Showing papers on "Silicate minerals published in 2018"

High-precision 41K/39K measurements by MC-ICP-MS indicate terrestrial variability of δ41K

Abstract: Potassium is a major component in continental crust, the fourth-most abundant cation in seawater, and a key element in biological processes. Until recently, difficulties with existing analytical techniques hindered our ability to identify natural isotopic variability of potassium isotopes in terrestrial materials. However, measurement precision has greatly improved, and a range of K isotopic compositions has now been demonstrated in natural samples. In this study, we present a new technique for high-precision measurement of K isotopic ratios using high-resolution, cold plasma multi-collector mass spectrometry. We apply this technique to demonstrate natural variability in the ratio of 41K to 39K in a diverse group of geological and biological samples, including silicate and evaporite minerals, seawater, and plant and animal tissues. The total range in 41K/39K ratios is ca. 2.6‰, with a long-term external reproducibility of 0.17‰ (2σ, N = 108). Seawater and seawater-derived evaporite minerals are systematically enriched in 41K compared to silicate minerals by ca. 0.6‰, a result consistent with recent findings. Although our average bulk-silicate Earth value (−0.54‰) is indistinguishable from previously published values, we find systematic δ41K variability in some high-temperature sample suites, particularly those with evidence for the presence of fluids. The δ41K values of biological samples span a range of ca. 1.2‰ between terrestrial mammals, plants, and marine organisms. Implications of terrestrial K isotope variability for the atomic weight of K and K-based geochronology are discussed. Our results indicate that high-precision measurements of stable K isotopes, made using commercially available mass spectrometers, can provide unique insights into the chemistry of potassium in geological and biological systems.

Silica Solubilization Potential of Certain Bacterial Species in the Presence of Different Silicate Minerals

Abstract: Silicate solubilisng bacteria (SSB) are distributed in soil, water, aquatic sediments and in silicate minerals but their population is smaller than the total bacteria indicating their uniqueness. Pond sediment showed a higher SSB with 40×104 g−1 while sugarcane field soil recorded 31×104 g−1 dry weight. SSB population in silicate minerals varied with the highest in phyto-sil followed by muscovite but very low in quartz and illite. The silicate solubilisation potential of the six isolates tested on eight minerals varied with the isolates and mineral. Magnesium trisilicate was more easily solubilised than, quartz, or muscovite. Four of the elite isolates were characterized by 16S r RNA sequencing and were found to be Bacillus flexus, B. mucilaginosus, B. megaterium and Pseudomonas fluorescens. The GC/MS analysis of organic acids produced in the medium containing feldspar and quartz by Bacillus flexus and B. muicilaginosus showed variation with the minerals. The release of silica in solution serves as a nutrient for life forms.

Comparison of static and mineralogical ARD prediction methods in the Nordic environment

Abstract: Acid rock drainage (ARD) is a major problem related to the management of mining wastes, especially concerning deposits containing sulphide minerals. Commonly used tests for ARD prediction include acid–base accounting (ABA) tests and the net acid generation (NAG) test. Since drainage quality largely depends on the ratio and quality of acid-producing and neutralising minerals, mineralogical calculations could also be used for ARD prediction. In this study, several Finnish waste rock sites were investigated and the performance of different static ARD test methods was evaluated and compared. At the target mine sites, pyrrhotite was the main mineral contributing to acid production (AP). Silicate minerals were the main contributors to the neutralisation potential (NP) at 60% of the investigated mine sites. Since silicate minerals appear to have a significant role in ARD generation at Finnish mine waste sites, the behaviour of these minerals should be more thoroughly investigated, especially in relation to the acid produced by pyrrhotite oxidation. In general, the NP of silicate minerals appears to be underestimated by laboratory measurements. For example, in the NAG test, the slower-reacting NP-contributing minerals might require a longer time to react than is specified in the currently used method. The results suggest that ARD prediction based on SEM mineralogical calculations is at least as accurate as the commonly used static laboratory methods.

Hydrochemistry and water quality of Rewalsar Lake of Lesser Himalaya, Himachal Pradesh, India

Abstract: The present research is to study hydrochemistry and water quality of Rewalsar Lake during pre-monsoon, monsoon, and post-monsoon seasons. The Ca2+ and Na+ are observed as the dominant cations from pre- to post-monsoon season. On the other hand, HCO3− and Cl− are observed dominant anions during pre-monsoon and monsoon seasons, whereas HCO3− and SO42− during post-monsoon season. The comparison of alkaline earth metals with alkali metals and total cations (Tz+) has specified that the carbonate weathering is the dominant source of major ions in the water of lake. The HCO3− is noticed to be mainly originated from carbonate/calcareous minerals during monsoon and post-monsoon, but through silicate minerals during pre-monsoon. The SO42− in Rewalsar Lake is produced by the dissolution of calcite and dolomite etc. The alkali metals and Cl− in the lake can be attributed to the silicate weathering as well as halite dissolution and anthropogenic activities. Certain other parameters like NO3−, NH4+, F−, and Br− are mainly a result of anthropogenic activities. The alkaline earth metals are found to surpass over alkali metals, whereas weak acid (HCO3−) exceed to strong acid (SO42−). The Piper diagram has shown Ca2+–HCO3− type of water during all the seasons. The water quality index has indicated that the water quality of the lake is unsuitable for drinking from pre- to post-monsoon. Several parameters like salinity index, sodium adsorption ratio, sodium percent, residual sodium carbonate, magnesium hazard etc. have revealed the water of Rewalsar Lake as suitable for irrigation.

On the relative timing of listwaenite formation and chromian spinel equilibration in serpentinites

Abstract: Ultramafic rocks exposed at the Earth's surface generally record multiple stages of evolution that may include melt extraction, serpentinization, carbonatization, and metamorphism. When quantitative thermometry based on mineral chemistry is applied to such rocks, it is often unclear what stage of their evolution is being observed. Here, in peridotites with extensive replacement of silicate minerals by carbonates (listwaenites), we present a case study that addresses the timing of carbonate formation relative to closure of exchange reactions among relict primary minerals. Massive and schistose serpentinized peridotites of Neoproterozoic age outcrop at Gabal Sirsir, South Eastern Desert, Egypt (northwestern corner of the Arabian-Nubian Shield or ANS). Petrography, bulk composition, and mineral chemistry are all consistent with a strongly depleted mantle harzburgite protolith for the serpentinites. Bulk compositions are low in Al_2O_3 and CaO and high in Mg# [molar Mg/(Mg+Fe) = 0.89–0.93]. Relict spinel has high Cr# [molar Cr/(Cr+Al)] and low Ti, while relict olivine has high Mg# and NiO contents. Based on compositions of coexisting relict olivine and chromian spinel, the protolith experienced 19 to 21% partial melt extraction. Such high degrees of partial melting indicate a supra-subduction zone environment, possibly a forearc setting. Along thrust faults and shear zones, serpentinites are highly altered to form talc-carbonate rocks and weathering-resistant listwaenites that can be distinguished petrographically into Types I and II. The listwaenitization process took place through two metasomatic stages, associated first with formation of the oceanic crustal section and near-ridge processes (∼750–700 Ma) and subsequently during obduction associated with the collision of East and West Gondwana and escape tectonics (∼650–600 Ma). In the first stage, Mg# of chromian spinel in the serpentinites continuously changed due to subsolidus Mg–Fe^(2+) redistribution, while the Mg# of chromian spinel in the Type I listwaenites was frozen due to the absence of coexisting mafic silicates. During the second stage, the Type II listwaenites formed along shear zones accompanied by oxidation of relict chromian spinel to form ferritchromite and Cr-bearing magnetite in both serpentinites and listwaenites. The high Cr# of chromian spinel relics in both serpentinites and listwaenites preserves primary evidence of protolith melt extraction, but divalent cations are more easily mobilized at low temperature. Hence, relict chromian spinel in listwaenites shows significantly higher Mg# and lower MnO than that in serpentinite, suggesting that nearly complete alteration of ultramafic rocks to form listwaenite took place prior to re-equilibration between chromian spinel and the surrounding mafic minerals in serpentinites. Furthermore, the ferritchromite in the serpentinites has higher Mn content (1.1–2.1 wt%) than that in the listwaenites (0.6–0.9 wt%), indicating its formation after carbonatization since carbonate minerals are a favorable sink for Mn.

Onboard experiment investigating metal leaching of fresh hydrothermal sulfide cores into seawater

Abstract: We observed the initial release rate of metals from four fresh (i.e., without long time exposure to the atmosphere) hydrothermal sulfide cores into artificial seawater. The sulfide samples were collected by seafloor drilling from the Okinawa Trough by D/V Chikyu, powdered under inert gas, and immediately subjected to onboard metal-leaching experiments at different temperatures (5 °C and 20 °C), and under different redox conditions (oxic and anoxic), for 1–30 h. Zinc and Pb were preferentially released from sulfide samples containing various metals (i.e., Mn, Fe, Cu, Zn, Cd, and Pb) into seawater. Under oxic experimental conditions, Zn and Pb dissolution rates from two sulfide samples composed mainly of iron disulfide minerals (pyrite and marcasite) were higher than those from two other sulfide samples with abundant sphalerite, galena, and/or silicate minerals. Scanning electron microscopy confirmed that the high metal-releasing sample contained several galvanic couples of iron disulfide with other sulfide minerals, whereas the low metal-releasing sample contained fewer galvanic couples or were coated by a silicate mineral. The experiments overall confirmed that the galvanic effects with iron disulfide minerals greatly induce the initial release of Zn and Pb from hydrothermal sulfides into seawater, especially under warm oxic conditions.

Mineralogy, Crystallography and Structural Complexity of Natural Uranyl Silicates

Abstract: Naturally occurring uranyl silicates are common constituents of the oxidized parts (i.e., supergene zone) of various types of uranium deposits. Their abundance reflects the widespread distribution of Si4+ in the Earth’s crust and, therefore, in groundwaters. Up to date, 16 uranyl silicate minerals are known. Noteworthy is that the natural uranyl silicates are not extremely diverse regarding their crystal structures; it is a result of possible concentrations (activity) of Si4+ in aqueous solutions derived from dissolution of primary Si minerals or the composition of late hydrothermal fluids. Therefore, in natural systems, we distinguish in fact among two groups of uranyl silicate minerals: uranophane and weeksite-group. They differ in U:Si ratio (uranophane, 1:1; weeksite, 2:5) and they form under different conditions, reflected in distinctive mineral associations. An overview of crystal-chemistry is provided in this paper, along with the new structure data for few members of the uranophane group. Calculations of the structural complexity parameters for natural uranyl silicates are commented about as well as other groups of uranyl minerals; these calculations are also presented from the point of view of the mineral paragenesis and associations.

The Role of Halogens in Terrestrial and Extraterrestrial Geochemical Processes: Surface, Crust, and Mantle

Abstract: Terrestrial worlds on which H2O-bearing fluids are stable tend to show a higher degree of chemical differentiation than similar worlds on which such fluids are not present or stable Other than CO2 and S, important components of these H2O-bearing fluids are the halogens, specifically the more common F and Cl and, to a lesser degree, the less common Br and I When taken together in combination with H2O, F and Cl are potentially powerful ligands with respect to solution-enabled mass transport of most metal cations This characteristic has indirect implications with respect to the reactivity of halogen-bearing fluids with many mineral species from among the silicates, oxides, sulfides, and orthophosphates and with melts The low wetting angle of Cl-bearing solutions along grain boundaries, coupled with a high reactivity, allow them to flow easily along and through grain boundaries under mid-crustal to lower crustal/lithospheric mantle P-T conditions, thus aiding both mineral re-equilibration and mass transfer on a local (cm’s) as well as on a large (km’s) scale during metamorphism The low H2O activity of these brines under high-grade conditions enables them to co-exist with nominally anhydrous Fe-Mg silicate minerals characteristic of the lower crust and the lithospheric mantle, such as orthopyroxene, clinopyroxene, and olivine Evidence for the presence of these brines is seen in the fluid and melt inclusions found in the minerals from igneous rocks, ore deposits, sedimentary rocks, lower crustal rocks, and the lithospheric mantle In magmas, halogens play an important role with regard to the solubility of various metal cations as well as affecting crystallization temperatures During crystallization of plutonic magma bodies, while F tends to be retained in felsic melts, fluorapatite, and micas, Cl is expelled as a NaCl-KCl-CaCl2-bearing fluid into the surrounding country rock taking with it a large variety of other metal cations These are subsequently deposited in associated fissures, veins, and pegmatites as ore minerals/deposits during crystallization of the main magma body The presence of such fluids has been confirmed by extensive fluid inclusion and experimental studies Beyond the Earth, halogens play an important role in the geochemistry of other terrestrial worlds in the solar system on which H2O can exist as a fluid whether on or beneath the surface These include Mars, volatile-rich asteroids, and those ice moons of Jupiter and Saturn believed to contain subsurface oceans In each of these cases both direct observation and terrestrial analogues allow for a more complete understanding of the role that halogens play in the various H2O-aided geochemical processes present on these worlds The goal of this volume is to bring together a diverse group of geoscientists with long-range interests, knowledge, and experience concerning the role that halogens play in or during a variety of geochemical and hydrothermal processes in the crust and mantle of the Earth as well as on volatile-rich asteroids, Mars, and the ice moons of Jupiter and Saturn in a series of review chapters The range and depth of knowledge contained within these chapters regarding the role of halogens in geochemical processes, both terrestrial and extraterrestrial, outline and provide a firm foundation on which to base our current understanding of how halogens contribute to the geochemical/geophysical evolution and stability of liquid H2O-bearing terrestrial worlds in the solar system overall

Origin of Reverse Zoned Cr-Spinels from the Paleoproterozoic Yanmenguan Mafic–Ultramafic Complex in the North China Craton

Abstract: We conducted petrological and mineral chemistry investigations of Cr-spinel in ultramafic rocks of the Yanmenguan mafic–ultramafic complex in the North China Craton The Cr-spinel grains occur as inclusions in enstatite, tschermakite, phlogopite, and olivine, or as interstitial grains among the aforementioned silicate minerals, and show concentric or asymmetrical textures Back-scattered electron and elemental images and compositional profiles of the spinel grains indicate the presence of Cr- and Fe-rich cores and Al- and Mg-rich rims The host silicate minerals display a decrease in Al and Mg contents accompanied by an increase in Cr and Fe away from the spinel These textures and compositional variations suggest that subsolidus elemental exchange more likely gave rise to the compositional zonation, resulting in the transfer of Al and Mg from the silicate minerals to the spinel The Mn, Ni, and Ti contents in spinel and the major elements of olivine-hosted spinel are relatively stable during subsolidus elemental diffusion and thus are more reliable tracers of primary high-temperature processes The temperature estimates reveal that the subsolidus diffusion might have occurred at 600–720 °C, which could be linked to the regional metamorphic event

Processes Controlling Groundwater Chemistry from Mulakalacheruvu Area, Chittoor District, Andhra Pradesh, South India: A Statistical Approach Based on Hydrochemistry

Abstract: In India groundwater is an important source for domestic and agricultural purposes. Management of this resource is very important to meet the increasing demand of water. In this study, the ionic concentrations of groundwater from Mulakalacheruvu area, Chittoor district, Andhra Pradesh, South India have been calculated during May 2014 and the major ionic concentrations have been analyzed by hydrochemcial and statistical methods in order to trace the main processes controlling the groundwater chemistry. The results have suggested that these groundwater samples belong to Na-HCO3 (18 samples), Ca-HCO3 (16 samples) and Mg-HCO3 (6 samples) types. The hydrochemical methods suggest that the rocks in the aquifer system are sources of the major ions in the groundwater, and the silicate is the main mineral phase. Three sources viz. mafic silicate, felsic silicates and easily soluble minerals have been identified as responsible for the chemical variations of the groundwater. Further, the source contributions of silicate minerals for the groundwater hydrochemistry have been analyzed. These results suggest that the hydrochemistry of the groundwater in the south-eastern area is mainly controlled by weathering of felsic silicate (61-100%), whereas the north-western area is principally contributed by mafic silicate minerals (48-100%).

Biogenic Weathering: Solubilization of Iron from Minerals by Epilithic Freshwater Algae and Cyanobacteria

Abstract: A sandstone outcrop exposed to freshwater seepage supports a diverse assemblage of photosynthetic microbes. Dominant taxa are two cyanophytes (Oscillatoria sp., Rivularia sp.) and a unicellular green alga (Palmellococcus sp.). Less abundant taxa include a filamentous green alga, Microspora, and the desmid Cosmarium. Biologic activity is evidenced by measured levels of chlorophyll and lipids. Bioassay methods confirm the ability of these microbes to dissolve and metabolize Fe from ferruginous minerals. Chromatographic analysis reveals citric acid as the likely chelating agent; this low molecular weight organic acid is detectable in interstitial fluid in the sandstone, measured as 0.0756 mg/mL. Bioassays using a model organism, Synechoccus elongates strain UTEX 650, show that Fe availability varies among different ferruginous minerals. In decreasing order of Fe availability: magnetite > limonite > biotite > siderite > hematite. Biotite was selected for detailed study because it is the most abundant iron-bearing mineral in the sandstone. SEM images support the microbiologic evidence, showing weathering of biotite compared to relatively undamaged grains of other silicate minerals.