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Relations of feldspar and mica with water at low temperature and pressure
... Merapi samples, sample quantity was limited, so whole lapilli were not utilized, but an additional disaggregation step was performed prior to the crushing stage (referred to as "broken" to obtain particles 5-10 mm in diameter). For the purposes of this study, we have not determined the surface area of the samples following the various stages of disaggregation, as previous dissolution experiments have revealed that the geometric surface area and the reactive surface area are not synonymous [e.g., Garrels and Howard, 1959] and may vary greatly depending on initial grain size of the samples [e.g., Anbeek, 1992]. Following sonication in deionized water for 60 min, each sample was centrifuged at 7000 rpm for 30 min, filtered (0.45 μm cellulose acetate membrane), and the water analyzed with the PerkinElmer Optima 3000 DV inductively coupled plasma-optical emission spectrometer (ICP-OES) to determine the concentration of elements. ...
... Armstrong [1940] observed an increase in K and Na of solutions using ground feldspar in water. Garrels and Howard [1959] performed experiments using K-feldspar and mica samples, and observed a rapid reaction with water at room temperature, where both OH À and K + releases were controlled by the available surface area of the minerals. These various experiments, and many other following studies [e.g., Anbeek, 1992;Wollast and Chou, 1992;Stillings and Brantley, 1995], were primarily concerned with the dissolution of feldspar minerals and the relative roles of water pH, temperature, initial solute composition, feldspar crystal structure, and grain size in order to constrain rates of chemical weathering. ...
... The data presented here show that the longer-duration experiments (48 h and 1 week sample-water exposure) did not produce elevated element concentrations compared to the 1 h experiments. This suggests that the rapid exchange mechanism occurred within the 1 h time frame, and further exchange was halted, either due to saturation of solution or formation of a protective leached layer on grain surfaces [e.g., Garrels and Howard, 1959;Muir et al., 1990]. ...
Volcanic ash deposition following explosive eruptions can pose significant hazards for water quality, human health, agriculture, and infrastructure functionality. Many studies have examined how fresh ash deposition may lower the pH of, and introduce a range of potentially toxic elements into, exposed surface waters. However, no study has yet determined the effects on water composition as a result of mechanical pyroclast disaggregation and production of new fresh particle surfaces and increasingly fine grained particles. Such disaggregation could result from natural posteruptive processes such as debris avalanches, lahars, or fluvial/aeolian transport and human activities such as cleanup efforts or mining of pyroclastic deposits. The posteruption time scales of pyroclast disaggregation may vary from months in moist tropical or temperate environments to years or decades in arid settings. Here we show, for the first time in experimental studies, that mechanical milling of pyroclasts will introduce a range of elements into exposed waters, including Al, which can be toxic at elevated levels, and Na, which increases the electrical conductivity of solutions. The pH of leaching solutions also increases by several log units. Such dramatic changes on the experimental scale may have implications for surface water composition in posteruptive settings, necessitating longer-term risk assessments for ecosystem health and consideration of the role of pyroclastic deposits in element cycling in volcanically active regions.
... Chemical weathering experiments in the laboratory, both in the past and at present, generally involve chemically dilute, undersaturated solutions at temperatures below 100°C in acid to circum-neutral pH solutions (see extensive compilation in Bandstra et al., 2008;older compilations in Blum and Stillings, 1995;Brantley, 2003). In laboratory studies at acid to circum-neutral pH, multi-cation silicates are characterized by the apparent non-stoichiometric, preferential release of interstitial cations, as well as Al (Nash and Marshall, 1956;Garrels and Howard, 1957;Wollast, 1967;Luce et al., 1972;Paces, 1973;Chou and Wollast, 1985;Muir et al., 1989Muir et al., , 1990Inskeep et al., 1991;Hellmann, 1994Hellmann, , 1995Schweda et al., 1997;Lee et al., 2008;Kameda et al., 2009), leading to the formation of chemically distinct surface altered zones, commonly called 'leached layers'. Based on both aqueous data and surface sensitive analytical techniques, it is thought that the leached layer mechanism is primarily controlled by two separate processes operating simultaneously (schematically shown in Fig.1): ...
... a. charged-balanced, ion exchange via solid-state volume interdiffusion of cations from the mineral with protons (H + or H 3 O + ) from the bulk solution (e.g. Garrels and Howard, 1957;Wollast, 1967;Luce et al., 1972;Paces, 1973;Muir et al., 1989Muir et al., , 1990Casey et al., 1988;Muir et al., 1989;Petit et al., 1989;Casey and Bunker, 1990;Banfield et al., 1995;Hellmann, 1997;Hellmann et al., 1997;Schweda et al., 1997;Yang et al., 2009) (possibly accompanied by the inward diffusion of water-see Petit et al., 1990) b. chemical hydrolysis reactions release Si and O into the bulk solution at the outer interface (Casey et al., 1988(Casey et al., , 1993Petit et al., 1989;Schweda et al., 1997;Banfield et al., 1995;Hellmann, 1995;Yang et al., 2009). ...
... In this paper we use geochemical modelling to test the hypothesis that quartz cement (and possibly other authigenic minerals) in the Magnus Field, Clay Minerals (2000) 35, 57±67 Northern North Sea, was sourced from the reaction of detrital K-feldspar (KAlSi 3 O 8 ) with source rockd e r i v e d C O 2 p r o d u c i n g q u a r t z , i l l i t e (KAl 3 Si 3 O 10 (OH) 2 ), kaolinite (Al 2 Si 2 O 5 (OH) 4 ) and ankerite ((Mg,Ca,Fe)(CO 3 )) via the following series of reactions (Garrels & Howard, 1959;Smith & Ehrenberg, 1989;Bjùrlykke & Aagaard, 1992 The decision to assess the impact of CO 2 on feldspar decay reactions in the model was based upon: (1) the present-day Magnus Field associated gas that contains up to 2 vol.% CO 2 , (2) the d 13 C of this CO 2 is in the range: À13 to À16% PDB, indicating an organic source (Magnus Field gas data made available by BP Production Co. Ltd.); and (3) the arkosic Magnus sandstone reservoir displays signs of K-feldspar dissolution and replacement. ...
... In fact, it seems that an early source rock maturation product of the offstructure KCF (i.e. CO 2 ) has initiated diagenesis in the reservoir by creating the conditions necessary for K-feldspar dissolution, i.e. low pH (Garrels & Howard, 1959) via reactions R1À3, (Fig. 3b). By using stable carbon isotope data, Macaulay et al. (1993) concluded that the ankerite in the MSM was derived from an organic source of HCO À 3 (with the d 13 C of ankerite being in the range À7.7 to TABLE 2. Input parameters used to calculate minimum CO 2 available for reaction in the Magnus Field. ...
A reaction path model was constructed in a bid to simulate diagenesis in the Magnus Sandstone, an Upper Jurassic turbidite reservoir in the Northern North Sea, UKCS. The model, involving a flux of source rock-derived CO2 into an arkosic sandstone, successfully reproduced simultaneous dissolution of detrital K-feldspar and growth of authigenic quartz, ankerite and illite. Generation of CO2 occurred before and during the main phase of oil generation linking source rock maturation with patterns of diagenesis in arkosic sandstones and limiting this type of diagenesis to the earlier stages of oil charging. Independent corroborative evidence for the model is provided by formation water geochemical data, carbon isotope data from ankerite and produced gas phase CO2 and the presence of petroleum inclusions within the mineral cements. The model involves a closed system with respect to relatively insoluble species such as SiO2 and Al2O3 but is an open system with respect to CO2. There are up to seven possible rate-controlling steps including: influx of CO2, dissolution of K-feldspar, precipitation of quartz, ankerite and illite, diffusive transport of SiO2 and Al2O3 from the site of dissolution to the site of precipitation and possibly the rate of influx of Mg2+ and Ca2+. Given the large number of possible controls, and contrary to modern popular belief, the rate of quartz precipitation is thus not always rate limiting.
... Cation exchange on the mica surface may be the most important factor. Garrels and Howard (1959) have demonstrated that H ÷ will readily exchange with K ÷ on muscovite surfaces. Silanol groups in the mica may also be capable of ionizing and donating or accepting a proton. ...
... Even so, if only a minor (0.001%) amount of new surfaces are formed, a substantial pH increase could result in the pore fluid. The elevated temperatures of the San Joaquin occurrences would probably also reduce, to an unknown extent, the H ÷ ~ K ÷ exchange on biotite based on results of Garrels and Howard (1959) for muscovite (at 25°C, Kexch = 107's; at 65°C, Kexch = 10s.s). ...
The close spatial association of detrital biotite with diagenetic carbonate, prehnite, or laumontite is attributed to attraction of H+ to the mica surface, resulting in an increase in pore-water pH and stabilization of carbonate or calcium aluminosilicates in the adjacent pore space.Experimental results at 25°C with biotite and muscovite show that in the pH range of 10-4 biotite adsorbs H+ whereas muscovite does not adsorb H+ until pH is less than ∼6.5 (for 0.01 M NaCl solutions) or less than ∼5.8 (for 0.5 M NaCl solutions). Increasing salinity reduces the H+ affinity of the micas. At seawater-type salinities (0.5 M NaCl), biotite is still capable of adsorbing from ∼1· 10−5 mol (pH = 7) to 2 · 10−5 mol (pH = 6). During late-stage diagenetic compaction, if iotite/pore fluid ratios are high, this effect can substantially raise pore-fluid pH.Biotite may also have an inherited net negative charge in the framework, or develop such a charge by incongruent loss of K+ from cleavage surfaces. The theoretical cation distribution adjacent to such a charged surface has been evaluated by the Guoy—Chapman double-layer theory. Calculations show that within 10 Å of the biotite surface pH can be several orders of magnitude lower than in the bulk fluid, and ratios may be more than three times higher than the bulk fluid.
... Fluid flow along fractures in crystalline basement rocks is generally accompanied by chemical reactions between the aqueous fluid and the rock exposed along the fracture. The reactions occur because the fluid is rarely in chemical equilibrium with the mineral assemblages of granites and gneisses predominantly present in the continental crust (Garrels & Howard 1959). At the generally relatively low temperature above the ductile-brittle boundary (<400°C), reactions involving primary silicates are generally slow (Browne et al. 1989;Brantley 2004;Yadav & Chakrapani 2006) and most high-grade minerals are not stable in the presence of H 2 O at the prevailing conditions. ...
The permeability (κ[m2]) of fractured crystalline basement of the upper continental crust is an intrinsic property of a complex system of rocks and fractures that characterizes the flow properties of a representative volume of that system. Permeability decreases with depth. Permeability can be derived from hydraulic well test data in deep boreholes. Only a handful of such deep wells exist on a worldwide basis. Consequently, few data from hydraulically tested wells in crystalline basement are available to the depth of 4–5 km. The permeability of upper crust varies over a very large range depending on the predominant rock type at the studied site and the geological history of the drilled crystalline basement. Hydraulic tests in deep boreholes in the continental crystalline basement revealed permeability (κ) values ranging over nine log-units from 10−21 to 10−12 m2. This large variance also decreases with depth, and at 4 km depth, a characteristic value for the permeability κ is 10−15 m2. The permeability varies with time due to deformation-related changes of fracture aperture and fracture geometry and as a result of chemical reaction of flowing fluids with the solids exposed along the fractures. Dissolution and precipitation of minerals contribute to the variation of the permeability with time. The time dependence of κ is difficult to measure directly, and it has not been observed in hydraulic well tests. At depths below the deepest wells down to the brittle ductile transition zone, evidence of permeability variation with time can be found in surface exposures of rocks originally from this depth. Exposed hydrothermal reaction veins are very common in continental crustal rocks and witness fossil permeability and its variation with time. The transient evolution of permeability can be predicted from models using fictive and simple starting conditions. However, a geologically meaningful quantitative description of permeability variation with time in the deeper parts of the brittle continental crust resulting from combined fracturing and chemical reaction appears very difficult.
... The relative stability of muscovite in quartz-bearing systems compared to the aluminosilicates (pyrophyllite, kaolinite, andalusite), and K-feldspar has been the subject of a number of studies (Appendix A; Hemley, 1959;Garrels and Howard, 1959;Althaus et al., 1970;Usdowski and Barnes, 1972;Shade, 1974;Kerrick, 1972;Montoya and Hemley, 1975;Wintsch et al., 1980;Gunter and Eugster, 1980;Haselton et al., 1988;Sverjensky et al., 1991;Haselton et al., 1995;Frank et al., 1998;Frank and Vaccaro, 2012) at various T ; P , and chloride molalities. We have chosen to use data in this system provided by Sverjensky et al. (1991) because their experiments were almost entirely sampled using extraction techniques, as opposed to measurements on quenched fluids or determination of fluid compositions based on speciation calculations. ...
Internal consistency of thermodynamic data has long been considered vital for confident calculations of aqueous geochemical processes. However, an internally consistent mineral thermodynamic data set is not necessarily consistent with calculations of aqueous species thermodynamic properties due, potentially, to improper or inconsistent constraints used in the derivation process. In this study, we attempt to accommodate the need for a mineral thermodynamic data set that is internally consistent with respect to aqueous species thermodynamic properties by adapting the least squares optimization methods of Powell and Holland (1985). This adapted method allows for both the derivation of mineral thermodynamic properties from fluid chemistry measurements of solutions in equilibrium with mineral assemblages, as well as estimates of the uncertainty on the derived results. Using a large number of phase equilibria, solubility, and calorimetric measurements, we have developed a thermodynamic data set of 12 key aluminum-bearing mineral phases. These data are derived to be consistent with Na+ and K+ speciation data presented by Shock and Helgeson (1988), H4SiO4(aq) data presented by Stefánsson (2001), and the Al speciation data set presented by Tagirov and Schott (2001). Many of the constraining phase equilibrium measurements are exactly the same as those used to develop other thermodynamic data, yet our derived values tend to be quite different than some of the others’ due to our choices of reference data. The differing values of mineral thermodynamic properties have implications for calculations of Al mineral solubilities; specifically, kaolinite solubilities calculated with the developed data set are as much as 6.75 times lower and 73% greater than those calculated with Helgeson et al. (1978) and Holland and Powell (2011) data, respectively. Where possible, calculations and experimental data are compared at low T, and the disagreement between the two sources reiterates the common assertion that low-T measurements of phase equilibria and mineral solubilities in the aluminum system rarely represent equilibrium between water and well-crystallized, aluminum-bearing minerals. As an ancillary benefit of the derived data, we show that it may be combined with high precision measurements of aqueous complex association constants to derive neutral species activity coefficients in supercritical fluids. Although this contribution is specific to the aluminum system, the methods and concepts developed here can help to improve the calculation of water–rock interactions in a broad range of earth systems.
... However, the acidity of these fluids would have been quickly neutralized by the many H + consuming reactions (Table 1). Acidity may have been further reduced by instantaneous surface exchange (Garrels and Howard, 1959), in which alkalis on surfaces newly exposed by grain size reduction exchange for H + ions in solution. This process, called abrasion pH by Stevens and Carron (1948), can produce very high alkalinities and drive fluid compositions deep into the feldspar stability field (Wintsch, 1975a;Wintsch and Knipe, 1983). ...
Microstructures associated with cataclasites and mylonites in the Red River shear zone in the Diancang Shan block, Yunnan Province, China show evidence for both reaction hardening and softening at lower greenschist facies metamorphic conditions. The earliest fault-rocks derived from Triassic porphyritic orthogneiss protoliths are cataclasites. Brittle fractures and crushed grains are cemented by newly precipitated quartz. These cataclasites are subsequently overprinted by mylonitic fabrics. Truncations and embayments of relic feldspars and biotites show that these protolith minerals have been dissolved and incompletely replaced by muscovite, chlorite, and quartz. Both K-feldspar and plagioclase porphyroclasts are truncated by muscovite alone, suggesting locally metasomatic reactions of the form: 3K-feldspar + 2H+ = muscovite + 6SiO2(aq) + 2K+. Such reactions produce muscovite folia and fish, and quartz bands and ribbons. Muscovite and quartz are much weaker than the reactant feldspars and these reactions result in reaction softening. Moreover, the muscovite tends to align in contiguous bands that constitute textural softening. These mineral and textural modifications occurred at constant temperature and drove the transition from brittle to viscous deformation and the shift in deformation mechanism from cataclasis to dissolution–precipitation and reaction creep. These mylonitic rocks so produced are cut by K-feldspar veins that interrupt the mylonitic fabric. The veins add K-feldspar to the assemblage and these structures constitute both reaction and textural hardening. Finally these veins are boudinaged by continued viscous deformation in the mylonitic matrix, thus defining a late ductile strain event. Together these overprinting textures and microstructures demonstrate several oscillations between brittle and viscous deformation, all at lower greenschist facies conditions where only frictional behavior is predicted by experiments. The overlap of the depths of greenschist facies conditions with the base of the crustal seismic zone suggests that the implied oscillations in strain rate may have been related to the earthquake cycle.
... Several different processes have been proposed to explain kaolinite genesis and evolution in nature. The most accepted have been the transformation from feldspar to kaolinite with an intermediate stage of mica (Garrels and Howard, 1959), the transformation through amorphous stages (allophanes), hydrated halloysite and metahalloysite (Kinoshita and Muchi, 1954: Sudo and Takahashi, 1956; Ponder and Keller, 1960; Galan and Martin Pozas, 1971). The direct precipitation of kaolinite by feldspar hydrolysis, theoretically possible, seldom has been considered as a probable formation mechanism. ...
... The slighly higher percentages of 2M1 illites in greywackes than in pelites, and also higher in the silt fraction than in the clay fraction, indicate the existence of detrital illite. Nevertheless, the dominant polytype is 1Md which might have originated by erosion and weathering of micas from the source area (Yoder & Eugster, 1954; Velde, 1965) or as an intermediate product from the alteration of K-feldspar to kaolinite (Garrels & Howard, 1959), and in both cases 1Md illite would be inherited. It is more likely that it was a diagenetic phase, produced by aggradation from other phyllosilicates such as smectites or even kaolinites. ...
Two profiles in Devonian marine deposits have been studied, consisting of pelites, subgreywackes, greywackes and quartzites. Quartz and clay minerals are major components and feldspar and calcite are minor ones. Phyllosilicates in the fine fractions are kaolinite and illite; kaolinite has a high degree of ordering; illite is predominantly of a 1 M d polytype, with low Na content and poor crystallinity and has a phengitic composition in greywackes, whereas in pelites it is muscovitic in composition. Both phyllosilicates may be inherited from a source area with intensive weathering processes, although illite may also be a diagenetic phase. These mineral characteristics indicate that the Devonian rocks did not reach the anchizone boundary in their post-depositional evolution.
The chemical composition of pelites and subgreywackes reveals a high degree of chemical maturity. Chondrite-normalized REE patterns indicate a higher degree of weathering of these Devonian sediments than of Post-Archaean Australian Shales (PAAS), possibly as a consequence of sedimentary recycling processes. The REE patterns of the Devonian rocks in addition to the high Th/Sc, La/Sc and Th/Co ratios suggest a felsic composition of the primitive source area, probably a K-rich granite.
... During this deformation, muscovite will recrystallize , reducing the surface area of the muscovite and lowering the aK +faH + ratio. However, surface exchange is an instantaneous reaction (Garrels and Howard, 1959 ), but the recrystallization of muscovite is not. The rate of deformation must therefore be relatively slow for the recrystallization of muscovite to counter the effect of surface exchange on the aK +fa 11 + ratio in the fluid phase. ...
A model for metamorphic differentiation relying on the contrasting equilibria between the intergranular solution and both mineral recrystallization reactions and surface exchange reactions has been developed. Deformation at constant temperature and pressure will tend to increase the surface area of the minerals present, which in turn will increase the relative influence of surface reactions on the chemistry of the intergranular solution. In the examples described, cation exchange on muscovite surfaces shifts the alkali/H+ activity ratio of the solution into a feldspar stability field. This results in the crystallization of feldspar.
... Note that it is well-known that pristine fresh feldspar surfaces undergo rapid ion-exchange with cations (e.g., K + , H 3 O + ) in the solution at the onset of the experiments (e.g., Garrels and Howard, 1957). This phenomenon was also observed in the Alek97 experiments. ...
This paper explores how dissolution and precipitation reactions are coupled in batch reactor experimental systems at elevated temperatures. This is the fourth paper in our series of “Coupled Alkali Feldspar Dissolution and Secondary Mineral Precipitation in Batch Systems”. In our third paper, we demonstrated via speciation–solubility modeling that partial equilibrium between secondary minerals and aqueous solutions was not attained in feldspar hydrolysis batch reactors at 90–300 °C and that a strong coupling between dissolution and precipitation reactions follows as a consequence of the slower precipitation of secondary minerals (Zhu and Lu, 2009). Here, we develop this concept further by using numerical reaction path models to elucidate how the dissolution and precipitation reactions are coupled. Modeling results show that a quasi-steady state was reached. At the quasi-steady state, dissolution reactions proceeded at rates that are orders of magnitude slower than the rates measured at far from equilibrium. The quasi-steady state is determined by the relative rate constants, and strongly influenced by the function of Gibbs free energy of reaction ( ) in the rate laws.
... The pH of the water and acid-leach solutions increased by 0.1 to 2 pH units upon contact with most fresh ash samples (Table VII). The increase in pH is probably caused by rapid exchange of dissolved hydrogen (as H30 ÷ ions) for alkali or alkaline earth ions at the surface of ash particles (Garrels and Howard, 1959;Luce et al., 1972;Petrovic et al., 1976). The decrease in pH caused by some ash samples is more difficult to explain. ...
A study of leaching of freshly erupted basaltic and dacitic air-fall ash and bomb fragment samples, unaffected by rain, shows that glass dissolution is the dominant process by which uranium is initially mobilized from air-fall volcanic ash. Si, Li, and V are also preferentially mobilized by glass dissolution. Gaseous transfer followed by fixation of soluble uranium species on volcanic-ash particles is not an important process affecting uranium mobility. Gaseous transfer, however, may be important in forming water-soluble phases, adsorbed to ash surfaces, enriched in the economically and environmentally important elements Zn, Cu, Cd, Pb, B, F, and Ba. Quick removal of these adsorbed elements by the first exposure of freshly erupted ash to rain and surface water may pose short-term hazards to certain forms of aquatic and terrestrial life. Such rapid release of material may also represent the first step in transportation of economically important elements to environments favorable for precipitation into deposits of commercial interest.
... (i.e., those in the ␣ bilayer) are removed, (2) dangling Si atoms at Si-O bonds broken during cleavage become attached to O or OH, and (3) complex structural relaxation develops in the outermost 4 -5 unit-cell layers at the orthoclase (001)-water interface. The rapid removal of K ϩ ions at the surface is consistent with numerous experimental dissolution studies (e.g., Garrels and Howard, 1957;Busenberg and Clemency, 1976;Chou and Wollast, 1984;Blum and Lasaga, 1991;Wollast and Chou, 1992;Stillings et al., 1995) showing rapid and complete exchange of near-surface cations for protons (or hydronium ions) upon exposure to aqueous solution. The attachment of O or OH to surface Si atoms suggests dissociative chemisorption of H 2 O; cleavage of a water molecule provides an OH group that attaches to a dangling Si atom, forming a terminal silanol (Si-OH) group and a proton that can be exchanged for K ϩ at the surface. ...
In situ X-ray specular reflectivity and atomic force microscopy were used to determine the structure of the orthoclase (001) cleavage surface in contact with deionized water at 25°C. These are the first in situ measurements of the orthoclase–water interface structure performed to Ångström-scale resolution. The orthoclase (001) cleavage surface has minimal roughness, and only one of two possible surface terminations is exposed. The X-ray data show that (1) the silica network at the orthoclase surface is terminated by an oxygen-containing species (e.g., O or OH) having a coverage of 1.9 ± 0.25 ML (the expected coverage is 2.0 ML, where 1 ML = 1 atom/55.76 Å2), (2) the outermost layer of K+ ions have been removed with a derived coverage of 0.0 ± 0.08 ML (the bulk truncated K+ coverage is 1.0 ML), and (3) a complex relaxation profile affecting the near-surface structure propagates ∼26 Å into the orthoclase with a maximum relaxation of ∼0.15 Å near the surface. These data are inconsistent with K+ ion depletion below the topmost K+ layer. These results provide a new baseline for understanding the initial steps of the feldspar dissolution process, demonstrate the power of combining X-ray scattering techniques with scanning probe microscopies for understanding the intrinsic characteristics of complex mineral–water interface systems, and suggest a new approach for understanding feldspar dissolution mechanisms.
Results of influence of mineralogy on physico-chemical and some geotechnical properties of soils developed over granite, schist, and migmatite gneiss around Minna in central Nigeria are presented. A region around Minna was studied to obtain the structure and petrography of the rocks. Three trial pits taken to depths of 5.0, 3.5, and 4.0m were dogged within region of granites, schists, and migmatite gneiss rocks, respectively, all of which belong to Basement Complex of northern Nigeria, to study the mineralogical, physico-chemical, and some geotechnical properties of overburden soils over these rocks. In each of the trial pits, soil samples were collected at interval of 0.5m depths to terminal depths of the pits. X-ray diffraction, index properties, and compaction tests were carried out the samples, and the results were analyzed to determine the influence of mineralogy on physico-chemical and some geotechnical properties of the soils. The result showed that rate of weathering is higher in granite rocks, where primary minerals altered to secondary minerals including smectite minerals. This is followed by overburden over schists, which was observed to alter to secondary minerals like illite and kaolinite. Overburden soil over migmatite gneiss did not showed alteration to secondary minerals but conversion of the carbon element to sodium carbonate salt. Higher fines content, cation exchange capacity, and Atterberg limits, recorded from soils developed over granite rocks, probably resulted from the smectite minerals recorded within this profile. The non-plastic nature of soils, on schist rocks, despite presence of secondary minerals, and judging from the low natural moisture content observed within this profile, indicated that the minerals developed under poor drainage condition. It was therefore concluded that mineralogy of overburden soils over basement complexes and the drainage condition during formation affect their geotechnical properties.
Statistical studies of clay mineral associations in local sedimentary series lead to lithostratigraphical conclusions. The distribution of clay minerals over a wider area, in sediments of one particular stage, illustrates the parts played by heritage, neo-formation, and diagenesis.
The present paper deals with the weathering mechanisms of silicate minerals from a fundamental point of view. The goal of such a study is to understand the main factors controlling the rates of this process and to predict its end products. The chemical weathering of silicates is one of the major processes responsible for the transfer of dissolved and eventually particulate components from land to sea.
It is a fact that mankind’s domain of influence at the surface of the planet is roughly that of clay mineral formation: soils, weathered rocks, diagenetic series, continental and marine sediments, geothermal fields. These clay resources have been exploited since the discovery of fire. It is now important, for environmental studies, to know as well as possible, how and where these minerals form. Curiously, among the numerous works published until now, only a few are devoted to the mechanisms of clay formation at the scale of a soil profile, i.e. the metric scale in temperate zones. Indeed, more is known at the scale of a country (km) or the mineral-fluid interface (nm). For example, at the scale of a country, weathering can be considered as a homogeneous process. As a consequence, it is possible to model chemical transfers and clay-mineral stability fields using calculation codes. On the other extreme, the intimate dissolution-recrystallization mechanisms at the fluid-mineral interface scale are studied on isolated pure crystals in order to simplify the chemical system.
This monograph presents the fundamentals of the mineralogy and geology of clays. The chemical or isotopic composition and the crystalline structure of clays is based on some relatively simple laws. Abundant illustrations help to understand the subject and can be used in courses. The organization of the book follows a logical structure. From the representation of the structure of minerals, the determination of their crystalline state by using X-ray diffractometer measurements (and other techniques) the thermodynamic and kinematic conditions of their formation is reconstructed. Then the genesis and alteration of the clays at their natural occurrences is discussed. In the last chapter it is stressed that clay minerals can form in variety of different environments: meteorites, lavas, subduction zones for example. The book is aimed for graduate students and scientists (geologists, geographers, engineers, environmental and soil scientists) who want to get a comprehensive introduction in the topic.
The dissolution kinetics of albite were studied using a continuous flow reactor based on the fluidized bed technique. The influence of pH and concentration of dissolved Na, Al and Si was investigated. The results indicate that the mechanism in- volves three successive steps: 1) rapid exchange of Na+ with H+, 2) build-up of a residual layer depleted in Na and also depleted in Al under acidic conditions, and 3) a steady-state and congruent dissolution stage where the rate is controlled by a surface reaction between the residual layer and the solution. It might be possible to describe the kinetics of this steady-state stage in terms of activated surface complexes, but their nature is certainly more complicated than previously considered.
Silica in free and combined forms is a dominant component of the solid material of many soils, and dissolved silica is commonly a major solute of soil solutions. Breakdown of primary silicates, translocation of silica in solution, and deposition of secondary silica-containing substances are involved in the development of soils. Ions essential for the growth of plants are released to the soil solution as silicates weather, and these ions may be held against leaching at exchange sites on other silicates. Silica is absorbed in appreciable quantities by some plants and is returned to the surface of the soil as the plants decay. The nature and transformations of silica and silicates in soil are, thus, fundamental to an understanding of many aspects of soil and plant sciences.
Feldspar weathering occurs via dissolution of all components into solution, with the subsequent precipitation of secondary minerals from solution, and it is the feldspars dissolution rate which controls the overall rate of feldspar weathering. The rate of feldspar dissolution is controlled by the kinetics of surface reactions at the mineral-water interface, not by mass transfer processes, either in solution or through a protective surface layer. At neutral to basic pH conditions, the entire range of feldspars compositions appears to dissolve nearly stoichiometrically, although a thin Al enriched surface layer (<20Å) may form, and cations, particularly Na⁺, may be exchanged with H⁺ to depths of several 100 Å. The exchange of cations with H⁺ appears to be reversible, with cation occupancy favored in the basic pH region. It is not clear whether the observed Al enrichment on the surface is a consequence of slightly non-stoichiometric dissolution, or readsorption of Al from solution at charged surface sites. At acidic solution pH’s, a silica-enriched surface layer 100’s to 1000’s of Å thick may form. This layer is highly hydrated and disordered, and analogous to an amorphous SiO2 gel. The silica-enriched surface layer does not provide a diffusional barrier to the transport of Al and cations to solution, and does not appear to effect the destruction rate of the feldspar tetrahedral lattice.
Hydrothermal experiments on an andesite have been carried out under the condition of 110 °C, autogeneous pressure, 0.05 M H 2 SO 4 solution and renewal of acid solution every 6 hours.
The experimental results indicate that the plagioclase and pyroxenes in the treated samples show micropits and microfractures. Small crystals readily suffered from alteration, as compared with large ones, and the susceptibility of the minerals to the acid solution is decreased in the order of plagioclase, augite, hypersthene and opaque minerals. Plagioclase, which is the most reactive mineral in the experiments, shows an increase of SiO 2 , but a decrease of Al 2 O 3 , CaO and Na 2 O as the experiments proceeded. The chemical change of the treated andesite, it indicates that the relative amounts of SiO 2 , TiO 2 , MgO, ΣFeO, MnO and K 2 O increase, while those of Al 2 O 3 , CaO, Na 2 O and P 2 O 5 decrease with an increase of the experimental duration.
The relative mobility of chemical elements listed in decreasing order is P, Ca, Al, Na, Si, K, Mg, Fe, Mn and Ti in terms of the K value. It is no wonder that Ca, Al and Na are more mobile than others because these elements are readily released into solution from the more reactive plagioclase, while Mg, Fe etc. still stay in the less reactive pyroxenes and opaque minerals.
The incongruent dissolution of feldspar is associated with the formation and evolution of Si-rich amorphous interfacial structures during its chemical weathering. The stoichiometry of dissolution is compositionally dependent, and this dependence reflects the nature of dissolution reaction as a combination of surface renewal and heterogeneous chemical reaction. We hypothesize that during continuous surface renewal, reactive sites on a feldspar's surface will inherit characteristics from the bulk structure of the primary mineral. Hence, the dissolution rate of feldspar depends not only on water chemistry but also on its crystallographic properties. We propose a new formalism to quantify the dependence of the dissolution rates of feldspars on their crystallographic properties. A correlation between the degree of Al/Si ordering and the stoichiometry of feldspar dissolution is predicted based on this formalism. This correlation is verified by a combination of water chemistry analysis, synchrotron X-ray diffraction with structure refinement (HR-XRD), and Fourier transform infrared spectroscopy (FTIR). We find that the rates of dissolution and the degree to which dissolution is incongruent depend on the frequency of SiOSi and SiOAl linkages (chemistry) and the crystallographic characteristics of these linkages (bond lengths, bond angles, and ordering) in the dissolving mineral. The knowledge of Al/Si ordering provides a means to quantify the relative abundances of Si atoms with different reactivities. Using a C1bar-based structure model, we show that the distribution of Al within the Al-rich (T1) type of tetrahedral sites has only a minor effect on dissolution stoichiometry, whereas the distribution of Al between the T1-type and the Si-rich (T2)-type of sites affects dissolution incongruence predominantly. A greater extent of disorder results in higher dissolution incongruence. As an implication, our results suggest that the formation of interfacial structures during silicate dissolution should at least be partially affected by reactivity differences among the framework atoms of a silicate.
This study examines the composition and development of a complex stratigraphic section from the Sunwapta Pass, Jasper National Park. An analysis of the chemical and physical characteristics of the various stratigraphic component revealed the presence of a sequence of deposits including three volcanic ash layers interspersed with deposits of colluvial/alluvial and aeolian origin. On the basis of electron microprobe analysis of glass shards from each of the three ashes, Bridge River, St. Helens Y and Mazama ashes were identified. The identifications were corroborated in part by a radiocarbon date of 6170 +/- 100 B.P. obtained on charcoal fragments from a layer overlying the Mazama ash. Pedogenic degradation of the volcanic ashes resulted in the production of amorphous iron and aluminium silicates. The clay mineralogy of the section, dominated by illite, chlorite, vermiculite and intergrade material, showed development of an expanding clay mineral specifically associated with the volcanic ash. Laboratory simulation of the ash degradation under the influence of chelating agent (EDTA) showed that the ash degraded and released substantial amounts of iron and aluminium which may then be involved in clay mineral formation. The presence on the section of palaeosols formed in association with the ash and aeolian deposits together with various layers containing charcoal fragments clearly attests to a complex history at the site, including episodic phases of alluviation/colluviation and aeolian deposition including volcanic ash from at least three separate eruptions. In consequence, the section is important in providing a basis for palaeoenvironmental reconstruction in the Sunwapta Valley during the Holocene.
The hydrogen content of dioctahedral and trioctahedral micas has been investigated by means of infrared spectroscopy and radioactivo measurements. Dioctahedral and trioctahedral micas have been treated with deuterium oxide and tritium-bearing water. The introduced isotopes of hydrogen have been determined after various thermal treatments.
It is concluded that dioctahedral hydrous micas may represent mixed crystals between K(Si 3 Al)Al 2 O 10 (OH) 2 and H 2 O(Si 3 Al)Al 2 O 9 (OH) 3 , whereas trioctahedral micas seem to represent mixed layer minerals between phlogopite and vermiculite.
The recent literature on the kinetics of water-rock interactions is reviewed. The data are then extended to provide a quantitative framework for the description of weathering and alteration. The available experimental data on dissolution of silicates verifies quantitatively the usual mineral stability series in sedimentary petrology. The rate of hydration of carbonic acid is shown to be a possible limiting factor in water-rock interactions. The framework is developed to enable use of laboratory dissolution experimental results and thermodynamics to arrive at a rate law applicable up to equilibrium and therefore applicable to natural systems. The kinetic justification for the significance of a water-rock ratio is discussed. With a proper treatment of fluid flow, the equations are applied to the weathering profile leading to the development of bauxites from nepheline syenites.
The many tools with which one can probe the atomic-scale structures of surfaces include electron-, ion-, and X-ray based techniques (e.g., low energy electron diffraction, Rutherford ion backscattering, X-ray diffraction, photoelectron diffraction), as well as scanning probe microscopies (e.g., scanning tunneling microscopy and atomic force microscopy) (Somorjai 1981; Woodruff and Delchar 1986; Zangwill 1988; van Hove 1999). These tools have been extremely valuable for revealing surfaces structures and processes at ultra-high vacuum conditions. However, most of these surface-sensitive techniques suffer from the substantial shortcoming, from the perspective of mineral-fluid interface studies, that they cannot be applied to surfaces in contact with water. It is preferable to measure mineral-fluid interface structures in situ for direct insight into the geochemical phenomena of interest because there is no reason to assume that a mineral surface can be removed from an aqueous solution without substantially modifying the surface structure or properties.
X-rays are an ideal probe of mineral-water interfaces. X-rays readily penetrate macroscopic amounts of water and can therefore investigate the mineral-water interface directly, in situ . X-rays can measure atomic scale structures, such as the separation of individual atoms or molecules, because X-ray wavelengths are comparable to atomic sizes (Warren 1990). In fact, the interaction of X-rays with matter is known at a very fundamental level, and X-ray based techniques can provide truly quantitative data concerning the arrangements of atoms through a variety of approaches, such as crystallography and X-ray absorption spectroscopy (Als-Nielsen and McMorrow 2001). These characteristics can also be used to study the structure of the mineral-fluid interface (e.g., atomic locations, bond lengths) with sub-Angstrom precision.
Of the many X-ray based techniques available, a very powerful approach for probing interfacial structures is based on the measurement of X-ray reflectivity. The X-ray reflectivity is simply defined as the ratio …
Micas are described from the kaolin type Tertiary weathering crusts formed on granites and gneisses of Lower Silesia, Poland. In the weathering crust, large scale processes of transformation of micas (muscovite, biotite) take place. Weathering of micas is a gradual transformation of their structures by removal of the mobile cations Mg2+, Fe2+, Fe3+ and finally K+. Muscovite transforms into kaolinite through the intergrowths of two phases, mica and kaolinite. A degradation series, muscovite → mica/montmorillonite → montmorillonite → kaolinite also is observed. Biotite passes directly into kaolinite, but at higher concentrations of K and Al one observes the following sequence of biotite degradation: biotite → dioctahedral mica rich in Fe → dioctahedral mica poor in Fe → kaolinite. This process involves the exchange of Mg2+ and then Fe2+ of octahedral layers by Al3+ less mobile under weathering conditions. In the next stage of degradation, K+ removal and kaolinite formation takes place. The micas representing different stages of this transformation, dark green, light-green, and silver-white, have been separated and investigated.
Experimental investigations of chemical weathering in which powders of potassium feldspar, albite, leucite, muscovite, tremolite, olivine, and volcanic glass are treated with pure water and with dilute solutions of sulfuric, carbonic, and hydrochloric acid have been performed in an apparatus in which the mineral powder is exposed to a circulating water flow. The experiments have been continued recently by treating kaolinite and montmorillonite. The course of decomposition of these minerals depends on water flow rate, grain size, temperature, and pH of the solutions. These experiments in open systems are compared with investigations reported in the literature and with the conditions of natural weathering.
Experiments on the conversion of feldspar to illite reveal that variation of the fluid/rock ratio (flow rate) has a significant effect on the kinetics of feldspar dissolution and illite formation. Dissolution of albite in near-neutral KCl solution at 200°C and 500 bar shows that the Si and Na release rate per unit surface area of albite is faster with higher fluid/rock ratio. These results have application to the interpretation of secondary porosity formation. The experiments also reveal a special set of conditions under which rapid illite formation can occur. In contrast to the slow process of illite formation in neutral solution, there can be mass nucleation and growth of illite platelets on albite surfaces in an initially acidic solution with low fluid/rock ratio. In similar experiments with higher fluid/rock ratio, kaolinite forms instead of illite because there is insufficient solid to titrate the large volume of solution. This study suggests that illitization of feldspar or kaolinite may be triggered by a decreased rate of acidic fluid-influx during burial diagenesis.
The effect of sandstone-framework instability during diagenesis on porosity and permeability is compared for quartz, arkosic and lithic arenites. Of the cases investigated, quartz frameworks are the most stable and suffer only from mechanical compaction and pressure solution. Arkosic frameworks have variable stability involving potential widespread alteration of feldspars. The least stable are the lithic sandstone frameworks which are susceptible to all four main porosity and permeability-reducing processes: mechanical compaction, plastic deformation, pressure solution, and mineralogical alteration of framework constituents.
Albite feldspar was hydrolyzed over a wide-range of pH conditions at 100, 200, and 300°C. The release rates of Na, Al, and Si were measured as a function of time from the initial, pre-steady-state phase to the attainment of steady-state, congruent dissolution conditions. Leached layers were developed during the initial stages of dissolution due to the preferential release of Na with respect to Al and Si at nearly all pH and temperature conditions. The preferential release of Na is due to the higher rate of ion exchange reactions vs. hydrolysis reactions associated with the release of Si and Al. The depths. of Na preferential leaching show a pH dependence. Maximum depths on the order of 1500 and 1200 Å were recorded at acid and basic pH conditions, respectively, whereas minimum depths were observed in the neutral pH range. Leached layers deficient in Al (with respect to Si) were recorded at acid and neutral pH conditions. At mildly basic pH conditions, Al and Si were congruently released, or alternatively, either Al or Si was preferentially released. At more extreme basic pH conditions, only Al was preferentially released. The depths of Al preferential leaching were not determinable under all conditions due to the precipitation of an Al surface phase; a maximum recorded depth of 250 Å was determined at basic pH conditions. The preferential release behavior of Al and Si is ascribed to the pH dependency of the speciation of AlOH and SiOH groups at the surface and within the leached layers. However, at very basic pH conditions, where AlO− and SiO− groups are postulated to predominate, the preferential release of Al is probably due to the intrinsically greater reactivity of Al-bridging oxygen bonds.
The aim of this study is to gain a better understanding of the nature and distribution of surface species occurring during the initial stage of weathering of albite. Instead of the classical acidbase titration experiments used extensively in previous work, the surface of freshly ground mineral was titrated by adding increasing amounts of solid to pure water. The aqueous phase was analyzed for all the elements present after a well-defined short reaction time. Compared to the classical titration experiment, this approach has the advantage of defining the entire composition of the system and limits the interferences of the dissolution process with surface reactions. The results confirm the rapid exchange of Na + by H + at the surface of albite, leading to the formation of an H-feldspar and a deprotonation producing negatively charged surface species. Addition of a large excess of Na + to the aqueous solution demonstrates the reversibility of the exchange reaction. The exchange reaction and the deprotonation can be depicted by apparent equilibrium constants which describe the density of the negatively charged species as a function of pH. These results strongly suggest that the negatively charged surface species are closely related to Na sites at the feldspar surface where three species can be identified: NaAlSi 3 O 8 , HAlSi 3 O 8 , and AlSi 3 O 8 - . The fractional dependence of pH obtained (0.35) for the density of the surface charge is close to the fractional order of the dissolution rate in the same pH region (pH > 5.5). Additional experiments show that the fresh albite surface is able to react with gaseous CO 2 which results in a more disturbed surface layer.
Well-characterized samples of rhyolitic obsidian, perlite and felsite from a single lava flow are leached of U by alkaline oxidizing solutions under open-system conditions. Pressure, temperature, flow rate and solution composition are held constant in order to evaluate the relative importance of differences in surface area and crystallinity. Under the experimental conditions U removal from crushed glassy samples proceeds by a mechanism of glass dissolution in which U and silica are dissolved in approximately equal weight fractions. The rate of U removal from crushed glassy samples increases with decreasing average grain size (surface area). Initial rapid loss of a small component (≈ 2.5%) of the total U from crushed felsite. followed by much slower U loss, reflects variable rates of attack of numerous uranium sites. The fractions of U removed during the experiment ranged from 3.2% (felsite) to 27% (perlite). An empirical method for evaluating the relative rate of U loss from contemporaneous volcanic rocks is presented which incorporates leaching results and rock permeability data.
Although steady state dissolution of feldspar occurs stoichiometrically in acidified dilute solutions, leached layers form during dissolution and are maintained at constant thickness by the balance between leaching of Al3+ and Mb+ and silica network hydrolysis. The feldspar surface in contact with dilute acid solution is penetrated by hydrogen species whose concentration is pH-dependent. Because we have observed that dissolution rate and ion exchange within the feldspar surface decrease with increasing concentration of cations in solution, but proton adsorption is not dependent upon salt concentration, we present two rate models which assume that ion exchange of H+ or H3O+ for K+, Na+, or Ca2+ enhances the hydrolysis of Si within the leached, hydrated surface layer. Specifically, the pH-dependent ion-exchange reactions are assumed to accelerate hydrolysis of AlOSi bonds in the surface, causing an increase in the concentration of ≡SiOH sites throughout the leached, hydrated layer and a consequent increase in the rate of silica network hydrolysis. The silica network hydrolysis reaction (depolymerization), enhanced by ion exchange, is therefore pH-dependent and rate-limiting, in contrast to network hydrolysis of quartz at low pH. Assuming that dissolution rate is dependent upon the surface concentration of protonated exchange sites, which we model with a Langmuir isotherm, we predict the following rate equation: R = knnsT (KH{H+}/1 + KH{H+} + KM{Mb+} + ∑i Ki{C+i})n Here, R is the area-normalized rate of feldspar dissolution at steady state, k is the rate constant, ns is the fraction of total sites that are AlOSi (exchange) sites at the water-feldspar interface, T is the number of surface sites per unit area, KH and KM refer to the adsorption constants for H+ and Mb+ adsorption onto the AlOSi site respectively, and the braces refer to activities of dissolved species. The term ∑i Ki{C+i} includes the effect of adsorption of any other species Ci onto the AlOSi (exchange) site, including Al3+ adsorption. The value of n is 1 if only AlOSi sites on the surface contribute to dissolution but equals 0.5 if sites throughout the hydrated surface layer contribute. We have used the model with n = 0.5 to fit dissolution rate data of five feldspars as a function of NaCl concentration.
Dry ground phlogopite was placed in deionized water saturated with COs at room temperature and pressure. The bulk solution was buffered between a pH of 5 and 6 which is close to the pH of natural weathering systems. The conditions simulated a closed system. After 1010 hr, 2.0% of the total K, 0.95% of the Mg, 0.54% of the Si, and 0.74% of the F had been released, indicating that the dissolution was incongruent. Most of the K was released within 3 min, apparently by a rapid surface exchange with hy- drogen ion. One-third of the cation-exchange capacity of this phlogopite arises from cations released from the outer surfaces, while two-thirds arises from the release of more deeply seated cations. All cations exhibited decreasing release with time, the slowest being Si. The rate-controlling "factor" in the later stages is related to the release of Si. It is difficult to distinguish linear from parabolic kinetics in the later stages because of the slow rate of dissolution; however, linear kinetics is most likely. If linear kinetics is applicable, the dissolution rate of Si was 3.8 10 -17 mole/cm2/sec. Conclusions may be affected by the length of the experimental run.
Delayed flow waters are solute-rich and exhibit high p(CO2) characteristics. The slow transit of these waters through a distributed drainage system and the predominance of relatively rapid reactions, such as sulphide oxidation and carbonate dissolution, in this environment maximize solute acquisition. Quick-flow waters are dilute, both because of their rapid transit through ice-walled conduits and open channels, and because the weathering reactions are fuelled by relatively slow gaseous diffusion of CO2 into solution, despite solute acquisition being dominated by rapid surface exchange reactions. As a consequence, quick flow usually bears a low or open-system p(CO2) signature. Bulk meltwaters are more likely to exhibit low p(CO2) values when suspended-sediment concentrations are high, which promotes post-mixing reactions. This conceptual model suggests that the composition of both quick flow and delayed flow is likely to be temporally variable, since kinetic, rather than equilibrium, factors determine the composition. -from Authors
Two variables must be considered when calculating exchange free energies (AG~ for 2:1 clays: (1) anionic field strength, as expressed by equivalent anionic radius (ra), and (2) interlayer water content, as expressed by interlayer molality. For smectites that are in a state of high hydration, interlayer molality is determined by the cations undergoing exchange. Thus AG~ for an exchanging cation pair can be cal- culated solely from measurements of r~. r~ is related to layer charge per half unit cell (C) and ab unit cell area(A)by:ra = (-A/8~rC)lJz. Thelayerchargenecessaryforcationfixationcanbepredictedbycalculating the ra at which cation exchange with an illite structure expresses a AG~ equal to that of exchange with a smectite structure. The theory can also be applied qualitatively to understand the high selectivity of illite for Cs +, the fixation of K + rather than Na § in shales during diagenesis, the stability of illite over muscovite in the weathering environment, and cation segregation in smectite.
Abstract Basal ice samples were collected from ice exposures in a natural subglacial cavity beneath an outlet glacier of Øksfjordjøkelen, North Norway. Sediment and cation (Ca2+, Mg2+, Na+, K+) concentrations were then determined, and indicate stacking of basal ice units producing a repeat pattern of ‘clean firnification ice’ overlying sediment-rich ice. All measured cations show correlation with sediment concentration indicating weathering reactions to be the dominant contributor of cations. Regressions of specific sediment surface area per unit volume with cation concentration are performed and used to predict cation concentrations. These predicted values provide an indication of cation relocation within the basal ice sequence. The results suggest limited melting and refreezing resulting in the relocation of predominantly monovalent cations downward through the profile. Exchange of cations into solution during the melting of sediment-rich ice samples has previously been suggested as a source of error in such investigations. Analyses of sediment-free regelation ice spicules formed at the bed show cation concentrations above firnification ice levels and comparable, in many instances, to the basal ice samples.
High-resolution in situ X-ray specular reflectivity was used to measure the structures of orthoclase (001) and (010) cleavage surfaces in contact with deionized water at 25°C. X-ray reflectivity data demonstrate a high degree of structural similarity between these two orthoclase-water interfaces. Both interfacial structures include cleavage along the plane of minimal bond breakage resulting in surfaces terminated by non-bridging oxygens; structured water within 5 Å of the orthoclase surface (consisting of adsorbed species at the surface and layered water above the surface), with a featureless water profile beyond 5 Å; substitution of outermost K+ ions by an oxygen containing species (presumably H3O+); and small structural displacements of the near surface atoms. The interfacial water structure, in comparison with recent results for other mineral-water interfaces, is intermediate between the minimal structure found at calcite-, barite-, and quartz-water interfaces and the more extensive structure found at the muscovite-water interface.
Waters were sampled from 17 boreholes at Haut Glacier d'Arolla during the 1993 and 1994 ablation seasons. Three types of concentrated subglacial water were identified, based on the relative proportions of Ca2+, HCO3- and SO42- to Si. Type A waters are the most solute rich and have the lowest relative proportion of Si. They are believed to form in hydrologically inefficient areas of a distributed drainage system. Most solute is obtained from coupled sulphide oxidation and carbonate dissolution (SO-CD). It is possible that there is a subglacial source of O-2, perhaps from gas levels found (up to 1200 mueq/L) are greater than could be bubbles released during regelation, because the high SO42- achieved if sulphides are oxidized by oxygen in saturated water at 0degreesC (c, 414 mueg/L). A more likely alternative is that sulphide is oxidized by in anoxic environments. If this is the case, exchange reactions involving Fe-III and Fe-II from silicates are possible. These have the potential to generate relatively high concentrations of HCO3- with respect to SO42-. Formation of secondary weathering products, such as clays, may explain the low Si concentrations of Type A waters. Type B waters were the most frequently sampled subglacial water. They are believed to be representative of waters flowing in more efficient parts of a distributed drainage system. Residence time and reaction kinetics help determine the solute composition of these waters. The initial water-rock reactions are carbonate and silicate hydrolysis, and there is exchange of divalent cations from solution for monovalent cations held on surface exchange sites. Hydrolysis is followed by SO-CD. The SO42- concentrations usually are
Leaching of freshly erupted air-fall ash, unaffected by rain, from the May 18, 1980, eruption of Mount St. Helens volcano, Washington, shows that Ca2+, Na+, Mg2+, SO
42−, and Cl− are the predominant chemical species released on first exposure of the ash to water. Extremely high correlation of Ca with SO4 and Na with Cl in water leachates suggests the presence of CaSO4 and NaCl salts on the ash. The amount of water soluble material on ash increases with distance from source and with the weight fraction of small (less than 63 micrometers) ash particles of high-surface area. This suggests that surface reactions such as adsorption are responsible for concentrating the soluble material. CaSO4, NaCl, and other salts are probably formed as microscopic crystals in the high-temperature core of the eruption column and are then adsorbed by silicate ash particles.
The environmentally important elements Zn, Cu, Cd, F. Pb, and Ba are released by a water leach in concentrations which could pose short-term hazards to some forms of aquatic life. However, calculated concentrations are based on a water-to-ash ratio of 4:1 or less, which is probably an underestimation of the regionally operative ratio. A subsequent leach of ash by warm alkaline solution shows dramatic increases, in the amount of dissolved SiO2, U, and V, which are probably caused by increased dissolution of the glassy component of ash. Glass dissolution by alkaline ground water is a mechanism for providing these three elements to sedimentary traps where they may coaccumulate as uraniferous silica or U-V minerals.
Leaching characteristics of ash from Mount St. Helens are comparable to characteristics of ash of similar composition from volcanoes in Guatemala. Ashes from each locality show similar ions predominating for a given leachate and similar fractions of a particular element in the ash removed on contact with the leach solution.
During equilibration of K-feldspar, quartz and muscovite with dilute KCl-solutions, the change in pH of the solution was measured as a function of time. The resulting equilibrium constant, K
T = aK + /aH +, is 104.210.06, 105.860.03 and 106.010.03 at 300, 60 and 30 C respectively (standard states at 1 bar) and are consistent with the best higher temperature data. At 30 C this constant is consistent with the aK + /aH + ratio of seawater. From K
T and the activity of K
+ in seawater, a pH of 8.2 is calculated, essentially identical with the pH which results from dissolution of CaCO3 under atmospheric CO2-pressure. Consequently, detrital K-feldspar, quartz, muscovite, and calcite are stable in seawater. Apparently, the seawater pH is controlled by CaCO3 as well as K-feldspar, quartz and muscovite. Independently both equilibria show virtually the same pH, within the variability due to disordering, solid solution and surface energy effects.Assuming that the K-concentrations of pore solutions vary between about 4000 and 40 ppm, these solutions have alkalic pH-values in the temperature range between 30 and 300 C if K-feldspar, quartz and muscovite are present. In limestones the pH is fixed by the dissociation of CaCO3; the occasionally observed formation of K-feldspar in these rocks requires a minimum K-concentration of approximately 4 ppm.
Aqueous dissolution experiments with the vitric phase of a rhyolitic tuff were performed at 25°C and constant pH in the range 4.5–7.5. Results suggest interchange of aqueous hydrogen ions for cations situated both on the surface and within the glass.At time intervals from 24 to 900 hr., dissolution kinetics are controlled by ion transport to and from sites within the glass.Experimental data indicate that parabolic diffusion rate of a chemical species from the solid is a nonlinear function of its aqueous concentration. A numerical solution to Fick's second law is presented for diffusion of sodium, which relates it's aqueous concentration to it's concentration on glass surface, by a Freundlich adsorption isotherm. The pH influence on sodium diffusion in the model can be accounted for by use of a pH-dependent diffusion coefficient and a pH-independent adsorption isotherm.
It has been widely accepted that a chemically altered, protective surface layer regulates the dissolution, and hence the weathering, of plagioclase feldspars under Earth's surface conditions. In this study, we examine this hypothesis in detail with the aid of scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Using these techniques, we have been unable to find any direct evidence suggesting the presence of a chemically altered coating on feldspar surfaces which have been weathered in the lab. Instead, our results suggest that the mechanism controlling feldspar weathering is a surface controlled reaction.Based on SEM observations and measurements of rate of release of silica, we postulate that the process of feldspar dissolution proceeds in two stages in the lab. Initially, one observes the dissolution of ultrafine (⪡ 1 μm diameter) particles which are produced during grinding of the sample, and which adhere tenaciously to the surfaces of larger grains. This is the stage which results in the non-linear rates of dissolution which are commonly observed in the lab (parabolic kinetics). Secondly, the dissolution occurs at sites of excess surface energy such as at dislocations or similar crystal defects. This process yields linear rates of dissolution, and it dominates the weathering of feldspars in the field.
Analysis of aluminosilicate steady-state dissolution/precipitation rates indicate that in contrast to what is commonly assumed, the constant pH rates are not independent of chemical affinity at far from equilibrium conditions. Rather, the logarithm of these rates for albite and kaolinite are linear functions of the logarithm of aqueous Al concentration over wide ranges of saturation states. Consideration of both the steady-state rates and the surface chemistry of these minerals following dissolution indicates that these rates are consistent with their control by the decomposition of an Al-deficient, silica-rich surface precursor complex. Taking account of reactions written to form this complex leads to a rate equation for the dissolution/precipitation of these minerals that accurately describes their variation on pH, aqueous Al concentration, and chemical affinity. By analogy, it appears likely that the rates of numerous other aluminosilicate dissolution/crystallization reactions are also consistent with their control by the decomposition of similar precursor complexes. It follows from these observations that 1.(1) the generation of steady state dissolution rate constants from experiments performed in batch type reactors,2.(2) the interpretation of the pH dependence of aluminosilicate dissolution reactions requires explicit account of the effects of aqueous Al concentration on these rates.
The exchange reaction on the wollastonite surface was investigated at 25°C with both short-term (<2.5 h) and long-term (>48 h) dissolution studies. In acidic solutions, the dissolution of wollastonite is nonstoichiometric with a greater release of Ca than Si relative to the wollastonite stoichiometry. Both short-term and long-term exchange reaction stoichiometries are 0.5.Rapid desorption of Ca2+ from the surface of untreated wollastonite caused a rise of the suspension pH to about 10 in a couple of minutes. Therefore, potentiometric titrations were performed with an acidreacted wollastonite where most surface detachable Ca2+ had been removed. Addition of alkali and alkaline earth metal chloride solutions to the acid-reacted wollastonite suspension results in a pH decrease with K+> Na+ > Ba2+ > Mg2+ > Ca2+ in equal molal solutions. This suggests that the cations in these solutions are adsorbed to the wollastonite surface. Surface protonation properties of the acid-reacted wollastonite are found to be similar to those of microporous silica but with the point of zero salt effect (pzse) of 4.5–5.5 rather than the 3.0 of microporous silica. The surface protonation-deprotonation as a function of pH is modeled with a one-site double layer model which includes Na adsorption from the background electrolyte to reasonable accuracy.The adsorption of CrO42−, MoO42−, Ca2+, Mg2+, Ba2+, and Na+ from aqueous solutions to the acidreacted wollastonite/water interface was determined as a function of the pH and ionic strength of the solution. CrO42− and MoO42− were not adsorbed to the wollastonite surface at pH above 3. The extent of cation adsorption increases with increasing pH and decreases with increasing ionic strength. Ca2+ adsorption depends on both the surface area of wollastonite and total amount of Ca2+ in the suspension. For alkaline earth metals at the same concentration, the adsorption sequence is Ba2+> Ca2+> Mg2+. At pH 8.5, the maximum Ca2+ adsorption density on the acid-reacted wollastonite is about 0.83 μmol m2−.
The concentration of H⁺ which reacts with an adularia surface, [HS⁺], was measured with acid-base titrations of adularia powder-water suspensions. Due to the complexity of feldspar surface reactions, it was necessary to calculate a H⁺ mass balance in order to separate the fractions of H⁺ involved in cation exchange reactions, [Hex⁺]; dissolution reactions, [Hdis⁺]; and adsorption at surface hydroxyl sites, [Hads⁺]. Reproducibility of acid and base titrations of HS⁺ was pH-dependent, ranging from ±3 μmol H⁺ m⁻² at pH 4 to ±1.5 μmol H⁺ m⁻² at pH > 6.5. This departure was due to the exchange of Kfsp⁺ for Haq⁺, which was not completely reversible under the conditions of our experiment. Reproducibility of acid and base titration curves for [Hads⁺] vs. pH was ± 1.5 μmol m⁻², suggesting the H⁺ adsorption reaction was reversible.
Samples of albite feldspar were dissolved at 300C and 170 bars for periods up to 24 h in flow-through reactors at acid, neutral, and basic pH conditions. Three MeV ion beam techniques, Resonant Nuclear Reaction Analysis (RNRA), Rutherford Backscattering Spectrometry (RBS), and Elastic Recoil Detection Analysis (ERDA) were employed to obtain elemental depth profiles and information on the composition of the near-surface region after dissolution. Based on the anti-correlative trends of the H and Na profiles obtained by RNRA, Na loss and H permeation are coupled by an ion exchange process in acidic and neutral pH solutions. At basic pH conditions, the evidence is ambiguous as to whether there is a limited degree of ion exchange between aqueous cations and Na, as based on RBS spectra and Na RNRA profiles.
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