leakage scenarios for porous aquifers and slow or moderate groundwater flow velocities. However, the selection of the thermodynamic database has an impact on the dissolved calcium concentration, leading to an uncertainty in the simulation results. The resulting uncertainty, which applies also to the calculated propagation of an aquifer zone depleted in calcite through dissolution, seems negligible for shallow aquifers of approximately 60 m depth, but amounts to 35 % of the calcium concentration for aquifers at a depth of approximately 400 m.
... The formation and evolution of soils, rocks, and landscapes (Buhmann and Dreybrodt, 1985;Brantley, 2008;Jin et al., 2010), the efficiency of carbon capture in deep geological reservoirs (Matter and Kelemen, 2009), and the weathering of man-made materials and structures are all highly dependent on the rates at which minerals dissolve. However, mineral dissolution rates measured in laboratory experiments are often found to be far greater than those measured in field settings, even after the rates have been normalized to account for different environmental conditions (Casey et al., 1993;Baxter and DePaolo, 2000;Malmstrom et al., 2000;White and Brantley, 2003;Yoo and Mudd, 2008;Ma-her et al., 2009;Zhu and Lu, 2009). Due to the inherent difficulty of directly observing slow mineral reactions within complex porous media, evaluating the mechanisms that have been proposed to account for this apparent discrepancy has represented a significant challenge. ...
The rate at which limestone dissolves determines the resistance of
buildings and monuments to weathering, the efficiency of carbon capture
in deep geological reservoirs, and the processes by which soils, rocks,
and landscapes form and evolve. However, the normalized rates of mineral
dissolution measured in laboratory experiments are often found to be far
greater than those measured in field settings. Here, we use atomic force
microscopy (AFM) measurements to demonstrate experimentally that the
rate of calcite dissolution within micron-size pores at the surface of a
limestone sample is much lower than the rate of dissolution in the
surrounding calcite surface. In addition, we use numerical simulations
to show that this difference cannot be explained using a simple
diffusion-surface reaction model. We suggest that the observed
heterogeneous reaction rates could instead be due to the elevated
density of reactive high curvature features on the polished surface
surrounding the pore. These high curvature features can strongly affect
local interfacial free energy, making such surfaces more prone to
dissolution. As a result, polished surfaces should be more reactive than
pore surfaces that have effectively been smoothed during prolonged
contact with natural fluids. As rate experiments routinely use polished
and powdered samples, the results may help to explain the widely
reported discrepancy between lab and field-based dissolution rates.
This article overviews connections between nanoscale weathering and geomorphology. Nanoscale processes are on one side of a fundamental threshold between the coarser microscale (micrometers and up) and the finer nanoscale with different molecular dynamics. Nanoscale processes impact a variety of geomorphic research including Arctic and alpine mineral decay, biotic weathering as an explanation for deviations from Goldich's weathering series, carbon sequestration related to silicate dissolution, case hardening, detachment limited erosion, dirt cracking, geochemical pollution, the meteoric ¹⁰Be cosmogenic nuclide, rock coating behavior, silt production, and tafoni.
Metal exposure to terrestrial organism is influenced by the reactivity of the solid-phase metal pool. Aging is one of the important factors that control the reactivity of the solid-phase metal pool in soil. In this study, the selected 13 soils were collected from different locations of China, representing different soil types. The reactivity variation of spiked Cu and Zn with aging was assessed in these 13 soils, and their comparative toxicity potentials (CTPs) were also calculated. The median reactive fractions (freactive) of Cu and Zn with 95% confidence intervals were 1.6 × 10-2 (3.5 × 10-6 to 2.2 × 10-1) and 0.10 (9.1 × 10-4 to 0.44) kgreactive/kgtotal, and the median CTPs for Cu and Zn were 2.09 (8.1 × 10-4 to 2.2 × 104) and 0.85 (8.5 × 10-4 to 7.2 × 102) m3/kg day, respectively. The statistical analysis indicated that aging variability in the CTP of Cu and Zn was mainly associated with the variability in soil organic carbon and pH. These results stress the importance of dealing with aging in the calculation of CTPs for terrestrial ecotoxicity of metals.
In the context of the French research law dedicated to the radioactive waste management, several ceramics were proposed for the specific conditioning of actinides. Among them, monazites and monazite/cheralite solid solutions were particularly considered in this work.Thus, monazites LnPO4 (Ln = La → Gd) were prepared by thermal conversion of low-temperature rhabdophane precursors, LnPO4·0.667H2O. From synchrotron experiments, the crystal structure of rhabdophane was solved. It was found to be monoclinic (space group C2) as monazite (space group P21/n). The multiparametric study of the kinetics of dissolution of monazites LnPO4 (Ln = La → Gd) was performed in nitric acid solutions and using dynamic conditions. The normalized dissolution rates remained very low whatever the acidity, the temperature, and the lanthanide element considered. The partial order of the reaction related to the protons activity varied from 0.7 ± 0.2 to 1.5 ± 0.3. Moreover, the apparent activation energy of the dissolution mechanism was found to vary with temperature, suggesting a change in the rate-limiting step. The decrease of the apparent activation energy for T 313 K was assigned to the impact of saturation processes with respect to the rhabdophane.Thus, the solubility products as well as thermodynamic data associated to the formation of rhabdophanes were evaluated from over- and under-saturation conditions. The similar values obtained by both approaches confirmed the reversibility of the equilibrium associated to the rhabdophane precipitation. Solubility products as well as thermodynamic data only slightly varied along the lanthanide elements series : –2151 ± 13 ≤ ∆f Ho (298K) ≤ –2130 ± 12 kJ.mol-1, –2004 ± 2 ≤ f Go (298K) ≤ –1984 ± 2 kJ.mol-1 and –504 ± 11 ≤ ∆f So (298K) ≤ – 473 ± 12 J.mol-1.K-1, excepted for europium that presented the highest values of free energy (‒1896 ± 2 kJ.mol-1), enthalpy (‒2057 ± 9 kJ.mol-1) and entropy (–538 ± 11 J.mol-1.K-1) of formation. The comparison of the data obtained for rhabdophanes and monazites showed that the values associated to rhabdophane can be deduced from those of monazite by adding the contribution of 0.667 water molecule. It confirms either the monoclinic structure of the rhabdophane and the number of water molecules present in the structure.For the first time, structural and microstructural evolution of monazite pellets was monitored during dissolution thanks to several complementary surface analysis techniques (ESEM, AFM, GI-XRR and GI-XRD). This study confirmed the very good chemical durability of these ceramics (only 0.04 wt.% and 0.3 wt.% of dissolved ceramic in 0.1 M HNO3 and 0.25 M HNO3, respectively, at 363 K after 300 days of dissolution). These different techniques also highlighted that pre-existing defects (pores, cracks, polishing marks) acted as preferential dissolution zones. However, no secondary phase was clearly evidenced onto the surface of the samples even if the solutions were found to be close to equilibrium with rhabdophane.Finally, an original protocol of precipitation of Th-doped rhabdophanes, Nd1-2xCaxThxPO4·0.667H2O, was developed during this work. The first estimation of the apparent solubility constants, performed in under-saturated conditions in 0.25 M HCl solution, did not evidence any significant variation in the temperature range investigated.
Agrominerals are finely ground rocks and minerals used as low-cost fertilisers, and they have received more attention in recent years as sustainable development and climate change mitigation have come to the forefront of societal concerns. Here, we summarise progress in agromineral research over the last 20 years, and discuss the challenges and opportunities of this discipline. The idea of agrominerals has been around since the early 19th century. However, widespread application is subject to economic practicality. In recent years, two big trends have dominated agromineral research. First, some global warming mitigation strategies, such as 'enhanced chemical weathering' and bio-energy carbon capture and storage call for the application of rock powders in arable land on a massive scale. This gives agromineral research an urgency and significance. Second, advances in knowledge of mineral weathering kinetics are poised to transform predictions of agronomic effectiveness from mere empirical studies to more quantitative evaluation. We now have a much better understanding of the factors that influence weathering and nutrient release rates. We forecast that rapid advances in some areas of biogeochemistry will enable advances in the study of agrominerals. In particular, we will be able to measure weathering and nutrient release rates at the field scale, and ultimately to predict kinetic processes of mineral dissolution or precipitation in soil-water-plant systems and the cycling of nutrients and toxic elements in agricultural land.
Water (ice, liquid, or vapor) is a critical driver of future exploration, and methods of its detection and characterization are a high priority for upcoming lunar missions. Thus, we assess the potential for alteration products resulting from water-ice liberated during various impact events in the lunar polar regions. In this work, we estimate the maximum amount and duration of melted, vaporized, or sublimed water-ice during representative post-impact environments using a model of bulk heat transfer. Our model is sensitive to heat loss by radiation, initial and final near-surface temperatures, and pre-existing water-ice abundance and distribution. Mineral dissolution rates in aqueous solution are used as a metric for potential chemical alteration in the presence of liberated water-ice following an impact. We find that the modeled timescales and potential for water liberation and reactivity are compatible with near-surface chemical alteration in some lunar post-impact environments. While initial surface temperatures less than ∼110 K are adequate to maintain near-surface ice reservoirs at the lunar poles, when heated, pore pressures below a depth of ∼35 cm are potentially adequate to sustain liquid water. Mild near-surface environments (e.g., ∼5 °C) lasting a few decades, allow for aqueous alteration of sensitive minerals such as olivine, apatite, and glassy materials. Higher temperatures favor degassing of H2O, but vapor-phase interactions may occur. The limited amounts of available water will likely result in reactions with only the most sensitive minerals such as glasses and Fe-metal. Over time, secondary mineralization would be mixed into the upper few meters of the lunar regolith through subsequent bombardment, assuming it escapes later intense heating events; however, surface exposures would be subjected to space weathering. Nonetheless, based on our modeling, future explorers should consider instrumentation capable of detecting minor to trace amounts of impact-induced chemical alteration in the upper few meters of the lunar surface.
The production of H2 by oxidation of FeO, taken here as model compound for steel slags, has been investigated both in pure water and under acidic aqueous conditions in the 373–573 K temperature range. Whereas after 65 h, H2 yield was negligible in pure water at 423 K, the reaction 3 FeO(s) + H2O(l) → Fe3O4(s) + H2(aq) reached near completion at the same temperature within 10 h in a solution containing 0.05 mol/l acetic acid. Increasing acetic acid concentration by one order of magnitude did not yield significantly more H2. At identical initial pH, acetic acid was found to be more efficient than oxalic acid and hydrochloric acid at enhancing H2 production. Acidic conditions increased FeO dissolution kinetics and, consequently, improved H2 yield. The specific efficiency of acetic acid resides in its thermal stability as well as in the potential of ligand-promoted Fe(II) dissolution. We show that the positive kinetics effect of mild acetic acid solutions over H2 yield evidenced on FeO does not apply directly to steel slags which buffer the pH to high values due to the presence of large amounts of CaO.
Die Gesteinsverwitterung umfasst im Wesentlichen physikalische und chemische Prozesse. Die mechanische Zerkleinerung des Gesteins wird als Physikalische Verwitterung, die Veränderung der mineralogischen und chemischen Zusammensetzung als Chemische Verwitterung bezeichnet. Sind Lebewesen an den Gesteinsveränderungen beteiligt, spricht man, unabhängig vom Mechanismus, von Biologischer Verwitterung. Bei der chemischen Verwitterung wird zwischen thermodynamischen und kinetischen Aspekten unterschieden. Thermodynamische Gleichgewichtskonzepte stehen im Vordergrund bei konstanten Bedingungen, bei Partikeln mit hoher spezifischer Oberfläche, bei langer Kontaktzeit und bei rasch reagierenden Mineralien. Kinetische Betrachtungen werden relevant, wenn die Kontaktzeit kurz, die zugängliche Oberfläche begrenzt und die Reaktionen der beteiligten Mineralien langsam sind. Wichtigste Reaktionsmechanismen bei der kinetisch kontrollierten Verwitterung von Silikaten sind die protonen- und die ligandeninduzierte Auflösung. Enthält ein Mineral ein oder mehrere Elemente, welche in verschiedenen Oxidationsstufen vorkommen können, kann es bei der Verwitterung, je nach Bedingungen, zu einer Oxidation oder Reduktion der entsprechenden Elemente kommen. Pedologisch wichtige Beispiele sind die oxidative Verwitterung von Pyrit, die Oxidationsverwitterung von Biotit und die reduktive Auflösung von Fe(III)-Oxiden. Die neueren Erkenntnisse zur Verwitterung basieren praktisch ausschließlich auf Laborexperimenten unter kontrollierten Bedingungen. Die Gesteinsverwitterung im Boden verläuft aber unter dauernd wechselnden Bedingungen. Aus Felddaten abgeschätzte Verwitterungsgeschwindigkeiten sind etwa 1 bis 2 Zehnerpotenzen geringer als die entsprechenden Laborwerte. Eine Modellierung der Feldverhältnisse unter Verwendung von Labordaten wird erst dann möglich sein, wenn es gelingt, die Effekte von Temperatur, pH, Ionenstärke, Zusammensetzung der Bodenlösung, Sättigungsbedingungen und vor allem der hydrologischen Vorgänge auf die Auflösungs- und Ausfällungsgeschwindigkeiten quantitativ zu beschreiben.
Scale is a fundamental theoretical issue in geomorphology, and while it has received considerable attention from process geomorphologists as a group, it has received substantially less attention from periglacial geomorphologists. While issues of temporal and spatial scale are inextricably linked, the focus of this paper is on spatial scale issues and on spatial scale linkage in the context of periglacial landform and landscape evolution. It has been widely argued within process geomorphology that landforms, landscapes, and geomorphic processes are scale dependent, and consequently consistent representations of form and process across the Earth's surface cannot be meaningfully extended from representations at any given scale. This circumstance arises from the widely held belief that emergent variables appear at larger scales which are not identifiable at smaller scales and thus preclude upscaling as a means of understanding large scale systems. Despite the potential pitfalls associated with scale linkage, various approaches have been adopted to extend research findings at one scale to another scale. These include the application of methodologies which specifically exclude scale variability such as the use of dimensionless values, smoothing of small scale data to reveal large scale patterns, reformulation of small scale data using a synthetic approach, and the averaging of process patterns. This paper argues that a more promising approach to dealing with the dilemma of upscaling is to adopt a factorial approach, which acknowledges problems of emerging variables, and instead focuses on process-control factors. Three systematic areas: weathering, hillslope processes, and patterned ground are selected to demonstrate the application of factor-based scale linkage within periglacial geomorphology.
Altered perthites from a weathered pegmatite in the Spruce Pine District, North Carolina, were characterized by electron microprobe as a K-rich microcline host with lesser Na-rich plagioclase having a lamellar morphology. Light-optical and transmission electron microscopy (TEM) show microtextural elements such as phase boundaries, holes and microfractures that could serve as potential nucleation sites for alteration to clay minerals.
The host microcline contains albite and pericline twinning textures that vary in character; the amount of each twinning type and/or the size of twin individuals changes on a μm scale. Plagioclase ranges from large lamellar vein and film albite (visible in the light microscope) to cryptoperthite whose size ranges from μm to perhaps 100 Å. The smallest-scale albite appears to be a late-stage phase of exsolution in which lamellae have nucleated heterogeneously on albite-twin composition planes in the microcline.
Alteration is concentrated in vein and film albite, especially along grain boundaries with microcline. Powder X-ray diffraction (XRD) patterns of intensely altered pegmatite show halloysite. Holes, microfractures, vein albite/host microcline boundaries and microcline/halloysite boundaries trend parallel to the traces of (010) and {110}, suggesting that these directions are pathways along which fluids migrate. Cleavage and microfractures occur along, and holes are bounded by, these directions. Holes are associated with dislocations and the latter are observed at feldspar/clay boundaries. Twin domains and cryptoperthitic albite are less susceptible to alteration than coarse lamellar albite and regions containing negative crystals and microfractures. However, microtextures in some areas containing halloysite suggest that once fluids penetrate the crystal, alteration may proceed preferentially in more strongly twinned regions.
The rates of abiotic mineral dissolution are commonly measured in batch or flow-through chemical reactors using established techniques and models. Interpretation of biogeochemical mineral dissolution rates in the presence of intact cells is more difficult and techniques for such approaches are not as well established. However, measurement and interpretation of the kinetics of enzyme-catalyzed reactions using both Michaelis-Menten and Monod models for purified enzymes and cell-containing systems respectively is well established for soluble substrates and may provide paradigms for analysis of such reactions with insoluble substrates. Two problems hinder the use of such approaches for biogeochemical mineral reaction systems. First, enzymes acting upon insoluble substrates for environmentally important reactions have generally not been isolated and purified. Second, little is known concerning how to model and interpret such characteristics as the catalytic efficiency of an enzyme that is transferring electrons to or from an insoluble substrate. These and other questions await new techniques and paradigms of study for mineral reactivity. Despite the complexities, a few generalizations can be made by comparing abiotic and biotic rates, and by comparing rates measured in vitro (i.e. fractions of cells interacting with mineral surfaces) and in vivo (i.e. intact cells interacting with mineral surfaces). This chapter summarizes approaches for analysis of both abiotic and biotic mineral reaction systems with examples drawn from reactivity of Fe (III) oxides. The rates of abiotic ligand-promoted, abiotic reductive, and biotic dissolution of goethite are shown to be slower than the rate predicted for abiotic proton-promoted dissolution of Fe(II) oxide, suggesting that this latter dissolution rate may be an upper limit for all reductive and nonreductive dissolution of Fe(III) oxides, including dissimilatory Fe(III) reduction. While comparison of in vivo to in vitro studies will become more useful in the future as environmentally important enzymes are isolated and purified, comparisons of systems using membrane fractions to systems using whole cells demonstrate how it might be possible to scale up rates of biotic reactions from enzymes to cultures. In this endeavor, computer-based (in silico) investigation of complex reactions must be utilized in order to identify rate mechanisms that are consistent with experimental observations.
This chapter presents the first overview of the connection between nanoscale weathering and geomorphology, where three overarching themes recur. First, nanoscale processes are on one side of a fundamental threshold between the coarser microscale (micrometers and up) and the finer nanoscale with its dramatically different molecular dynamics. Second, nanoscale processes do impact a variety of prior geomorphic research, including threads related to ongoing instability in mineral weathering, silt production, rock coating behavior, geochemical pollution, thermal weathering from wildfires, and biotic weathering as an explanation for deviations from Goldich's weathering series. Third, it is possible to link the nanoscale to more classic geomorphic concerns through scaling up quantitatively by digital image processing of microscope imagery and conceptually through connections to weathering forms such as rock splintering.
The use of mineral and glass dissolution rates measured in laboratory experiments to predict the weathering of primary minerals and volcanic and nuclear waste glasses infield studies requires the construction of rate models that accurately describe the weathering process over geologic timescales. Additionally, the need to model the long-term behavior of nuclear waste glass for the purpose of estimating radionuclide release rates requires that rate models be validated with long-term experiments. Several long-term test methods have been developed to accelerate the glass-water reaction [drip test, vapor hydration test, product consistency test B, and pressurized unsaturated flow (PUF)], thereby reducing the duration required to evaluate long-term performance. Currently, the PUF test is the only method that mimics the unsaturated hydraulic properties expected in a subsurface disposal facility and simultaneously monitors the glass-water reaction. PUF tests are being conducted to accelerate the weathering of glass and validate the model parameters being used to predict long-term glass behavior. A one-dimensional reactive chemical transport simulation of glass dissolution and secondaryphase formation during a 1.5-year-long PUF experiment was conducted with the Subsurface Transport Over Reactive Multiphases (STORM) code. Results show that parameterization of the computer model by combining direct benchscale laboratory measurements and thermodynamic data provides an integrated approach to predicting glass behavior over the length of the experiment. Over the 1.5-year-long test duration, the rate decreased from 0.2 to 0.01 g/(m 2-day) based on B release for low-activity waste glass LAWA44. The observed decrease is approximately two orders of magnitude higher than the decrease observed under static conditions with the SON68 glass (estimated to be a decrease by four orders of magnitude) and suggests that the gel-layer properties are less protective under these dynamic conditions.
Faults in the upper crust initiate from pre-existing (inherited) or precursory (early-formed) structures and typically grow by the mechanical interaction and linkage of these structures. In crystalline rock, rock architecture, composition, cooling, and exhumation influence the initiation of faults, with contrasting styles observed in plutonic rocks, extrusive igneous rocks, and foliated metamorphic rocks. Brittle fault growth in granitic rock is commonly controlled by the architecture of inherited joints or preexisting dikes. In basalt, abundant joints control the surface expression of faulting and, enhanced compliance due to abundant joints leads to folding and deformation asymmetry in the fault zone. Highly reactive mafic minerals likely become rapidly evolving fault rocks. In foliated metamorphic rocks, fault initiation style is strongly influenced by strength anisotropy relative to the principal stress directions, with fracturing favored when the foliation is aligned with the directions of principal stress. The continuity of micas within the foliation also influences the micromechanics of fault initiation. Brittle kink bands are an example of a strain-hardening precursory structure unique to foliated rock. Each of these fault initiation processes produces different initial fault geometry and spatial heterogeneity that influence such properties as fault permeability and seismogenesis.
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Atomic force microscopy has been used to observe the surface dynamics during dissolution of polished fluorite surfaces with different orientations. These surfaces, with an initially high density of atomic scale defects, showed fast changes during the first seconds in contact with a solution. Different types of structures developed on each surface, depending on its initial orientation and solution composition. These structures dissolved slower than the main surface persisting for at least 67.5 days of continuous dissolution. A new interpretation of traditional kinetic and thermodynamic models of dissolution applied to surfaces with a high density of steps is proposed to explain the observations. The new model includes the following: (a) fast initial dissolution at defect sites, (b) formation of a fluid boundary layer at the mineral–solution interface enriched in the dissolving ions, and (c) precipitation of more stable fluorite structures nucleated at surface defects. This model highlights the importance of considering surface defects and crystal orientation for advancing our understanding of processes happening at the mineral–solution interface and for developing more accurate kinetic dissolution and crystal growth models essential in Earth and material sciences.
The solubility and weathering reactions of ettringite, (Ca6Al2(SO4)3(OH)12·26H2O), were used to study the geochemical equilibria of the Ca(OH)2–Al2(SO4)3–H2O system at environmental pH conditions. Ettringite is a stable mineral above a pH of 10.7 and dissolved congruently with a log Ksp of −111.6 (±0.8). Between pH 10.7 and 9.5, ettringite underwent incongruent dissolution to gypsum and Al-hydroxides and controlled Ca2+, Al3+, and SO42− activities. At near neutral pH, Al-hydroxy sulfates precipitated in addition to gypsum and Al-hydroxide. These Al-hydroxy sulfate phases exhibited prismatic and anhedral shapes and had variable Al/S ratios. In addition, some new poorly crystalline Ca–Al-hydroxy sulfate phases were identified in microscopic studies when the pH was acidic (pH∼5). The activities of Ca2+, Al3+, and SO42− suggest that the geochemistry of the Ca(OH)2–Al2(SO4)3–H2O system in the pH range of 7 to 10 is simple and its component Ca(OH)2–SO3–H2O and Al2(SO4)3–H2O systems behave independently of each other. The precipitation of Al-hydroxy sulfates below pH 7.0 significantly influenced Ca2+ and SO42− activities. This effect was pronounced when Ca–Al-hydroxy sulfate phases started precipitating (pH<5.0). The lack of thermodynamic data on the newly identified Al, and Ca–Al-hydroxy sulfates makes it difficult to interpret the geochemistry of Ca(OH)2–Al2(SO4)3–H2O system for pH≤5.0. Reaction path calculations conducted using the EQ6 computer code predicted ion activities close to the experimental values above pH 5.0. The observed differences between thermodynamic modelling and actual experimental data below this pH can be explained by the formation of Al–/Ca–Al-hydroxy sulfate phases in the system, as detected by electron microscopy and X-ray elemental analysis. These reactions are relevant and useful to the prediction of Al, and Ca geochemistry in natural systems.
Acidithiobacillus ferrooxidans are commonly present in acid mine drainage (AMD). A. ferrooxidans derive metabolic energy from oxidation of Fe²⁺ present in natural acid solutions and also may be able to utilize Fe²⁺ released by dissolution of silicate minerals during acid neutralization reactions. Natural and synthetic fayalites were reacted in solutions with initial pH values of 2.0, 3.0 and 4.0 in the presence of A. ferrooxidans and in abiotic solutions in order to determine whether these chemolithotrophic bacteria can be sustained by acid-promoted fayalite dissolution and to measure the impact of their metabolism on acid neutralization rates. The production of almost the maximum Fe³⁺ from the available Fe in solution in microbial experiments (compared to no production of Fe³⁺ in abiotic controls) confirms A. ferrooxidans metabolism. Furthermore, cell division was detected and the total cell numbers increased over the duration of experiments. Thus, over the pH range 2–4, fayalite dissolution can sustain growth of A. ferrooxidans. However, ferric iron released by A. ferrooxidans metabolism dramatically inhibited dissolution rates by 50–98% compared to the abiotic controls.
This study elucidates the complicated mechanisms of granite weathering under acidic conditions. An experiment was carried out on uncrushed samples of granite and four minerals (single crystals of annite, albite, microcline, and quartz) in a closed system at room temperature for 56 days. The chemistry of the resulting surface was determined through XPS. The specific features of granite were confirmed by comparing them with the individual minerals. It was found that biotite played the central role in early stage weathering of the granite samples from a solid analytical viewpoint. Furthermore, plagioclase played the background role in near-neutral to acidic conditions. The increase of pH level due to plagioclase dissolution promoted Fe-oxide precipitation from biotite. Moreover, the precipitation of products including Al started slower, mainly because of its lower dissolution performance. This phenomenon led to an increase in the reactivity of biotite. Under more acidic conditions, the dissolution of biotite was dominant and constituent minerals had no influence on each other. The inter-mineral effect operated via pH fluctuation, as the weathering characteristics of the individual minerals depended on pH levels.
To elucidate the influence of constituent minerals in weathering of whole rock granite under acidic conditions, an artificial weathering experiment was carried out in a closed system using uncrushed samples of a piece of granite and four single crystals (annite, albite, microcline, and quartz). Thermodynamic analysis of rock (mineral)–solution equilibria was performed using pH–Eh and stability diagrams. Based on the solution analytical viewpoint, dominant mineral which played a central role depended on the pH of solutions in the early stage of weathering: the mineral was plagioclase under a slightly acidic condition (pH 5), and biotite under more acidic conditions (pH 4–1). In near-neutral to slightly acidic conditions (pH ≥ 5), pH increased remarkably due to plagioclase dissolution. As a result, the solution characteristics made Fe(OH)3 to precipitate easily. This H+ consumption enhanced an acid buffer capacity of the aqueous solution with granite.
Recent analyses from the Curiosity rover at Yellowknife Bay (Gale crater, Mars) show sedimentary rocks deposited in a lacustrine environment and containing smectite clays thought to derive from the alteration of olivine. However, little is known about the weathering processes of olivine under early martian conditions, and about the stability of smectite clays in particular. Here, we present a 3-month experiment investigating the weathering of forsteritic olivine powders (Fo90) under a dense CO2 atmosphere, and under present-day terrestrial conditions for comparison. The experiment also evaluates the potential effects of hydrogen peroxide (H2O2), as a representation of the highly oxidizing compounds produced by photochemical reactions throughout martian history. The weathered samples were characterized by means of near-infrared spectroscopy (NIR), X-ray diffraction (XRD), transmission electron microscopy with energy dispersive X-ray spectrometry (TEM-EDX), Mössbauer spectroscopy and thermogravimetry. The results show that a Mg-rich smectite phase formed from the weathering of olivine under CO2 conditions, although in lower abundance than under terrestrial conditions. The main secondary phase formed under CO2 turns out to be a silica-rich phase (possibly acting as a “passivating” layer) with a non-diagnostic near-infrared spectral signature. The use of H2O2 highlights the critical importance of both the redox conditions and Fe content of the initial olivine on the nature of the secondary phases.
A multitude of natural processes and technical applications require our ability to provide a reliable prediction of crystal surface reactivity. During the last decades, the detailed analysis of crystal surface reactions revealed the existence of intrinsic variability in surface reactivity. This reactivity acts in addition to extrinsic factors, such as inhibitors, background electrolytes, or pH variations. The nature of this intrinsic variability is poorly understood. In any case, it must represent complex energetic contributions from and interactions between reacting mineral surfaces and dissolved and particulate materials in the fluid.
Consequently, single rate constants and homogeneous surface area normalization are problematic constraints for the explanation and prediction of surface reactions of both, natural and artificial materials. Here, we review various aspects of intrinsic variability of crystal surface reactivity from experimental and modeling results including the fundamental role of microscopic kink site distribution. Important reaction processes include crystal dissolution and sorption of small particles, i.e., nanoparticles and colloids.
A key means of understanding these variations lies in understanding the relationship between surface energy distribution and surface roughness. Rough surfaces are well-known to control the retention of colloids and nanoparticles under unfavorable adsorption conditions. This review provides an integrated view on an important feedback process that includes surface reactivity, kink site distribution, surface roughness, particle retention, and surface inhibition as critical components. Also, we review briefly the methods used to quantify surface roughness variability over a wide range of scales. It is our goal to highlight the multiple facets that may both reflect or influence the intrinsic variability of reactions over time.
The formation of iddingsite by the oxidative weathering of Fo//8//0 olivine begins by solution of Mg from planar fissures, 20 A wide and spaced 200 A apart, parallel to (001). Oxidation of Fe within the remaining olivine provides nuclei for the topotactic growth of goethite. cleavage cracks less than 50 A in diameter allow Na, Al, and Ca from adjacent minerals, particularly plagioclase, to enter the altering olivine while Mg and Si diffuse away. In the early stages of weathering, strips of Fe-rich smectite (saponite), 20-50 A wide and 1-7 layers thick, form bridges 50-100 A long across the planar fissures. Dioctahedral smectite crystallizes on the margins of wider cleavage-controlled fissures; with further weathering halloysite is formed away from the fissure walls. In the ultimate stages of alteration, the saponite and dioctahedral smectite are lost, leaving a porous, oriented aggregate of goethite crystals.
The influence of temperature, pH, acid type, and surface area on the kinetics of the acid dissolution of natural and synthetic
willemites and natural hemimorphites has been investigated. Specific rate constants, based upon areas determined by krypton
adsorption measurements, were estimated from the experimental data obtained. For both willemite and hemimorphite, the rates
of dissolution in different acids are shown to be related to the relative strengths of zinc-acid anion complexes. The reactivity
of willemite toward acids increases with increasing replacement of zinc by manganese. Mixed chemical/diffusion control is
responsible for the observed rates of willemite dissolution under the conditions studied (HNO3, HCl, HClO4, H3PO4, H2SO4, pH 0.31 to 3.00,T 21 to 94 °C). Estimates of the relative contributions of chemical and diffusional resistances to the overall rate have been
made for the dissolution of manganese-free willemite in sulfuric acid solutions. The experimentally measured rates have been
demonstrated to be in reasonable agreement with predicted overall dissolution rates. Proposals are made regarding the nature
of the diffusion and chemical steps involved in the dissolution process. Hemimorphite was found to be considerably more reactive
than willemite and its dissolution is primarily diffusion controlled under the conditions studied (T 20 to 76 °C, pH 2 to 3.5).
In order to understand more quantitatively the causes of ordering in the divalent-cation silicate olivines, the authors have modeled eight ordered and anti-ordered binary olivine pairs (Fe-Mg, Mg-Ni, Fe-Mn, Mg-Ca, Mg-Co, Mg-Mn, Mn-Ca, and Fe-Ca) using distance least-squares computer modeling. Relative structure energies were calculated for the hypothetical ordered and anti-ordered intermediate olivine structures using an electrostatic model with Coulomb and repulsive terms. Observed and model cell parameters agree very well, and changes in order/anti-order are reflected primarily in variations in the b cell parameter. Details of the structures demonstrate that the interatomic distances of any given M1 coordination polyhedron depend not only on its chemical occupancy, but also on the occupancy of the M2 polyhedron; likewise for the M2 polyhedral geometry. The calculated structure energies of the optimum distance models correctly and semi-quantitatively predict the observed site preferences in all eight cases, in contrast to the approximation using linear combinations of end-member site energies. The calculated energies are an approximation to the cation exchange energies to which ligand field stabilization energies and any covalency or polarization contributions may be added.
The defect microstructure and oxidation state of Fe in metasomatized olivine crystals from peridotite xenoliths from Dish Hill, California, have been studied by transmission electron microscopy (TEM) and Mossbauer spectroscopy. Mossbauer data indicate that between 1 and 6% of the Fe in the olivine crystals is Fe3+. TEM revealed the presence of layers 0.6 nm wide and parallel to (001) of olivine. The layers formed by alteration of olivine. TEM images from many areas revealed a very fine domain structure. Some dislocations are associated with trails of submicrometer-sized fluid inclusions that contain iron oxide crystals a few tens of nanometers wide. These features may be healed fractures that provided pathways for oxidized Fe-bearing fluids. -from Authors
Laihunite has been produced by heating single-crystals of synthetic fayalite in the air at 400, 600 and 700oC. Opaque complexes produced at the surfaces and internal defects of the heated crystals consist of iron oxides, amorphous silica and laihunite. No laihunite has been observed in the crystals heated above 800oC. Chemical analysis of the heated fayalite by analytical EM shows that diffusion of Fe2+ ions to the surfaces and internal defects took place during the oxidation process, and produced iron oxide-silica complexes. -J.A.Z.
Die Geschwindigkeit der Auflösung von Berylliumoxyd in Schwefelsäure, Salzsäure und Oxalsäure folgt einer Beziehung der Form
magnified image
Die Geschwindigkeitskonstante k hängt von der Art der Säure und von der Vorbehandlung (Glühtemperatur) des Oxyds ab. Die Adsorption von Oxalsäure an BeO läßt sich durch eine Freundlich‐Isotherme
magnified image
beschreiben. Die Exponenten b und β für Oxalsäure sind identisch. Es wird auf einen Mechanismus geschlossen, dessen geschwindigkeitsbestimmender Schritt in der Reaktion der adsorbierten Säure mit der BeO‐Oberfläche besteht.
Surface species control the mechanism and rate of low-temperature
silicate dissolution and precipitation reactions in dilute solutions.
This has been repeatedly emphasized1-7. The interaction
between dissolved species and silicate surfaces involves exchange and
adsorption phenomena, which equilibrate rapidly, and are accessible to
determination by surface titration8,9. Here we report a
comparison of surface species concentrations with the dissolution rates
of olivine and albite which indicates that while the dissolution rates
are a complex function of solution pH, the dissolution rates have a
simple first-order dependence on surface concentrations of specific
surface species. Thus, the interpretation of the kinetic data for
olivine and albite dissolution becomes simple when the chemical
speciation at the mineral surface is incorporated, and this approach
holds great promise for unifying the rate data for many important
silicates.
The dissolution kinetics of most slightly soluble oxides and silicates are controlled by chemical processes at the surface. The reaction controlling steps can be interpreted in terms of a surface coordination model.In dilute acid solutions, in the absence of complex-forming ligands, the dissolution kinetics are controlled by the surface bound protons. The rate of the proton-promoted reaction of δ-Al2O3 is RH = kH(CH3)3 where Ch3 is the proton concentration per unit area on the oxide surface. The mechanism can be described by the attachment of three protons to the reaction site prior to the detachment of an Al species into the solution. The dissolution rate of BeO is proportional to (CH2)2. For δ-Al2O3 at pH ⩽ 3.5 dissolution rate is independent of pH; at this pH maximum surface concentration of protons is reached.The organic ligand-promoted dissolution, RL, is of first order with respect to concentration of surface chelates: where {ML} is the concentration of surface chelates per unit area. Detachable surface complexes result from surface coordination of metal ions of the hydrous oxides with bidentate ligands. Especially efficient are bidentate ligands that form mononuclear surface complexes. The sequence of rate constants shows that five- and six-membered chelate rings (oxalate, catechol, malonate and salicylate) enhance the dissolution reactions to a greater extent than seven-membered rings (phthalate, succinate). Monodentate ligands (benzoate ion), though readily adsorbed, do not enhance dissolution rates. However, they can inhibit dissolution by displacing ligands that catalyze this reaction.
The dissolution rates of Fo100 and Fo91 in aqueous solutions in the pH range 2–12.4 at 25°C have been measured using fluidized bed and batch reactors. Rates depend upon the pH, the partial pressure of CO2, and the presence of organic ligands. At low PCO2 (≤10−4.5 atm) with no organic ligands present, the rate of olivine dissolution, R, is given by , where aH+ is the activity of H+ in solution. However in basic solutions, when the partial pressure of CO2 is equal to atmospheric levels (PCO2 = 10−3.5 atm), olivine dissolution rates are nearly pH independent throughout the pH range 6–12 and are about equal to the minimum rate of dissolution under CO2 purged conditions. At pH 11 the presence of atmospheric levels of CO2 reduces the dissolution rate by over an order of magnitude (to 10−14.1 mol cm−2 s−1). Apparently, positive charge on the olivine surface can be neutralized by increasing PCO2. In contrast, experiments conducted in the acidic and near neutral pH ranges indicate that organic ligands chelate surface Mg causing an increase in the olivine dissolution rate when present. Organic ligand effects are greatest in the near neutral pH domain. For example, at pH 4 dissolution rates are increased by 0.75 log units (to 10−12.25 mol cm−2 s−1) in solutions of 10−3 molar ascorbic acid or 0.05 molar potassium acid phthalate over rates measured in organic free solutions. The chelation effect becomes less important as pH decreases. Rates at pH 2 in the presence of these organic ligands are indistinguishable from those measured without organics.
Acid dissociation and water exchange on (Mo{sub 3}O{sub 4}(OH{sub 2}){sub 9}){sup 4+} have been studied by {sup 17}O NMR and UV-visible spectrophotometry in noncomplexing acidic aqueous solution. Exchange at the d-H{sub 2}O ligands, trans to be {mu}{sub 2}-bridging oxygen atoms of the core, occurs readily (k{sub OH}{sup 298}) = 1.6 {times} 10{sup 2} s{sup {minus}1} and much faster than that occurring at the c-H{sub 2}O, trans to the capping oxygen (k{sub OH}{sup 298} = 1.5 {times} 10{sup {minus}3} s{sup {minus}1}). A mechanism of exchange for both waters involving solely the same conjugate base (Mo{sub 3}O{sub 4}OH{sub 2}{sub 8{minus}}(OH)){sup 3+} is relevant. The positive {Delta}S{sup {double dagger}} value (+35 J K{sup {minus}1} mol{sup {minus}1}) for the d-H{sub 2}O exchange and comparison with available data for 1:1 anation reactions of (Mo{sub 3}O{sub 4}(OH{sub 2}){sub 9}){sup 4+} with NCS{sup {minus}} and HC{sub 2}O{sub 4}{sup {minus}} and support for an I{sub d} mechanism. Chemical shifts for both the c- and d-H{sub 2}O {sup 17}O NMR resonances followed as a function of H{sup +} concentration confirmed that the relevant deprotonation occurred uniquely at a d-H{sub 2}O. Values for K{sub a}{sup 298}(Mo{sub 3}O{sub 4}{sup 4+}), determined from the kinetic treatment (0.31 M) and from UV-visible spectrophotometry (0.24 M), were in satisfactory agreement and indicative of high acidity at the d positions. The compound (Mo{sub 3}O{sub 4}(OH{sub 2}){sub 9})(CH{sub 3}C{sub 6}H{sub 4}SO{sub 3}){sub 4} {times} 13H{sub 2}O crystallizes in the monoclinic space group Cc: a = 31.99 (1) {angstrom}, b = 9.886 (2) {angstrom}, c = 17.789 (5) {angstrom}, {beta} = 99.67 (2){degree}. The final agreement was R = 0.059 with 5183 observed reflections. The packing is mainly characterized by the large number of crystallization water molecules and H-bond interactions. It consists of alternate layers of opposite charge.
The effect of decreasing dissolution rate of nickel oxide per unit surface area in acid solution with increasing prior annealing temperature (700–1450°C) has been shown to occur over a wide range of pH. The linear dependence of log (rate) on pH may be explained on a model of non-oxidative dissolution in which the pH variation changes the overpotential at the surface. Electron microscopy shows a different mode of attack at pH < 0. The presence in solution of a strong oxidizing ion such as cobaltic causes a very large increase in dissolution rate (> 200 fold) for all prior annealing temperatures but the oxide annealed at 1450°C is still the slowest to dissolve in the presence of cobaltic ions. This is believed to be because it has the lowest kink site density, the role of the Co3+ being hole injection into the p-type semiconductor at kink sites. The general conclusion of the work is that the supply of the majority carriers (the holes) may be rate limiting in the dissolution process.
This book provides an updated look at crystal field theory, one of the simplest models of chemical bonding, and its applications. After an introductory section chapters go on to describe: an outline of crystal field theory; energy level diagrams and crystal field spectra of transition metal ions; measurements of absorption spectra of minerals; crystal field spectra of transition metal ions in minerals; crystal chemistry of transition metal-bearing minerals; thermodynamic properties influenced by crystal field energies; trace element geochemistry in the crust and mantle; remote sensing compositions of planetary surfaces; and covalent bonding of transition metals. -A.W.Hall
A QUANTITATIVE understanding of the rates and mechanisms of dissolution of crystalline solids in aqueous solutions is critical to the chemical modelling of many geochemical, environmental and industrial processes. Here I show that a linear free energy equation, developed recently1,2 for the prediction of the standard Gibbs free energies of formation of isostructural families of crystalline solids, can also be used for predicting the dissolution rates of solids. This equation bears a close analogy with the Hammett equation for aqueous organics3. Regression of data for the surface-reaction-controlled dissolution rates of isostructural families of divalent metal oxides and orthosilicates using the new equation yields coefficients characteristic of the specific crystal structure, whichturn out to be very close to the coefficients obtained by regression of standard free energy data for the same families. These results suggest that standard free energy coefficients can be used to predict dissolution rates.
Abert Lake in south-central Oregon provides a site suitable for the study of sequential weathering and diagenetic events. In this first of two papers, transmission electron microscopy was used to characterize the igneous mineralogy, subsolidus alteration assemblage, and the structural and chemical aspects of silicate weathering reactions that occur in the volcanic rocks (basalts, basaltic andesites, and dacitic/ rhyolitic extrusive and pyroclastics) that outcrop around the lake. Olivine and pyroxene replacement occurred topotactically, whereas feldspar and glass alteration produced randomly oriented smectite in channels and cavities. The tetrahedral, octahedral, and interlayer compositions of the weathering products, largely dioctahedral smectites, varied with primary mineral composition, rock type, and as the result of addition of elements released from adjacent reaction sites. Weathering of the highly evolved, Fe-rich Jug Mountain complex at the north end of the lake produced a homogeneous smectite assemblage that contrasts with the heterogeneous smectite assemblage replacing the volcanics along the eastern margin of the lake. The variability within and between the smectite assemblages highlights the microenvironmental diversity, fluctuating redox conditions, and variable solution chemistry associated with mineral weathering reactions in the surficial environment. Late-stage exhalative and aqueous alteration of the volcanics redistributed many components and formed a variety of alkali and alkali-earth carbonate, chloride, sulfate, and fluoride minerals in vugs and cracks. Overall, substantial Mg, Si, Na, Ca, and K are released by weathering reactions that include the almost complete destruction of the Mg-smectite that initially replaced olivine. The leaching of these elements from the volcanics provides an important source of these constituents in the lake water. The nature of subsequent diagenetic reactions resulting from the interaction between the materials transported to the lake and the solution will be described in part II ( et al., 1991).
The dissolution kinetics of the p-type semiconducting oxide MnO in dilute acid solutions have been investigated. For pure MnO, prepared by decomposition of the carbonate above 900 °C in a H2(5%)/N2 atmosphere, the specific dissolution rate at pH 2 and 30 °C is ca. 4 × 10–5 mol s–1 m–2. This is similar to the rate for ionic MgO, and two orders of magnitude faster than CoO. Like MgO and CoO, MnO exhibits an initial increase in rate with pH. Beyond this initial regime, the rate dependence on pH is consistent with rate control through the second protonation. The higher oxides, Mn3O4, Mn2O3 and MnO2, have been shown to dissolve about three orders of magnitude more slowly than MnO. The presence of these higher oxides as impurities in many preparations of MnO explains the apparently slower dissolution kinetics of such impure monoxide samples. Reduction from solution increases the dissolution rate of the higher oxides. It is concluded that the presence of Mn3+ associated with surface or bulk defects in MnO can be rate-controlling for the non-stoichiometric oxide.
Dissolution rates in acidic solution of nickel oxide prepared in air from carbonate and hydroxide salts at 550, 700, 1100 and 1450 °C are compared. The powders were pre-characterised by thermal analysis, B.E.T. surface area measurement, transmission electron microscopy, X-ray and electron diffraction and infrared spectroscopy. The carbonate decomposition process is considerably more energetic than that of the hydroxide. The rates per unit surface area for the ex-carbonate oxide are more than five times higher than those for the ex-hydroxide oxide at 550, 700 and 1100 °C annealing temperatures, but comparable at 1450 °C. The 700 °C ex-carbonate oxide shows anomalously high, initial rates, at <10 monolayer-equivalents removed, attributed to surface reduction. All samples show increases in rate with extent of dissolution owing primarily to surface roughening and increased kink site densities. The results are discussed in relation to excess (over equilibrium) surface and bulk defect concentrations introduced by decomposition proces, and modified by subsequent annealing.
The dissolution of p-type CoO has been investigated as part of the study of electronic and surface structural factors controlling dissolution kinetics of semiconducting oxides. Electron microscopy of surface morphology changes on annealing showed sintering but no preferential growth of particular low-index faces. Surface attack by acid produced considerable surface roughening but no evidence for formation of particular low surface energy faces. Increasing prior annealing temperature caused lowering of the dissolution rate per unit surface area. A linear relationship between log(rate) and pH with slope –0.6 was found. There is a general trend for the dissolution rate of CoO to be decreased with increasing oxidation potential of the solution. Doping with Li, La and Cr resulted in increased dissolution rates in all cases. The influence of defect structure, atomic surface detail, surface charge-carrier depletion and surface states are examined in relation to these observations.
Iddingsite rimming olivine in a basanite from the Limberg, Germany, is composed of saponite and goethite. Transmission electron microscopy of ion-thinned, oriented crystals suggests a two-stage alteration process. At first, the olivine breaks into a mosaic of 50-Å diameter {110} bounded needle-shaped domains which change to a metastable hexagonal phase having a = 3.1 Å and c = 4.6 Å, probably of close-packed, metal-oxygen octahedra. This reaction opens solution channels in the olivine which are detectable from about 20-Å diameter and are parallel to the olivine y -axis. Laths of smectite, one or two layers thick, 20 Å wide, and as much as 100 Å long parallel to their y-axis nucleate from the metastable phase and begin to fill in the solution channels. The laths orient with smectite (001) parallel to olivine (100). As the channels widen, prismatic {110} goethite crystals form directly from the metastable hexagonal phase. This first stage thus provides heterogeneous nuclei of smectite and goethite, formed epitactically and perhaps topotactically from a metastable intermediary.
In a second stage,these nuclei enlarge by deposition from solution as water migrates readily through the solution channels. A reduction in total volume allows smectite veins to form, misoriented with respect to the olivine. The reaction conserves iron, requires the addition of aluminum and water, and releases magnesium and silicon. Electron microprobe analyses of the iddingsite indicate that the smectite is saponite.
Chemical weathering processes, essentially caused by the interaction of
water and the atmosphere with the Earth's crust, transform primary
minerals into solutes and clays and, eventually, into sedimentary rocks;
these processes participate in controlling the global hydrogeochemical
cycles of many elements. Many mineral dissolution processes are
controlled by a chemical mechanism at the solid-water interface. The
reaction-controlling steps can be interpreted in terms of a surface
coordination model. The tendency of a mineral to dissolve is influenced
by the interaction of solutes—H+, OH-,
ligands, and metal ions—with its surface. The surface reactivity
is shown to depend on the surface species and their structural identity;
specifically, the dependence of dissolution rates on pH and on dissolved
ligand concentrations can be explained in terms of surface protonation
(and deprotonation) and of ligand surface complexes. A general rate law
for the dissolution of minerals is derived by considering, in addition
to the surface coordination chemistry, established models of lattice
statistics and activated complex theory.
Une etude comparative des vitesses de dissolution d'echantillons MgO presentant des structures de surface et des distributions de taille de particules differentes ne met pas en evidence des resultats tres differents
This paper compares the mineralogy and chemistry of clay minerals in sediments from various depths and positions in Abert Lake and surrounding playa with those of the weathered materials entering the lake in order to reveal the nature and extent of post-depositional mineralogical modification. Analytical electron microscope (AEM) data from individual clay particles reveal that each sample is comprised of a highly inhomogeneous smectite assemblage. The thin clay flakes (commonly less than 10 nm wide) display a complete range in octahedral sheet compositions from nearly dioctahedral to nearly trioctahedral. The very abundant Mg-rich lake smectites with an estimated composition K{sub 0.29} (Al{sub 0.23}Mg{sub 2.16}Fe{sub 0.30})Si{sub 3.80}Al{sub 0.20}Oââ(OH)â are not formed by weathering. This confirms the importance of diagenetic Mg uptake. Smectite compositions within individual samples were highly variable, yet source-related characteristics such as the abundance of Fe-rich smectite were apparent. Little evidence for systematic K or Mg enrichment with depth was identified in samples from depths of down to 16 feet below the sediment-water interface. The most magnesian assemblages are associated both with weathering sources of Mg-rich smectite and playa environments subjected to repeated wetting and drying cycles. Other diagenetic reactions in the sediment include recrystallization of Na-rich silica gel and diatom fragments. Abundant, submicron-sized, untwinned, euhedral crystals of K-feldspar are interpreted to be authigenic in origin.
This paper reports studies of the rates of dissolution of natural Brucite, optical grade , and commercial in aqueous solutions. Natural Brucite dissolves by means of a surface reaction requiring protons, and while some protonated ions accelerate the dissolution no inhibitors were found. Dissolution of less perfect is normally diffusion limited; several substances which reduced the rate of dissolution of such were found. first reacts with water to form an layer and the rate is controlled by dissolution of the .
The mechanism of the rate determining step (r.d.s.) for dissolution of well characterised, very perfect MgO “smoke” crystals, has been re-evaluated by studies of the dependence of log (rate) on pH. Surface potential barrier modification is the most likely r.d.s. when solution diffusion is not limiting. Electron microscopic studies of changes in surface structure of partly dissolved crystals have been related to rate changes during dissolution. There is a doubling of surface area per unit mass in the first 10% of dissolution, due to initial attack at defect sites, after which the increase in surface area per unit mass (to more than ten times the initial area) is due to both decreasing particle size and surface roughening.
It is shown that factors previously recognized, but not regarded as
critical, can dominate dissolution kinetics of ionic oxides. The use of
the nearly perfect{100} MgO surfaces of smoke cubes to obtain very
precise values of dissolution rates per unit surface area, in dilute
HCl, HClO_4 and HNO_3, has shown that rates extrapolated to zero
dissolution are almost independent of pH in the range 2.0-3.5.
Dissolution rates were measured by monitoring solution pH as a function
of time. This revealed increasing rates with increasing pH up to about
5% total dissolution, followed ultimately by a return to the linear
relation between lg(rate) and pH (slope ca. -0.5) normally expected. The
initial increase in rate is due to increasing [Mg2+] in
solution and is observed with [Mg^2+] as low as 1% of the [H^+]. A
linear relation between lg(rate) and [Mg2+] is found during
the early stages of dissolution. Other cations (Al3+, Na^+)
also increase the initial rate, to a similar extent. Electron-microscope
observations of the cubes show alteration of the surfaces to a
castellated structure (of \{100\}-based projections and intrusions) on
wetting before dissolution, and the development of facets having an
average \{110\}-nature during dissolution. The results are in conflict
with current theoretical models, and a qualitative account of the
mechanism of the establishment of a `stable' solution double layer is
given.
The dissolution rates of plagioclase in acids are very sensitive to the mineral composition. At pH = 2.0 and 25C, rates range from {approx} 1 {times} 10{sup {minus}15} feldspar-mol/cm{sup 2}/s for albite to greater than 1 {times} 10{sup {minus}12} feldspar-mol/cm{sup 2}/s for anorthite. At pH = 3.0, the respective rates range from {approx} 3 {times} 10{sup {minus}16} feldspar-mol/cm{sup 2}/s to {approx} 7 {times} 10{sup {minus}14} feldspar-mol/cm{sup 2}/s. The relationship between mineral composition and dissolution rate, however, is not simple. In order to better define such a relation at pH = 2, rates were measured on eight plagioclase minerals and compared to existing data. In general, the dissolution rates of a given plagioclase mineral estimated by different researchers generally agree to within a factor of two to five. The uncertainty is much larger for Ca-rich minerals such as bytownite and anorthite, and reasons for the disagreement are unclear. Even with this uncertainty, however, it is apparent that dissolution rates vary nonlinearly with composition and that the rates for the Ca-rich minerals vary more with composition than Na-rich minerals.
COMMON rock-and soil-forming minerals are complicated structures of varying
composition. Despite some encouraging progress1,2 there is as
yet no comprehensive rationale for predicting the dissolution rates of these
minerals. Here we test the hypothesis3 that dissolution rates of
compositionally distinct orthosilicate minerals scale in a fashion similar to
rates of water exchange around the corresponding dissolved, divalent cation.
Although dissolution rates span several orders of magnitude, the hypothesis is
sustained. Minerals containing alkaline-earth cations dissolve at
rates that correlate with ionic size, whereas minerals containing
first-row transition metals dissolve at rates that vary with the
number of cationd-electrons. Both types of behaviour are
consistent with the control of dissolution rate by the character of the bonds
between the divalent cation and neighbouring oxygen atoms. This result supports
the proposed link3–6 between the mechanisms of mineral
dissolution and the mechanisms by which a dissolved metal exchanges ligands.
With this link it may be possible to predict dissolution rates for other nearly
isostructural minerals that vary in composition.
With a simple theoretical derivation, we present a new correlation equation between a Raman v1 band and rM-O, where v1 is the frequency of the total symmetric stretching vibrations of aquated metal ions and rM-O is the distance between a metal cation and its coordinated water molecule. It is shown that there is a linear relationship between v1 and 1/rM-O2 for the cations of the same valence with the same hydration number. Although the electrostatic theory for ionic hydration can explain most of the hydrations of both divalent and trivalent metal ions in aqueous electrolyte solutions, it became clear that the discreteness of water molecules must be taken into account for a future successful theory of ionic hydration.
Olivine crystals in basaltic andesites which crop out in the Abert Rim, south-central Oregon have been studied by high-resolution and analytical transmission electron microscopy. The observations reveal three distinct assemblages of alteration products that seem to correspond to three episodes of olivine oxidation. The olivine crystals contain rare, dense arrays of coherently intergrown Ti-free magnetite and inclusions of a phase inferred to be amorphous silica. We interpret this first assemblage to be the product of an early subsolidus oxidation event in the lava. The second olivine alteration assemblage contains complex ordered intergrowths on (001) of forsterite-rich olivine and laihunite (distorted olivine structure with Fe3+ charge balanced by vacancies). Based on experimental results for laihunite synthesis (Kondoh et al. 1985), these intergrowths probably formed by olivine oxidation between 400 and 800C. The third episode of alteration involves the destruction of olivine by low-temperature hydrothermal alteration and weathering. Elongate etch-pits and channels in the margins of fresh olivine crystals contain semi-oriented bands of smectite. Olivine weathers to smectite and hematite, and subsequently to arrays of oriented hematite crystals. The textures resemble those reported by Eggleton (1984) and Smith et al. (1987). We find no evidence for a metastable phase intermediate between olivine and smectite (M — Eggleton 1984). The presence of laihunite exerts a strong control on the geometry of olivine weathering. Single laihunite layers and laihunite-forsteritic olivine intergrowths increase the resistance of crystals to weathering. Preferential development of channels between laihunite layers occurs where growth of laihunite produced compositional variations in olivine, rather than where coherency-strain is associated with laihunite-olivine interfaces.
Kinetics of etching of MgO crystals have been studied in H2SO4, HNO3 and HCl. The effects of etching time, acid concentration and temperature on the growth of hillocks, on the selective etch rate and on the rate of overall dissolution are demonstrated. It is observed that etch rates are independent of time, but are determined by the temperature and concentration of the acid. The etch rate-concentration curves show maxima which are characteristic of an acid. The values of activation energy for the processes of dissolution, selective etching and hillock growth and the corresponding frequency factors are computed. It is established that the process of dissolution in concentrated H2SO4 is diffusion controlled, while in H2SO4 with concentrations below 18 N and in HNO3 and HCl it is reaction rate controlled. The pre-exponential factor is found to be a function of acid concentration. The results are discussed from the standpoint of chemistry. A comment on the data published on MgO by previous workers is made.
Die chemischen Reaktionen an der Grenzflche Oxid/Lsung — Protolyse, Adsorption von Anionen und Kationen — lassen sich zu einem Koordinationsmodell der Grenzflche zusammenfassen und mit Massenwirkungsausdrcken quantitativ beschreiben. Mit Hilfe dieses Modells gelingt es, die Auflsungskinetik oxidischer Festphasen unter einem einheitlichen Gesichtspunkt darzustellen: In Gegenwart adsorbierbarer Anionen verlaufen solche Reaktionen ber Oberflchenkomplexe, und die AuflsungsgeschwindigkeitR lt sich beschreiben mitR=k[H+]
A.
A ist der Oberflchenbedeckungsgrad mit einem anionischen Komplex. Das Koordinationsmodell wird auerdem auf die Adsorption anionischer Inhibitoren angewendet.Chemical processes at the hydrous oxide-solution interface — proton transfer, specific adsorption of cations and anions — can be interpreted in terms of a surface coordination model and quantified in terms of mass law equlibrium constants. With the help of this model the kinetics of dissolution of solid oxide phases can be formulated in a unifying way: In the presence of specifically adsorbable (surface-coordinating) anions the dissolution rateR depends critically on the relative concentration of surface complexesR=k[H+]
A, where
A is the relative surface coverage with an anionic surface complex. The coordination model is also used to interpret the adsorption of anionic inhibitiors.
Iron silicate minerals (bronzite, fayalite), exposed to aqueous dissolution in the laboratory for up to 60 days at room temperature and pH 1, 1.5, and 6, have been studied for evidence of changes in surface composition, using XPS, and these results compared with those obtained from solution chemical analysis. In the absence of dissolved O2 or at low pH (1–1.5) dissolution proceeds congruently after the initial formation of a thin (<10 Å) protonated surface layer depleted in Fe relative to Si. This layer is unstable and does not grow with time as attested to by long term congruent dissolution and by the formation of an amorphous silica surficial breakdown product at pH 1 and 1.5. In bronzite the layer is also slightly depleted in Mg but much less than it is in Fe due to the preferential occupation by Fe⁺² of more weakly bonded M2 sites. The behavior of the layer is similar to that found earlier on iron-free pyroxene (Schottet al., 1981); in other words, because of its thinness and instability it is not diffusion-inhibiting or protective toward dissolution.
Comparison of experimental and theoretical potentiometric titrations of kaolinite at 25°C indicates that the adsorption of H⁺ and OH⁻ ions to the mineral surface are metal cation specific and that the net adsorption can be modeled in terms of the constituent oxide components. We conclude, from the potentiometric, electrophoretic, and dissolution rate data presented in this paper, that the pH-dependence of the dissolution rates of slightly soluble oxide and silicate minerals is controlled by the adsorption of H⁺ and OH⁻ ions to specific metal cation surface reaction sites.
The relative dissolution rates of some simple divalent oxides in acids rank in a similar fashion as the rates of water exchange into the hydration sphere of the dissolved metal ion. This observation suggests that the local bonding environment of the metal ion and oxygen is the most important factor controlling the dissolution rate. Thus those variables which strongly influence rates of ligand exchange may also be important in controlling rates of oxide and silicate dissolution. This approach is used to predict the relative dissolution rates of simple orthosilicate minerals.
Acid dissociation and water exchange on [Mo3O4(OH2)9]4+ have been studied by 17O NMR and UV-visible spectrophotometry in noncomplexing acidic aqueous solution. Contrary to an earlier report, the oxygen atoms of the "core" are extremely inert with no exchange observed on either over a 2-year period. Exchange at the d-H2O ligands, trans to the μ2-bridging oxygen atoms of the "core", occurs readily (kOH298 = 1.6 × 102 s-1) and much faster than that occurring at the c-H2O, trans to the capping oxygen (kOH298 = 1.5 × 10-3 s-1). A mechanism of exchange for both waters involving solely the same conjugate base [Mo3O4(OH2)8-(OH)]3+ is relevant. The positive ΔS≠ value (+35 J K-1 mol-1) for the d-H2O exchange and comparison with available data for 1:1 anation reactions of [Mo3O4(OH2)9]4+ with NCS- and HC2O4- add support for an Id mechanism. Chemical shifts for both the c- and d-H2O 17O NMR resonances followed as a function of H+ concentration confirmed that the relevant deprotonation occurred uniquely at a d-H2O. Values for Ka298(Mo3O44+), determined from the kinetic treatment (0.31 M) and from UV-visible spectrophotometry (0.24 M), were in satisfactory agreement and indicative of high acidity at the d positions. The compound [Mo3O4(OH2)9](CH3C 6H4SO3)4·13H2O crystallizes in the monoclinic space group Cc: a = 31.99 (1) Å, b = 9.886 (2) Å, c = 17.789 (5) Å, β = 99.67 (2)°. The final agreement was R = 0.059 with 5183 observed reflections. The packing is mainly characterized by the large number of crystallization water molecules and H-bond interactions. It consists of alternate layers of opposite charge.
Acid dissociation and water exchange on (Mo{sub 3}O{sub 4}(OH{sub 2}){sub 9}){sup 4+} have been studied by {sup 17}O NMR and UV-visible spectrophotometry in noncomplexing acidic aqueous solution. Exchange at the d-H{sub 2}O ligands, trans to be {mu}{sub 2}-bridging oxygen atoms of the core, occurs readily (k{sub OH}{sup 298}) = 1.6 {times} 10{sup 2} s{sup {minus}1} and much faster than that occurring at the c-H{sub 2}O, trans to the capping oxygen (k{sub OH}{sup 298} = 1.5 {times} 10{sup {minus}3} s{sup {minus}1}). A mechanism of exchange for both waters involving solely the same conjugate base (Mo{sub 3}O{sub 4}OH{sub 2}{sub 8{minus}}(OH)){sup 3+} is relevant. The positive {Delta}S{sup {double dagger}} value (+35 J K{sup {minus}1} mol{sup {minus}1}) for the d-H{sub 2}O exchange and comparison with available data for 1:1 anation reactions of (Mo{sub 3}O{sub 4}(OH{sub 2}){sub 9}){sup 4+} with NCS{sup {minus}} and HC{sub 2}O{sub 4}{sup {minus}} and support for an I{sub d} mechanism. Chemical shifts for both the c- and d-H{sub 2}O {sup 17}O NMR resonances followed as a function of H{sup +} concentration confirmed that the relevant deprotonation occurred uniquely at a d-H{sub 2}O. Values for K{sub a}{sup 298}(Mo{sub 3}O{sub 4}{sup 4+}), determined from the kinetic treatment (0.31 M) and from UV-visible spectrophotometrymore » (0.24 M), were in satisfactory agreement and indicative of high acidity at the d positions. The compound (Mo{sub 3}O{sub 4}(OH{sub 2}){sub 9})(CH{sub 3}C{sub 6}H{sub 4}SO{sub 3}){sub 4} {times} 13H{sub 2}O crystallizes in the monoclinic space group Cc: a = 31.99 (1) {angstrom}, b = 9.886 (2) {angstrom}, c = 17.789 (5) {angstrom}, {beta} = 99.67 (2){degree}. The final agreement was R = 0.059 with 5183 observed reflections. The packing is mainly characterized by the large number of crystallization water molecules and H-bond interactions. It consists of alternate layers of opposite charge.« less
With the objective of elucidating the mechanisms of dioxygen activation by transition metal complexes, we have investigated functional models of some metalloenzymes, such as phenoxazinone synthase and catechol oxidase, based on bisdimethylglyoximatocobalt(II) and -iron(II) complexes. Kinetic studies and the identification of intermediates permitted to establish a free radical mechanism. The catalytic cycle involves H-atom abstraction from the catechol by a superoxo complex, generating a semi-quinone anion radical, which coordinates to the metal, affording new complexes. Useful free-radical intermediates in biological systems are not impossible since tyrosinase has been shown to contain coordinated tyrosyl radical as an integral part of the active enzyme. We have interpreted the structure-reactivity relationship observed in the reaction of the [Pd2Cl2(dppm)2] dimer with arenesulfonyl azides.
Laihunite--a new iron silicate mineral
- Laihunite Group
Laihunite Research Group, 1976. Laihunite--a new iron silicate mineral. Geochimica, 2:95-103 (in Chinese).
Dis-solution kinetics of manganese oxides
- C F Jones
- R Smart
- C St
- P S Turner
Jones, C.F., Smart, R.St.C. and Turner, P.S., 1990. Dis-solution kinetics of manganese oxides. J. Chem. Soc., Faraday Trans. 1, 86: 947-953.
Oxide surfaces in solution
- R L Segall
- R Smart
- C St
- P S Turner
Segall, R.L., Smart, R.St.C. and Turner, P.S., 1988. Oxide surfaces in solution. In: J. Nowotny and L.-C. Dufour (Editors), Surface and Near-Surface Chemistry of Ox-ide Materials. Elsevier, Amsterdam, pp. 527-576.
The dissolution behavior of corundum, kaolinite and andalusite: A surface complex reaction model for the dissolution of aluminosilicate minerals in diagenetic and weathering environments
- Carroll
Dissolution kinetics of manganese oxides
- Jones
Oxide surfaces in solution
- Segall
The Study of Kinetics and Mechanisms of Reactions of Transition Metal Complexes
- Wilkens