Showing papers on "Silicate minerals published in 2014"

Natural Weathering Rates of Silicate Minerals

Abstract: This chapter presents (1) the development of rates that quantitatively describe silicate mineral and rock weathering, (2) a summary of the available literature rate data for the weathering of several common silicate minerals, and (3) a discussion of the chemical, physical, and hydrologic processes that control silicate mineral weathering at the Earth's surface. Quantitative rates of weathering are important in understanding reaction mechanisms and in addressing a number of economic and environmental issues. Mass change, defined in terms of elements, isotopes, or mineral abundances, is determined from either solid-state (soil, regolith, and rock) or solute (pore water, groundwater, and surface water) compositions. Solid-state mass differences reflect weathering over geologic timescales while solute compositions reflect the residence time of the water. These mass losses or gains are normalized to surface area defined on a geographic, volumetric, or mineral-specific basis. The advantages of this approach are that such rates are related to reaction mechanisms and can be used as predictive tools in estimating how weathering will behave under various environmental conditions.

13.21 – Geochemistry of the Rare-Earth Element, Nb, Ta, Hf, and Zr Deposits

Abstract: Rare elements include rare-earth elements (REE), Nb, Ta, Zr, and Hf. As a group, these elements are concentrated in the crust and are incompatible (remain in the melt rather than substitute into silicate minerals). Carbonatites host the most important deposits of REE and Nb, although peralkaline granites and silica under saturated rocks also host deposits of these elements. By contrast, Ta is concentrated in peraluminous granites and pegmatites. The former types of deposits are characterized by rift and anorogenic tectonic settings, whereas the latter, most typically, are in collisional settings. Rare elements are concentrated by magmatic processes; in silicate melts one of the most important controls on rare element mineral solubility is the (Na + K)/Al ratio of the melt. Their solubilities are also temperature dependent and both these parameters may control magmatic ore deposition. Fluid–melt partition coefficients are typically low, but nonetheless the grades in many rare-element deposits are highest where hydrothermal alteration is strongly developed. Rare elements are hard acids in the Pearson sense and they prefer to bond electrostatically to form aqueous complexes with hard bases such as F − and OH − , as well as SO 4 2 − , CO 3 2 − , and PO 4 3 − . Fluid mixing may be the principal control of mineralization in hydrothermal systems.

Experimental Design and Data Analysis for Accurate Estimation of Reaction Kinetics and Conversion for Carbon Mineralization

Abstract: Silicate minerals such as olivine (Mg2SiO4) and serpentine [Mg3(OH)4(Si3O5)] can react with CO2 to form mineral carbonates to permanently store CO2. Despite significant advancements in carbon mineralization, major discrepancies in the reported kinetics exist because of inconsistencies among various experimental methodologies, the heterogeneity and aging of the minerals, and inadequate fast kinetics and morphological data to probe the reaction mechanisms. In this work, it was found that aged and freshly ground olivine produce very different carbonation yields. A new mineral cleaning protocol to remove fines (<5 μm) was developed. Fast and slow serpentine dissolution regimes were distinguished using a custom-built differential-bed reactor. Mineral carbonate formation using a bubble-column reactor was described. Different carbon analyses were evaluated for accurate estimation of the extent of carbon mineralization. Therefore, this study focused on the development of an experimental framework and a data analy...

Detection of solar wind-produced water in irradiated rims on silicate minerals

Abstract: The solar wind (SW), composed of predominantly ∼1-keV H+ ions, produces amorphous rims up to ∼150 nm thick on the surfaces of minerals exposed in space. Silicates with amorphous rims are observed on interplanetary dust particles and on lunar and asteroid soil regolith grains. Implanted H+ may react with oxygen in the minerals to form trace amounts of hydroxyl (−OH) and/or water (H2O). Previous studies have detected hydroxyl in lunar soils, but its chemical state, physical location in the soils, and source(s) are debated. If −OH or H2O is generated in rims on silicate grains, there are important implications for the origins of water in the solar system and other astrophysical environments. By exploiting the high spatial resolution of transmission electron microscopy and valence electron energy-loss spectroscopy, we detect water sealed in vesicles within amorphous rims produced by SW irradiation of silicate mineral grains on the exterior surfaces of interplanetary dust particles. Our findings establish that water is a byproduct of SW space weathering. We conclude, on the basis of the pervasiveness of the SW and silicate materials, that the production of radiolytic SW water on airless bodies is a ubiquitous process throughout the solar system.

Mössbauer parameters of iron in phosphate minerals: Implications for interpretation of martian data

Abstract: Phosphate minerals, while relatively rare, show a broad range of crystal structure types with linkages among PO 4 tetrahedra mimicking the hierarchy of polymerization of SiO 4 tetrahedra seen in silicate minerals. To augment previous Mossbauer studies of individual phosphate species and groups of species, this paper presents new Mossbauer data on 63 different phosphate samples, and integrates them with data on more than 37 phosphate species in 62 other studies from the literature. Variations in Mossbauer parameters of different sites in each mineral are then related to both the local polyhedral environment around the Fe cations and the overall structural characteristics of each species. The entire aggregated Mossbauer data set on phosphate minerals is juxtaposed against parameters obtained for spectra from the MIMOS spectrometers on Mars. This comparison demonstrates that signatures from many different phosphate or sulfate mineral species could also be contributing to Mars Mossbauer spectra. Results underscore the conclusion that unique mineral identifications are generally not possible from Mossbauer data alone, particularly for paramagnetic phases, although combining Mossbauer results with other data sets enables a greater level of confidence in constraining mineralogy. This study provides a wealth of new data on Fe-bearing phosphate minerals to bolster future analyses of Mossbauer spectra acquired on Mars.

High Resolution Core- and Valence-Level XPS Studies of the Properties (Structural, Chemical and Bonding) of Silicate Minerals and Glasses

Abstract: Core-level and valence-level X-ray Photoelectron Spectroscopy (XPS), developed in the late 1950’s and 1960’s by Siegbahn and coworkers (Siegbahn et al. 1969; Carlson 1975; Barr 1993; Fadley 2010) has become an invaluable tool over the last 40 years for studying mainly the surface properties and reactivity of a wide range of minerals, predominantly oxides (for reviews, see: Heinrich and Cox 1994; Chambers 2000; Salmeron and Schlogl 2008, and references in Bancroft et al. 2009; Newburg et al. 2011), sulfides (for reviews, see Hochella 1988; Bancroft and Hyland 1990; Nesbitt 2002; Murphy and Strongin 2009) and silicates (for a review see Hochella 1988; references in Biino and Groning 1998; Oelkers 2001; Zakaznova-Herzog et al. 2008). The large majority of these studies have focused on the first few surface monolayers of the minerals because of the surface sensitivity of the technique (~2–20 monolayers for photon energies of ≤ 1486 eV (Hochella 1988; Nesbitt 2002), and in many such cases, XPS has become the technique of choice for surface studies. Silicate XPS studies generally have focused on three surface applications outlined by Hochella (1988): (1) studies of the oxidation state of near surface atoms (e.g., Fe); (2) studies of sorption reactions on mineral surfaces; and (3) studies of the alteration and weathering of mineral surfaces. Fewer reports have focused on the fourth application of Hochella (1988), the study of the bulk atomic structure and chemical state properties of minerals and glasses. This is surprising perhaps, because the large majority (usually >90 %) of XPS line intensities comes from the bulk mineral in XPS studies using the typical laboratory Al K α X-ray sources (1486.6 eV). To emphasize this point, the surface S 2 p peaks from the …

Use of silicate minerals for pH control during reductive dechlorination of chloroethenes in batch cultures of different microbial consortia

Abstract: In chloroethene-contaminated sites undergoing in situ bioremediation, groundwater acidification is a frequent problem in the source zone, and buffering strategies have to be implemented to maintain the pH in the neutral range. An alternative to conventional soluble buffers is silicate mineral particles as a long-term source of alkalinity. In previous studies, the buffering potentials of these minerals have been evaluated based on abiotic dissolution tests and geochemical modeling. In the present study, the buffering potentials of four silicate minerals (andradite, diopside, fayalite, and forsterite) were tested in batch cultures amended with tetrachloroethene (PCE) and inoculated with different organohalide-respiring consortia. Another objective of this study was to determine the influence of pH on the different steps of PCE dechlorination. The consortia showed significant differences in sensitivities toward acidic pH for the different dechlorination steps. Molecular analysis indicated that Dehalococcoides spp. that were present in all consortia were the most pH-sensitive organohalide-respiring guild members compared to Sulfurospirillum spp. and Dehalobacter spp. In batch cultures with silicate mineral particles as pH-buffering agents, all four minerals tested were able to maintain the pH in the appropriate range for reductive dechlorination of chloroethenes. However, complete dechlorination to ethene was observed only with forsterite, diopside, and fayalite. Dissolution of andradite increased the redox potential and did not allow dechlorination. With forsterite, diopside, and fayalite, dechlorination to ethene was observed but at much lower rates for the last two dechlorination steps than with the positive control. This indicated an inhibition effect of silicate minerals and/or their dissolution products on reductive dechlorination of cis-dichloroethene and vinyl chloride. Hence, despite the proven pH-buffering potential of silicate minerals, compatibility with the bacterial community involved in in situ bioremediation has to be carefully evaluated prior to their use for pH control at a specific site.

Molecular-level mechanisms of quartz dissolution under neutral and alkaline conditions in the presence of electrolytes

Abstract: Copyright © 2014 by The Geochemical Society of Japan. Strandh et al., 1997), reactivity of different surface sites (e.g., Blum and Lasaga, 1988, 1991; Blum et al., 1990; Dove, 1994; Furrer and Stumm, 1986; Lasaga, 1992; Pelmenschikov et al., 2000; Raghavachari and Rohlfing, 1991; Bickmore et al., 2008; Wallace et al., 2010), the extent of undersaturation of the solution (e.g., Dove et al., 2005, 2008; Zhu, 2005; Zhu et al., 2006), and temperature (e.g., Brady and Walther, 1992; Casey and Sposito, 1992; Rimstidt, 1997; Rimstidt and Barnes, 1980). However, the molecular-level mechanisms for many of these driving forces remain elusive. Quartz is the simplest and an abundant silicate mineral in the earth’s crust. The study of its dissolution mechanism was expected to shed light on the dissolution mechanisms of other silicate minerals. However, it has been found that the dissolution mechanism of quartz is very different to that of other silicate minerals in several ways. For instance, the dissolution of most silicate minerals is insensitive in the presence of small amounts of electrolytes (NaCl, CaCl2). However, experimental data show that at near-neutral pH conditions, quartz will dissolve up to 100 times faster in the presence of small amounts of alkali and alkaline earth electrolytes (e.g., Berger et al., 1994; Dove and Crerar, 1990; Dove and Elston, 1992; Dove, 1994, 1999; Dove and Nix, 1997; Dove and Colin, Molecular-level mechanisms of quartz dissolution under neutral and alkaline conditions in the presence of electrolytes

Statistical analysis of hydrochemistry of groundwater and its implications for water source identification: a case study

Abstract: Hydrochemistry of groundwater is important for inrush water source identification in coal mines. For determining the source of inrush water from the 8101 working face in the Wolonghu coal mine, Northern Anhui Province, China, a total of 22 groundwater samples from three aquifer systems (loose layer—LA, coal bearing sandstone—CA, and the underlying limestone—TA), and eight samples from the 8101 working face were collected for analyzing major ion concentrations. The results suggest that major ion concentrations of the aquifer systems were different from each other, and they can be subdivided into Na–HCO3, Na–SO4, and Ca–SO4 types. Factor analysis indicates that their chemical compositions are mainly originated from two kinds of contributions: dissolution of soluble minerals (e.g., calcite, dolomite, and gypsum) and weathering of silicate minerals (e.g., plagioclase). Plots of factor scores and cluster analysis imply that LA was connected with CA, whereas TA was an isolated aquifer system. Moreover, the source of 8101 working face water was finally determined to be LA by using discriminant analysis.

Geographical variations in Sr and Nd isotopic ratios of cryoconite on Asian glaciers

Abstract: Cryoconite is a dark-coloured surface dust deposited on glaciers that consists of wind-blown mineral particles, as well as organic matter derived from microbes living on glaciers. In this paper, we analyse the Sr and Nd isotopic ratios of four mineral fractions (i.e., the saline, carbonate, phosphate, and silicate mineral fractions), as well as the organic fraction, of cryoconite samples obtained from six Asian glaciers (the Altai, Pamir, Tien Shan, Qillian Shan, and Himalayan regions), and discuss their geographical variations in terms of the geological origins of the mineral particles and the biogeochemical processes on the glaciers. The silicate mineral fraction showed lower Sr and higher Nd ratios for the glaciers located to the north (Altai, 87Sr/86Sr: 0.713?490?0.715?284, ?Nd(0): ?6.4 to ?5.6), while higher Sr and lower Nd ratios for the glaciers located to the south (Himalayas, 87Sr/86Sr: 0.740?121?0.742?088, ?Nd(0): ?16.4 to ?15.7); the ratios were similar to those of desert sand, loess, and river sediments in the respective regions of the glaciers. This result suggests that the silicate minerals within the cryoconites were derived from different sources depending on the geographical locations of the glaciers. The isotopic ratios of the saline, carbonate, and phosphate mineral fractions were distinct from those of the silicate fraction, and were similar to those of evaporites and apatite deposits from the Asian deserts, but also varied geographically, indicating that they are likely to reflect their geological origin. The Sr isotopic ratios of the organic fraction were similar to those of the saline and carbonate fractions from glaciers in the central area (Tien Shan and Qillian Shan), but were higher than those of the saline and carbonate fractions, and lower than the phosphate mineral fraction, in the northern and southern areas. The ratios of organic fraction may be determined from the mixing ratio of calcium sources incorporated by microbes on the glaciers.

Exfoliation and Characterization of Layered Silicate Minerals: a Review

Abstract: Exfoliated silicate minerals have attracted great attentions because of the dramatic improvement in properties This paper highlights the preparation of exfoliated silicate minerals, including physical, chemical, mixed physical and chemical methods The mechanisms by which silicates are exfoliated and the important influential factors are also summarized Finally, the instrumental techniques to characterize the exfoliated structure and exfoliation degree are presented

Silicate Minerals as a Source of Arsenic Contamination in Groundwater

Abstract: Mineral dissolution plays an essential role in controlling geogenic arsenic (As) contamination in groundwater. Although reductive dissolution of Fe oxyhydroxides is generally considered a key As release mechanism in many aquifers, some recent studies argue that silicate minerals, normally considered “inert” in As release, are the primary source of As contamination under certain conditions. The objective of this study is to determine As distribution in different minerals in a natural sediment and identify As release mechanisms and the role of silicate minerals in As release. A sediment sample was collected, characterized, and tested using leaching experiments at a range of pH and redox potentials. Our results showed that silicate minerals, which make up the bulk of the sediment, are the main As reservoir, containing 75 % of As. Fe–Mn oxyhydroxides, which are minor components in the sediment, are the second largest As reservoir and hold 16 % of As. Leaching experiments showed that silicate mineral dissolution is an important As-releasing mechanism and that high pH and low redox potential promoted silicate mineral dissolution and As release.

Effect of sulfate evaporate salt layer for formation of porphyrite iron ores in the Middle-Lower Yangtze River area

Abstract: The Middle-Lower Yangtze Polymetallic Ore Belt is one of the most important metallogenic belts in East China,comprising more than 200 polymetallic( Cu-Fe-Au,Mo,Zn,Pb,Ag) deposits. Ningwu and Luzong ore distrcts are the most important component of this belt. In these districts,volcanic-subvolcanic rocks,intrusions and subvolcanic rocks-related iron deposits which are well known as porphyrite iron deposits in China are widespread,during the Late Mesozoic. Based on the geological characteristics,spatial distribution and relationship with volcanic-subvolcanic rocks,the famous iron porphyrite deposit model has been established which improved the development of metallogenic theory and the effective guidance of porphyrite deposit prospecting greatly. This mineralization model emphasizes mainly magmatic-hydrothermal role, but the sulfate evaporate salt layers, at the top of the Zhouchongcun group in Middle Triassic,did not cause the attention in the role of mineralization. And this model mainly reflects the mineralization in shallow. The latest researches and prospecting results reveal that the Middle Triassic sulfate evaporates salt layers and mineralization has closely relationship. However,the ore-controlling mechanism of evaporate salt layers has still been unrevealed,"sulfate evaporate salt layers as oxidation barrier"in porphyry ore mineralization role rarely reported. In this paper,sulfur isotope characteristics of porphyrite iron deposits have been studied in Middle-Lower Yangtze River Polymetallic Ore Belt,and evaporate salt layers oxidation barrier in porphyry ore mineralization role has been revealed. Porphyrite iron deposits and Fe-S deposits commonly contain gypsum,and iron deposits,Fe-S deposits and gypsum deposits are paragenesis closely. In these deposits,the values of sulfideδ34SV-CDTare abnormally high,and the average values are higher than 5‰. Most of the values of gypsum δ34SV-CDTare about 20‰,which are similar to the value of marine sulfate. Sulfur isotopic composition of the deposits is closely related with the genesis types,with the reduced sulfur isotope value from ore magma type to ore magma-hydrothermal type to hydrothermal type. Such as the values of sulfide δ34SV-CDTare 10. 8‰ from Gushan deposit,7. 85‰ from Meishan deposit and 5. 01‰ from Washan deposit. The variation of sulfur isotopic composition of the deposits is mainly controlled by the sulfate reduced temperature and the proportion of original magma sulfur. The higher sulfate reduced temperature is,the higher sulfide δ34S value is. And the higher proportion percent of the original magma sulfur is the lower sulfide δ34S value. The calculate results obtained that most of sulfur is derived from sulfates in evaporate salt layers,and the proportion is approximately 60% ~ 80%. The reduction temperature is more than 450℃. The temperature of the sulfide precipitation was lower and relatively later. Thus,we infer that evaporate salt layers not just provide a large number of agents of mineralization for the sodium alteration,scapolitization and skarn alteration,like Na+,Cl-,CO32-and so on,but also course the Fe2 +transport as the complex( e. g. Na-Fe-Cl). On the other hand,the evaporate salt layer is the most important oxidation barrier in the depth crust,which could oxidize the Fe2 +into Fe3 +in the silicate magma and hydrothermal solution,and enrich the iron to be the iron deposit. It is a critical factor of the ore-forming of the porphyrite iron deposit. While the magma is assimilating evaporate salt layers( CaSO4),SO42-oxidize Fe2 +into Fe3 +in the silicate melt,which prevent Fe2 +to enter the lattice silicate minerals,with forming Fe3O4/ Fe2O3and poor iron silicate minerals like diopside,actinolite,tremolite and so on. The immiscibility occurs between iron oxideand silicate melt in magma chamber,by the effects of P,NaCl and volatile,forming the iron ore magma. The iron ore magma has strong viscous behavior,with short migration distance,penetrating along favorable structural parts,near the contact zone of intrution and salt layers. It forms ore magma type or like-skarn type iron deposit like Gushan and Meishan deposits. Metallogenic hydrothermal has strong mobility,transporting in the form of iron complex,with long migration distance,concentrating and precipitating in the distal the contact zone of intrution and salt layers,like the volcanic rocks covering the subvolcanic rocks. These two type iron deposits coexist in the porphyrite iron deposit family with a certain spatial zonation,forming the"double-metallogenic structure". In the contact zone of intrution and salt layers,there might present the Daye ore magma-skarn iron deposit,with high grade and high reserve,and the scale may exceed the iron deposit occurred in the shallow part of subvolcanic rocks or volcanic rocks. And,SO42-itself is reduced into S2-,while it is oxidizing the Fe2 +,and S2-combine with Fe2 +to form pyrite,in the top or side portion of the iron deposit. These are underlying reasons of paragenesis closely among iron deposits,Fe-S deposits and gypsum deposits.