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The vast majority of precious opal on the world market comes from opal fields in the Great Artesian Basin of Australia pointing to very special prerequisites for amorphous silica to consolidate in a way that leads to the famous play-of-color. We analyzed 20 opal-A samples from the Andamooka (South Australia) and Yowah (Queensland) precious opal fie...
Citations
... Amorphous silica is commonly present in nature and has little or no chronic adverse pulmonary effects [112][113][114][115][116] , such as in sedimentary rocks 117,118 , hot spring systems 119,120 , and soil 121 . Crystalline silica is a class I carcinogen 112 . ...
In this study, the core carcinogenic elements in Xuanwei Formation coal were identified. Thirty-one samples were collected based on the age-standardized mortality rate (ASMR) of lung cancer; Si, V, Cr, Co, Ni, As, Mo, Cd, Sb, Pb, and rare earth elements and yttrium (REYs) were analyzed and compared; multivariate statistical analyses (CA, PCA, and FDA) were performed; and comprehensive identification was carried out by combining multivariate statistical analyses with toxicology and mineralogy. The final results indicated that (1) the high-concentration Si, Ni, V, Cr, Co, and Cd in coal may have some potential carcinogenic risk. (2) The concentrations of Cr, Ni, As, Mo, Cd, and Pb meet the zoning characteristics of the ASMR, while the Si concentration is not completely consistent. (3) The REY distribution pattern in Longtan Formation coal is lower than that in Xuanwei Formation coal, indicating that the materials of these elements in coal are different. (5) The heatmap divides the sampling sites into two clusters and subtypes in accordance with carcinogenic zoning based on the ASMR. (6) PC1, PC2, and PC3 explain 62.629% of the total variance, identifying Co, Ni, As, Cd, Mo, Cr, and V. (7) Fisher discriminant analysis identifies Ni, Si, Cd, As, and Co based on the discriminant function. (8) Comprehensive identification reveals that Ni is the primary carcinogenic element, followed by Co, Cd, and Si in combination with toxicology. (9) The paragenesis of Si (nanoquartz), Ni, Co, and Cd is an interesting finding. In other words, carcinogenic elements Ni, Co, Cd, and Si and their paragenetic properties should receive more attention.
... Opal-A (microspheres or silica micro-size particles) has been deposited not only in Xuanwei Formation coal (Large et al. 2009;Downward et al. 2017) but also in all types of available surfaces on Earth (Rodgers et al. 2004;Channing and Butler 2007;Jones and Renaut 2007;Ishii et al. 2011;Liesegang and Milke 2014) and Mars (Sun and Milliken 2018). Therefore, the formation of opal-A is a common phenomenon in nature, and the predominant amorphous silica seems unlikely to cause environmental problems. ...
The study on the origin of quartz and silica in Xuanwei Formation coal in Northwest Yunnan, China, is helpful to understand the relationship between quartz and silica and the high incidence of lung cancer from the root. To address these questions, the mineralogy and microscopic studies of silica in Xuanwei Formation coal were performed. The following results were obtained: (1) silica in the late Permian Xuanwei Formation coal seams originated from detrital input, early diagenesis, and late diagenesis. (2) A more significant contribution comes from early diagenesis, which contains abundant authigenic quartz and amorphous silica. (3) Quartz and silica from inorganic silicon are more symbiotic with kaolinite and from biogenic silicon with chamosite. (4) Three silica polymorphs in coal samples have been identified: opal-A (amorphous silica), opal-CT/-C (cristobalite/tridymite), and α quartz. (5) Opal-A is ubiquitous, while opal-CT/-C and α quartz are rare. (5) Opal-A is an amorphous and nontoxic ordinary silica. (6) Since the toxicity of amorphous silica and its presence in coal is an emerging topic, it should be continuously monitored.
... Hydrothermal alteration occurs from 50 to 500 °C while weathering occurs below 50 °C (Pirajno 2009). Continental weathering of rocks also dissolves primary minerals and releases silica, which is then available for forming secondary minerals, including opal (Rondeau et al. 2012;Liesegang and Milke 2014;Chauviré et al. 2017Chauviré et al. , 2019. ...
Geochemistry of opal in Ethiopia
... Hydrothermal alteration occurs from 50 to 500 °C while weathering occurs below 50 °C (Pirajno 2009). Continental weathering of rocks also dissolves primary minerals and releases silica, which is then available for forming secondary minerals, including opal (Rondeau et al. 2012;Liesegang and Milke 2014;Chauviré et al. 2017Chauviré et al. , 2019. ...
A large amount of gem-quality opals is found in south Wollo/Delanta woreda, especially in Wegel Tena and Tsehay Mewucha localities in central Ethiopia. Petrographic investigation shows the host rock comprises porphyritic rhyolitic ignimbrite with quartz, plagioclase, and alkali feldspar phenocrysts. The matrix is composed of glass shards and is mostly weathered into clay, with a small proportion of biotite, hornblende, opaque minerals, and lithic fragments. Geochemically, the rhyolitic ignimbrite displays a pattern typical of silicic volcanic rocks from the area, with depletions of Ba, K, Sr, P, and Ti due to feldspar, apatite, and Fe-Ti oxide crystal fractionation. The rhyolitic ignimbrite is characterized by light REE-enrichment pattern ((La/Lu)N = 7.05–14.65) with slight negative Eu anomalies. The opal samples show lower REE than the rhyolitic ignimbrite, with stronger negative Eu anomalies and more positive Ce anomalies than the host rhyolitic ignimbrite. The Eu and Ce anomalies indicate that the fluid responsible for opal precipitation is associated in part with feldspar dissolution under variations in redox conditions, respectively. Therefore, as demonstrated in previous studies, we concluded that the Delanta opal is formed through intense weathering and alteration of rhyolitic ignimbrite before the eruption of the overlying thick and welded rhyolitic ignimbrite.
... Ages for this mineralization have been debated, with estimates including Late Cretaceous to Paleocene [136], continuously since Late Eocene [137], Mid-to late Miocene [138], and Recent [139]. The petrography of the deposits has been studied in detail [140][141][142]. ...
For many decades, wood silicification has been viewed as a relatively simple process of permineralization that occurs when silica dissolved in groundwater precipitates to fill vacant spaces within the porous tissue. The presence of specific silica minerals is commonly ascribed to diagenetic changes. The possibility of rapid silicification is inferred from evidence from modern hot springs. Extensive examination of silicified wood from worldwide localities spanning long geologic time suggests that these generalizations are not dependable. Instead, wood silicification may occur via multiple pathways, permineralization being relatively rare. Mineralization commonly involves sil-ica precipitation in successive episodes, where changes in the geochemical environment cause various polymorphs to coexist in a single specimen. Diagenetic changes may later change the mineral composition, but for many specimens diagenesis is not the dominant process that controls mineral distribution. Rates of silicification are primarily related to dissolved silica levels and permeability of sediment that encloses buried wood. Rapid silica deposition takes place on wood in modern hot springs, but these occurrences have dissimilar physical and chemical conditions compared to those that exist in most geologic environments. The times required for silicification are variable, and cannot be described by any generalization.
... EMP-WDX spot analysis of the silica spheres yields a composition (in wt%): SiO 2 ~ 96, Al 2 O 3 0.31, FeO total 0.08, and K 2 O 0.06, on average. Their spherical micromorphology and chemical composition are consistent with opal-A (e.g., Liesegang and Milke 2014). ...
The bituminous black pelite of the Messel UNESCO world heritage site is an exceptional palaeobiological archive from the Middle Eocene greenhouse climate. The pronounced homogeneity of the annually laminated pelite through a time interval of 640 kyr complicates the relative stratigraphic classification of fossil remains and is, thus, largely dependent on particular marker horizons within the uniform sedimentary column. We analyzed the most prominent marker horizon M using petrographic microscopy, X-ray powder diffraction, and electron probe microanalyses to identify and characterize its structure and phosphate-dominated mineralogy. Based on our analytical data, we suggest that this phosphatic marker horizon resulted from the exceptional combination of external tephra enclosed in bacteria and algae-rich layers, producing a coupled phosphorus and cation diffusion during diagenesis. Mantienneite (KMg2Al2Ti(PO4)4(OH)3 • 15H2O) is documented for the first time in the Messel fossil deposit. The diagenetic succession of messelite, montgomeryite, and mantienneite precipitation reflects the internal heterogeneities in primary mineral composition of an ash layer. Kerogen maturation and hydrocarbon migration produced acidic, reducing pore fluids with high P concentration, which enhanced the mobility of Al and Ti. The mantienneite-forming reaction marks the change from a regime of reducing to oxidizing conditions. With the deposition and diagenesis of the marker horizon M, a singular event is preserved in a sedimentary sequence otherwise regarded as largely uniform over a time span of about 640 kyr.
... The difference in the quartz peak intensity between the two samples was due to a greater or lesser inclusion of quartz grains. The lump with a maximum at 22 degrees was the most important feature; it is typical of opal-A [37] and was observed in Venezuelan tepui cave speleothems [27] (p. 84, Figure 79A). ...
... The Simpson index is also an estimator of both species richness and evenness, with lower values for higher diversity. As the reciprocal of the Simpson index is regarded as a different expression of the ENS, the highest Simpson index (lowest diversity) of 0.16 for GM2 was converted to an ENS of 6.25, and the lowest Simpson index (highest diversity) of 0.02 in GM1 yielded an ENS of 50 [37]. ...
The diversity of microorganisms associated with speleological sources has mainly been studied in limestone caves, while studies in silicate caves are still under development. Here, we profiled the microbial diversity of opal speleothems from a silicate cave in Guiana Highlands. Bulk DNAs were extracted from three speleothems of two types, i.e., one soft whitish mushroom-like speleothem and two hard blackish coral-like speleothems. The extracted DNAs were amplified for sequencing the V3–V4 region of the bacterial 16S rRNA gene by MiSeq. A total of 210,309 valid reads were obtained and clustered into 3184 phylotypes or operational taxonomic units (OTUs). The OTUs from the soft whitish speleothem were mostly affiliated with Acidobacteriota, Pseudomonadota (formerly, Proteobacteria), and Chloroflexota, with the OTUs ascribed to Nitrospirota being found specifically in this speleothem. The OTUs from the hard blackish speleothems were similar to each other and were mostly affiliated with Pseudomonadota, Acidobacteriota, and Actinomycetota (formerly, Actinobacteria). These OTU compositions were generally consistent with those reported for limestone and silicate caves. The OTUs were further used to infer metabolic features by using the PICRUSt bioinformatic tool, and membrane transport and amino acid metabolism were noticeably featured. These and other featured metabolisms may influence the pH microenvironment and, consequently, the formation, weathering, and re-deposition of silicate speleothems.
... Opal-A is a noncrystalline form of hydrous silica (SiO 2 ⋅nH 2 O) that often consists of submicrometersized spheres. In addition, the spheres in opal-A consist of 25−40 nm-sized subparticles and are most likely to formed by an aggregative growth process [66]. ...
Growing research interest in the use of diatomaceous biosilica results from its unique properties, such as chemical inertness, biocompatibility, high mechanical and thermal stability, low thermal conductivity, homogeneous porous structure with a large specific surface. Unlike the production of synthetic silica materials with a micro‐ or nano‐scale structure in an expensive conventional manufacturing process, diatomaceous biosilica can be produced in huge quantities without significant expenditure of energy and materials. This fact makes it an unlimited, easily accessible, natural, inexpensive, and renewable material. Moreover, the production of bio‐silica is extremely environmentally friendly, as there is essentially no toxic waste, and the process does not require more energy compared to the production of synthetic silica‐based materials. For all these reasons, diatoms are an intriguing alternative to synthetic materials in developing cheap biomaterials used in a different branch of industry. In review has been reported the state‐of‐art of biosilica materials, their characteristics approaches, and possible way of application. This article is protected by copyright. All rights reserved
... Although the origin of the Liesegang structures is not completely understood [56], they are usually explained as being due to the diffusion of reactants into the gel phase; their periodic supersaturation, nucleation, and precipitation in localized bands; and the depletion of reactants in adjacent zones [54,[57][58][59]. This process may be repeated many times to form the "zebra type" textures that often occur in opals [56,60]. As seen in Figure 6e,f, the internal voids at Rožná are filled with a mixture of fine-grained quartz and hematite, which are encircled by multiple Liesegang bands of hematite with synchysite crystals mostly growing on their outer rims (Figure 6g,h). ...
Synchysite was identified in the Rožná uranium deposit in a quartz–carbonate–sulfide vein, which is a part of the late (post-uranium and, post-Variscan) stage of the development of the hydrothermal system. The synchysite forms needles or lamellae, which are almost exclusively bound to the quartz filling of the veins. The structure of the quartz vein-filling, i.e., the preserved tubular syneresis crack pattern, Liesegang bands formed by hematite, chaotic grain size distribution of quartz grains, and ribbons of fibrous SiO2 grains, indicate that the synchysite crystallized in a silica gel. Its formation may be explained by the reaction of hydrothermal acid fluids rich in Fe2+ and rare earth elements (REEs) with alkaline Ca2+ HCO3− and F-rich fluids expelled from the gel during syneresis, or by its ageing. The subsequent recrystallization of the gel to form euhedral quartz grains was accompanied by the deformation of previously formed Liesegang rings, and the development of quartz rosettes. The study of fluid inclusions indicated that the silica gel originated at a very low temperature. The temperatures of the homogenization of two-phase inclusions in carbonate and quartz vein-filling varied between 38 and 74 °C, and the salinity ranged between 4 and 10 wt.% NaCl equiv. The δ13C carbonate values (from −4.65 to −5.21‰, PDB) indicate the deep-seated source of CO2, and δ18O values (from 14.76 to 18.22‰, SMOW) show that the source of the hydrothermal fluids was mainly surface water, with a possible admixture of fossil saline brines. The main sources of REEs are thought to have predominantly been uranium minerals (coffinitized uraninite and coffinite) that form a part of the breccia fragments embedded in the vein filling. The results illustrate the significant mobility of REEs in the late, low-temperature hydrothermal system, and they indicate the multiple remobilizations of REEs in the uranium deposits in general.
... Typically, XRD peaks are broad in all cases, particularly for opal-A [2,[4][5][6][7][8][9], suggesting a disordered "paracrystalline" environment. Other variability, such as in the amount of molecular water [7,[10][11][12][13], trace elements [14][15][16][17][18][19][20][21][22] and the mixture of silanols (Q3 site with one oxygen as a silanol [23][24][25] with the remainder bridged to other silicon atoms) with fully substituted (Q4 site all Si-O-Si bridged) species leads to a consensus that opal is not a true mineral. Despite extensive research, the structures of opal-A and opal-CT remain unresolved [3,12,26,27]. ...
... The minor and trace metal ion content of gem opals has been extensively analyzed for provenance reasons [16] or to explore diagenesis [20,21,67] while less attention has been paid to hyalites (opal-AN). Laser ablation/ion coupled plasma elemental analyses of selected samples (see Appendix B and Supplementary Materials) were conducted at Adelaide Microscopy using an Agilent 7900x (Agilent, Santa Clara, USA) with attached New Wave NWR213 laser ablation system with NIST612 (primary for calibration and drift correction) and NIST614 (secondary) standards. ...
... A range of delay times spanning 0-400 s was employed. After some experimentation the delays used were 1, 2, 3,5,7,10,15,20,25,35,50,75,100,200 and 400 s in addition to the zero-time value. Longer delay times are impractical as the 400 s experiment delay time experiment required almost 20 h of collection alone. ...
Single pulse, solid-state 29 Si nuclear magnetic resonance (NMR) spectroscopy offers an additional method of characterisation of opal-A and opal-CT through spin-lattice (T1) relaxometry. Opal T1 relaxation is characterised by stretched exponential (Weibull) function represented by scale (speed of relaxation) and shape (form of the curve) parameters. Relaxation is at least an order of magnitude faster than for silica glass and quartz, with Q3 (silanol) usually faster than Q4 (fully substituted silicates). 95% relaxation (Q4) is achieved for some Australian seam opals after 50 s though other samples of opal-AG may take 4000 s, while some figures for opal-AN are over 10,000 s. Enhancement is probably mostly due to the presence of water/silanol though the presence of paramag-netic metal ions and molecular motion may also contribute. Shape factors for opal-AG (0.5) and opal-AN (0.7) are significantly different, consistent with varying water and silanol environments, possibly reflecting differences in formation conditions. Opal-CT samples show a trend of shape factors from 0.45 to 0.75 correlated to relaxation rate. Peak position, scale and shape parameter, and Q3 to Q4 ratios offer further differentiating feature to separate opal-AG and opal-AN from other forms of opaline silica. T1 relaxation measurement may have a role for provenance verification. In addition , definitively determined Q3 / Q4 ratios are in the range 0.1 to 0.4 for opal-AG but considerably lower for opal-AN and opal-CT.
