Showing papers on "Silicate minerals published in 2010"

The silicate-mediated formose reaction: bottom-up synthesis of sugar silicates.

Abstract: Understanding the mechanism of sugar formation and stabilization is important for constraining theories on the abiotic origin of complex biomolecules. Although previous studies have produced sugars from small molecules through the formose and related reactions, the product mixtures are complex and unstable. We have demonstrated that simple two- and three-carbon molecules (glycolaldehyde and glyceraldehyde), in the presence of aqueous sodium silicate, spontaneously form silicate complexes of four- and six-carbon sugars, respectively. Silicate selects for sugars with a specific stereochemistry and sequesters them from rapid decomposition. Given the abundance of silicate minerals, these observations suggest that formose-like reactions may provide a feasible pathway for the abiotic formation of biologically important sugars, such as ribose.

The role of H2O in rapid emplacement and crystallization of granite pegmatites: resolving the paradox of large crystals in highly undercooled melts

Abstract: Granite pegmatite sheets in the continental crust are characterized by very large crystals. There has been a shift in viewing pegmatites as products of very slow cooling of granite melts to viewing them as products of crystal growth in undercooled liquids. With this shift there has been a renewed debate about the role of H2O in the petrogenesis of pegmatites. Based on data on nucleation of minerals and new viscosity models for hydrous granite melts, it is argued that H2O is the essential component in the petrogenesis of granite pegmatites. H2O is key to reducing the viscosity of granite melts, which enhances their transport within the crust. It also dramatically reduces the glass transition temperature, which permits crystallization of melts at hundreds of degrees below the thermodynamic solidus, which has been demonstrated by fluid inclusion studies and other geothermometers. Published experimental data show that because H2O drastically reduces the nucleation rates of silicate minerals, the minerals may not be able to nucleate until melt is substantially undercooled. In a rapidly cooling intrusion, nucleation starts at its highly undercooled margins, followed by inward crystal growth towards its slower-cooling, hotter core. Delay in nucleation may be caused by competition for crystallization by several minerals in the near-eutectic melts and by the very different structures of minerals and the highly hydrated melts. Once a mineral nucleates, however, it may grow rapidly to a size that is determined by the distance between the site of nucleation and the point in the magma at which the temperature is approximately that of the mineral’s liquidus, assuming components necessary for mineral growth are available along the growth path. Granite pegmatites are apparently able to retain H2O during most of their crystallization histories within the confinement of their wall rocks. Pegmatitic texture is a consequence of delayed nucleation and rapid growth at large undercooling, both of which are facilitated by high H2O (±Li, B, F and P) contents in granite pegmatite melts. Without retention of H2O the conditions for pegmatitic textural growth may be difficult to achieve. Loss of H2O due to decompression and venting leads to microcrystalline texture and potentially glass during rapid cooling as seen in rhyolites. In contrast, slow cooling within a large magma chamber promotes continuous exsolution of H2O from crystallizing magma, growth of equant crystals, and final solidification at the thermodynamic solidus. These are the characteristics of normal granites that distinguish them from pegmatites.

Petrology of Nepheline Syenite Pegmatites in the Oslo Rift, Norway: Zirconium Silicate Mineral Assemblages as Indicators of Alkalinity and Volatile Fugacity in Mildly Agpaitic Magma

Abstract: Agpaitic nepheline syenites contain complex zirconium silicate minerals as primary magmatic phases; agpaitic liquidus mineral assemblages are controlled by compositional parameters such as alkalinity and concentrations of water and halogens in the magma. The Larvik Plutonic Complex in the late Palaeozoic Oslo Rift, Norway consists of hypersolvus monzonite (larvikite) and different varieties of nepheline syenite. The nepheline syenite members have miaskitic mineralogy (i.e. zircon stable), except for a suite of late, mildly agpaitic pegmatites, which contain zirconium silicate minerals of the wohlerite, rosenbuschite and eudialyte groups, in some cases together with zircon. These minerals form part of the liquidus mineral assemblage of the nepheline syenite magma together with fluorite and the rock-forming minerals alkali feldspar, aegirine(–augite), alkali amphibole, biotite, nepheline and sodalite. Low-variance mineral assemblages with two or three zirconium silicate minerals, fluorite and the major rock-forming minerals can be modelled in terms of phase equilibria in a multi-component system involving the sodium disilicate component of the melt and the volatile components water, fluorine and chlorine. A semiquantitative isothermal and isobaric petrogenetic grid in log activity space constructed from the observed mineral assemblages and the compositions of the coexisting minerals indicate that there are at least three trends of evolution leading from a miaskitic to an agpaitic crystallization regime in peralkaline nepheline syenites: (1) the increasing alkalis trend, typically leading to eudialyte-dominated liquidus assemblages (and possibly further to hyperagpaitic residual liquids); (2) the increasing water trend, leading to catapleiite crystallization; (3) the increasing fluorine trend, giving transitionally agpaitic liquidus assemblages with wohlerite, lavenite and hiortdahlite, with or without zircon, but eventually with magmatic fluorite; this type of evolution is applicable to weakly agpaitic systems such as the Oslo Rift pegmatites. The evolution of such magmas will most probably terminate at liquidus boundaries also involving eudialyte and/or rosenbuschite, and they are unlikely to evolve to more strongly agpaitic compositions.

Plastic Deformation of Upper Mantle Silicate Minerals

Abstract: Summary Several minerals which from their occurrence in nodular inclusions in basalts and on theoretical grounds are believed to occur in the upper mantle have been deformed experimentally at high temperature and pressure. The glide mechanisms on which olivine crystals in naturally occurring aggregates are deformed operate in the experimentally deformed specimens in addition to other glide mechanisms which occur naturally only in meteorites. The glide mechanisms which predominate in olivine are shown to be a function of temperature and strain-rate. Diopside deforms chiefly by mechanical twinning in the experiments and enstatite by slip and shear-transformation to clinoenstatite.

Time-resolved remote Raman study of minerals under supercritical CO2 and high temperatures relevant to Venus exploration

Abstract: We report time-resolved (TR) remote Raman spectra of minerals under supercritical CO(2) (approx. 95 atm pressure and 423 K) and under atmospheric pressure and high temperature up to 1003 K at distances of 1.5 and 9 m, respectively. The TR Raman spectra of hydrous and anhydrous sulphates, carbonate and silicate minerals (e.g. talc, olivine, pyroxenes and feldspars) under supercritical CO(2) (approx. 95 atm pressure and 423 K) clearly show the well-defined Raman fingerprints of each mineral along with the Fermi resonance doublet of CO(2). Besides the CO(2) doublet and the effect of the viewing window, the main differences in the Raman spectra under Venus conditions are the phase transitions, the dehydration and decarbonation of various minerals, along with a slight shift in the peak positions and an increase in line-widths. The dehydration of melanterite (FeSO(4).7H(2)O) at 423 K under approximately 95 atm CO(2) is detected by the presence of the Raman fingerprints of rozenite (FeSO(4).4H(2)O) in the spectrum. Similarly, the high-temperature Raman spectra under ambient pressure of gypsum (CaSO(4).2H(2)O) and talc (Mg(3)Si(4)O(10)(OH)(2)) indicate that gypsum dehydrates at 518 K, but talc remains stable up to 1003 K. Partial dissociation of dolomite (CaMg(CO(3))(2)) is observed at 973 K. The TR remote Raman spectra of olivine, alpha-spodumene (LiAlSi(2)O(6)) and clino-enstatite (MgSiO(3)) pyroxenes and of albite (NaAlSi(3)O(8)) and microcline (KAlSi(3)O(8)) feldspars at high temperatures also show that the Raman lines remain sharp and well defined in the high-temperature spectra. The results of this study show that TR remote Raman spectroscopy could be a potential tool for exploring the surface mineralogy of Venus during both daytime and nighttime at short and long distances.

An Investigation into the Origin of Fe-Rich Presolar Silicates in Acfer 094

Abstract: Presolar silicate and oxide grains from primitive meteorites are recognized as “stardust” on the basis of their extremely anomalous O isotopic compositions. We report data on 48 O-anomalous grains that were identified in grain size separates of the ungrouped carbonaceous chondrite Acfer 094. A majority of these grains exhibit high 17 O/ 16 O isotopic ratios along with solar to sub-solar 18 O/ 16 O ratios and may have originated in low-mass stars with close-to-solar metallicity. Four silicate grains that contain 18 O enrichments were also measured for their Si isotopes. A comparison of their O and Si isotopic compositions with model predictions indicates that these 18 O-rich grains may have formed in supernova ejecta. Four of the O-anomalous grains are oxides while the remaining 44 are silicates, based on elemental compositions determined by Auger spectroscopy. The presolar oxides include a TiO2 grain and a grain with spinel stoichiometry. The silicate grains largely exhibit ferromagnesian compositions, although a few grains also contain small amounts of Ca and/or Al. Stoichiometric silicates were further classified as either olivine-like or pyroxene-like, and in this study pyroxene-like grains are more abundant than olivinelike ones. The majority of silicates contain more Fe than Mg, including a few grains with Fe-rich end-member compositions. Spectroscopic observations indicate the presence of Mg-rich silicates in the atmospheres of stars and the interstellar medium. Mg-rich minerals such as forsterite and enstatite form by equilibrium condensation in stellar environments. However, non-equilibrium condensation can result in higher Fe contents and the occurrence of such processes in the outflows of stars may account for the Fe-rich grains. Alternatively, secondary processes may play a role in producing the Fe enrichments observed in the presolar silicate grains identified in the matrix of Acfer 094.

Characteristics and formation mechanism of pedogenic hematite in Quaternary Chinese loess and paleosols

Abstract: Hematite is an important iron oxide mineral in loess–paleosol sequences in northwest China. Samples containing hematite and related minerals in the loess and paleosol units from Chinese Loess Plateau were investigated using powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and high resolution transmission microscopy (HRTEM). The results show that there are at least three genetic types of hematite in Quaternary loess–paleosol sequences of northern China: (1) Weathering products of Fe-bearing silicate minerals, such as chlorite and biotite, that precipitated as aggregates of hematite nano-crystals on mineral surfaces; (2) oxidation products of eolian magnetite grains with hematite nano-crystals closely associated with magnetite; (3) coarse (silt-sized) hematite grains present in the fresh loess. Hematite with a nanoporous texture indicates that dehydration of iron hydroxide ferrihydrite is the main mechanism for the formation of hematite coating during chemical weathering of Fe-bearing silicates. It is proposed that nano-phase hematite is primarily responsible for the red color of the paleosol units. Paleosol layers are also characterized by higher magnetic susceptibility and increased oxidation of eolian magnetite. It is proposed that the major contribution to magnetic susceptibility is from the nano-phase magnetite that may be related to microbial activity. This research demonstrates that the study of mineral characteristics, formation mechanisms and the relationship between hematite, other iron oxides, and Fe-bearing silicate minerals helps to understand better the geochemical process in loess, both before and after dust deposition. In addition our study helps to explain the correlation between magnetic susceptibility and paleoclimate in northern China during the Quaternary.

Chemical weathering and solute export by meltwater in a maritime Antarctic glacier basin

Abstract: Solute yields, laboratory dissolution data and both chemical and isotopic markers of rock weathering reactions are used to characterise the biogeochemistry of glacial meltwaters draining a maritime Antarctic glacier. We find that delayed flowpaths through ice-marginal talus and moraine sediments are critical for the acquisition of solute from rock minerals because delayed flowpaths through subglacial sediments are absent beneath this small, cold-based glacier. Here the mechanisms of weathering are similar to those reported in subglacial environments, and include sub-oxic conditions in the early summer and increasingly oxic conditions thereafter. Up to 85% of the NO3 − and 65% of the SO4 2− are most likely produced by bacterially mediated reactions in these ice marginal sediments. However, reactive pyrite phases are sparse in the host rocks, limiting the export of Fe, SO4 2− and cations that may be removed by weathering once pyrite oxidation has taken place. This means that dissolution of Ca2+ and Na+ from carbonate and silicate minerals dominate, producing moderate cationic denudation yields from Tuva Glacier (163 Σ*meq+ m−2 a−1) compared to a global range of values (94–4,200 Σ*meq+ km−2 a−1). Overall, crustally derived cations represent 42% of the total cationic flux, the rest being accounted for by snowpack sources.

Platy layer silicate minerals for controlling residual strength in landslide soils of different origins and geology

Abstract: We use a ring-shear apparatus to examine the relationship between the residual strength and mineralogical properties of landslide soils and find a significant relation between the residual strength parameter, φr, and the total content of layer silicate minerals prone to preferred orientation within the bulk soil. In this context, the total content of smectite, vermiculite, chlorite, and mica in the sub-425-μm soil fraction is a suitable mineralogical parameter for estimating the magnitude of φr. Plotting φr as a function of total mineral content forms a chair-shaped curve, based on which we classify landslide soils into three groups. In the first group of soils, sliding appears to be controlled by minerals such as quartz, feldspar, calcite, dolomite, and layer silicate minerals other than smectite, vermiculite, chlorite, and mica and their φr is almost constant at ∼32°. In the second group of soils, the controlling mineralogical factor for sliding quickly shifts from nonpreferred-orientation minerals to preferred-orientation minerals and φr decreases from 30° to 10°. In the third group of soils, sliding is well controlled by preferred-orientation layer silicate minerals and φr gradually decreases from 10° to 5°. The results show that the relationship can be used for predicting φr of a wide range of landslide soils that differ in geology, soil type, mineralogical properties, and shear strength.

Iron oxides in comet 81P/Wild 2

Abstract: We have used synchrotron Fe-XANES, XRS, micro Raman, and SEM-TEM analyses of Stardust track 41 slice and track 121 terminal area slices to identify Fe oxide (magnetite-hematite and amorphous oxide). Fe-Ti oxide, and V-rich chromite (Fe-Cr-V-Ti-Mn oxide) grains ranging in size from 200 nm to similar to ~10 μm. They co-exist with relict FeNi metal. Both Fe-XANES and microRaman analyses suggest that the FeNi metal and magnetite(Fe2O3FeO) also contain some hematite (Fe2O3). The FeNi has been partially oxidized (probably during capture), but on the basis of our experimental work with a light-gas gun and microRaman analyses, we believe that some of the magnetite-hematite mixtures may have originated on Wild 2. The terminal samples from track 121 also contain traces of sulfide and Mg-rich silicate minerals. Our results show an unequilibrated mixture of reduced and oxidized Fe-bearing minerals in the Wild 2 samples in an analogous way to mineral assemblages seen in carbonaceous chondrites and interplanetary dust particles. The samples contain some evidence for terrestrial contamination, for example, occasional Zn-bearing grains and amorphous Fe oxide in track 121 for which evidence of a cometary origin is lacking.

Diffusion in Quartz, Melilite, Silicate Perovskite, and Mullite

Abstract: This chapter presents diffusion data for a group of silicate minerals not covered by the other chapter categories. It is a diverse collection of minerals, ranging from silicate perovskite, a dominant phase in the Earth’s lower mantle, to quartz, a common crustal mineral, to melilite, an important constituent of calcium-aluminum-rich inclusions in primitive chondrite meteorites. A summary of the properties of minerals reviewed in this chapter is in Table 1⇓. This review will focus primarily on diffusion of cations, as diffusion of noble gases, hydrogen and oxygen are considered in other chapters in this volume (Baxter 2010; Farver 2010). View this table: Table 1. A list of minerals discussed in this chapter and some of their characteristics. Quartz is among the most abundant minerals in the continental crust, found in many igneous, metamorphic and sedimentary rocks. It also has various technological applications because of its piezoelectric properties (e.g., Ramus 1989; Lang 1993). Quartz is one of the most stable minerals, resistant to chemical attack and weathering. It can contain minor amounts of aluminum, alkali elements, and transition elements; quartz color, and in some cases luminescence, can be attributed to the presence of these elements and the defects they create. For example, tiny rutile needles or TiO2 in the colloidal state may be responsible for rose and blue quartz coloring, respectively, and the coloration of citrine is due to the presence of colloidal ferric hydroxide distributed submicroscopically (e.g., Deer et al. 1992). Diffusion of the major constituents of quartz, Si and O, has been extensively investigated under a broad range of conditions (Giletti and Yund 1984; Dennis 1984; Farver and Yund 1991; Sharp et al. 1991, Jaoul et al. 1995; Bejina and Jaoul 1996; Cherniak 2003). Since oxygen diffusion in minerals will be …

Iron isotope compositions of carbonatites record melt generation, crystallization, and late-stage volatile-transport processes

Abstract: Carbonatites define the largest range in Fe isotope compositions yet measured for igneous rocks, recording significant isotopic fractionations between carbonate, oxide, and silicate minerals during generation in the mantle and subsequent differentiation. In contrast to the relatively restricted range in δ56Fe values for mantle-derived basaltic magmas (δ56Fe = 0.0 ± 0.1‰), calcite from carbonatites have δ56Fe values between −1.0 and +0.8‰, similar to the range defined by whole-rock samples of carbonatites. Based on expected carbonate-silicate fractionation factors at igneous or mantle temperatures, carbonatite magmas that have modestly negative δ56Fe values of ~ −0.3‰ or lower can be explained by equilibrium with a silicate mantle. More negative δ56Fe values were probably produced by differentiation processes, including crystal fractionation and liquid immiscibility. Positive δ56Fe values for carbonatites are, however, unexpected, and such values seem to likely reflect interaction between low-Fe carbonates and Fe3+-rich fluids at igneous or near-igneous temperatures; the expected δ56Fe values for Fe2+-bearing fluids are too low to produced the observed positive δ56Fe values of some carbonatites, indicating that Fe isotopes may be a valuable tracer of redox conditions in carbonatite complexes. Further evidence for fluid-rock or fluid-magma interactions comes from the common occurrence of Fe isotope disequilibrium among carbonate, oxide, silicate, and sulfide minerals in the majority of the carbonatites studied. The common occurrence of Fe isotope disequilibrium among minerals in carbonatites may also indicate mixing of phenocyrsts from distinct magmas. Expulsion of Fe3+-rich brines into metasomatic aureols that surround carbonatite complexes are expected to produce high-δ56Fe fenites, but this has yet to be tested.

Dianionic species with a bond consisting of two pentacoordinated silicon atoms

Abstract: Silicon can form bonds to other tetracoordinated silicon atoms and these bonds form the framework of many organosilicon compounds and crystalline silicon. Silicon can also form a pentacoordinated anionic structure—a so-called ‘silicate’. No compounds containing a direct bond between two silicate moieties—‘disilicates’ where two silicate structures are combined in one species—have been reported because of the electronic repulsion between the anionic halves and difficulty preventing the release of anions. Here we report the synthesis of thermally stable and isolable disilicates by the reductive coupling reaction of a silane bearing two electron-withdrawing bidentate ligands. Two pentacoordinated silicons, positively charged despite the formal negative charge, constitute a single σ-bond and bind eight negatively charged atoms. They can be reversibly protonated, cleaving two Si–O bonds, to afford a tetracoordinated disilane. Their unique electronic properties could be promising for the construction of functional materials with silicon wire made up of silicate chains. Silicon, like carbon, favours a four-coordinate geometry and this underpins the frameworks of the wide range of inorganic and organosilicon compounds, from silicate minerals to polysilanes. Although some pentavalent silicon compounds have already been reported, this work presents the first example where two five-coordinate silicon atoms are bonded to each other.

Seismic velocity anisotropy of phyllosilicate-rich rocks: characteristics inferred from experimental and crack-model studies of biotite-rich schist

Abstract: SUMMARY Seismic velocity anisotropy of biotite schist (30 per cent-mode biotite) was measured under confining pressures up to 150 MPa. The rock shows weak orthotropy which was altered from transverse isotropy (TI) generated by biotite-preferred orientation. The orthotropy was caused by microfolding in the rock. The velocity increase under confining pressure indicates that most crack planes are aligned parallel to the cleavage planes (silicate sheet) of the oriented biotite minerals. The anisotropy of the rock is basically TI due to both the aligned biotite minerals and cracks, which have a common symmetry axis. We found that other sheet silicate-rich rocks have a similar anisotropy with the biotite schist, in which the TI-type anisotropy is characterized by the slow P- and S-wave velocities along the symmetry axis. This is caused by the preferred orientation of sheet silicate minerals and the extremely slow P- and S-wave velocities along the axis perpendicular to the silicate sheet compared to the directions along the silicate sheet. When rock contains a large percentage of highly oriented sheet silicates, the fast and slow shear waves exchange their polarities at some off-symmetry axis directions, indicating that the qSwave (quasi-S wave) velocity exceeds the SH-wave velocity. The phase velocity distribution of qS wave shows an asymmetry with respect to the angle from the symmetry axis, which is characterized by a bulge in this distribution located near the symmetric axis. This is inherent to most sheet silicate minerals. When crack density of aligned cracks increases, the P-wave velocity along the symmetry axis decreases considerably. The qS-wave phase velocity in the off-axis directions also decreases, in accordance with the decrease of the P velocity along the symmetry axis. The asymmetry of the qS-wave phase velocity distribution increases as the P-wave velocity decreases along the symmetry axis. This relationship can be well understood by means of Berryman’s extended Thomsen approach.

Cs-corrected HAADF–STEM imaging of silicate minerals

Abstract: Significant improvement of the resolution for electron microscopy by the development of the spherical aberration corrector (Cs-corrector) brings valuable information to understand local atomic structures in silicate minerals. Cation columns separated by ca. 1.5 A, which originate from the ionic radius of oxygen anion or half of the closest oxygen-oxygen distance, are common in silicates and they are easily resolved by a Cs-corrected transmission electron microscope (TEM) or scanning TEM (STEM) with a resolution close to 1 A. High-angle annular dark-field (HAADF) imaging using Cs-corrected STEM was applied to orthopyroxene (Opx) with augite lamellae, and cronstedtite, an iron-bearing sheet silicate. Noisy contrast in the HAADF images was compensated by advanced noise filtering techniques. All cation columns in the pyroxene structure were resolved in the HAADF images from the c-axis. A sub-angstrom difference in the position of the M2 site between Opx and augite, which is caused by the occupation of the site by Fe (Opx) and Ca (augite), was clearly detected in the HAADF image as well as different contrasts of the cation columns related to occupying elements. A pair of tetrahedral cation columns separated by ca. 1.5 A in cronstedtite observed along [100] directions frequently show unequal contrast, suggesting a difference of the amounts of substituted Fe(3+) between the two columns. Comparison between the experimental contrast with a simple simulation suggests the distribution of Fe(3+) in a tetrahedral sheet avoiding linkage of Fe(3+) coordinating tetrahedrons.

Sr, Nd and Pb stable isotopes of surface dust on Ürümqi glacier No. 1 in western China

Abstract: Stable-isotopic ratios of strontium (Sr), neodymium (Nd) and lead (Pb) provide a means of identifying a geological source of substances and are used as tracers of elements in biological and geochemical processes. We analyzed these isotopic ratios of surface dust (cryoconite) collected on Urumqi glacier No. 1, Tien Shan, China. The dust was separated chemically into five fractions (four minerals and organic matter), and the isotopic ratios of each fraction were measured. The Sr and Nd isotopic ratios in the fractions extracted with ultrapure water (saline minerals), hydrogen peroxide solution (organic matter) and acetic acid (carbonate minerals) were low and invariable, whereas those extracted by hydrochloric acid (phosphate minerals) and the residual fraction (silicate minerals) were higher. The difference was likely due to the original source of each fraction. The isotopic ratios of the surface dust collected from different sites showed no significant difference, suggesting that they were spatially uniform across the glacier. The isotopic ratios of the silicate fraction were closer to those of desert sand reported in China than those of the soil and bedrock around the glacier. This suggests that the silicate minerals on the glacier were derived from distant deserts. The isotopic ratios in saline, carbonate and phosphate fractions were also close to those of evaporites and apatite in that desert region, suggesting that these minerals were also derived from that source. The Sr isotopic ratios in the organic fraction were closer to ratios in the saline and carbonate fractions rather than the silicate or phosphate fractions and may therefore reflect the isotopic ratios of the elements when they are incorporated into living microbes on the glacier.

Silicate Minerals from Macedonia. Complementary Use of Vibrational Spectroscopy and X-Ray Powder Diffraction for Identification and Detection Purposes

Abstract: Review of the results of complementary use of vibrational spectroscopy (infrared and Raman) and X-ray powder diffraction in the process of detection and identification of the silicate minerals from the Republic of Macedonia is presented.

Partitioning of trace elements between garnet, clinopyroxene and diamond-forming carbonate-silicate melt at 7 GPa

Abstract: Concentrations of trace elements in coexisting garnet, clinopyroxene and completely miscible carbonate-silicate melt (formed at 7 GPa from the Chagatai silicocarbonatite rock known to be diamondiferous) were determined using LA-ICP-MS. The partition coefficients for Li, Rb, Cs, Ba, Th, U, Ta, Nb, La, Ce, Pb, Pr, Sr, Nd, Zr, Hf, Sm, Eu, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb, Lu, Sc and Zn were determined. The new experimental data for trace-element partitioning between garnet, clinopyroxene and carbonate-silicate melt have been compared with published data for partitioning between garnet, clinopyroxene and carbonatite melt, and garnet, clinopyroxene and silicate melt. The results show that the trace-element partitioning is not significantly altered by changes in melt composition, with HREE always concentrated in the garnet. Carbonate-silicate melt, as a diamond-forming medium, and carbonatite or silicate melt equilibrated with mantle silicate minerals, behave similarly in respect of trace-element distribution.

Novel fusion method for direct determination of uranium in ilmenite, rutile, columbite, tantalite, and xenotime minerals by laser induced fluorimetry

Abstract: A simple, rapid, effective and eco-friendly decomposition method is developed for the determination of uranium (U) by laser induced fluorimetry (LIF). The salts of sodium di-hydrogen phosphate (NaH2PO4) and di-sodium hydrogen phosphate (Na2HPO4) were used in the ratio of 1:1 (phosphate flux) for the decomposition and dissolution of refractory, non silicate minerals like ilmenite, rutile, columbite, tantalite, and xenotime. The effect of associated matrix elements (Ti, Fe, Nb, Ta, Mn and Y present in the sample) on quenching of uranyl fluorescence was studied. The flux used for the sample decomposition has several advantages. In the reported sample decomposition methods, α-hydroxy acids are used as complexing agents to prevent hydrolysis and to get clear and stable solution. This solution can not be directly used for U determination by LIF as α-hydroxy acids quench uranyl fluorescence, hence separation is required. In the present method no such separation is required. The flux itself acts as fluorescence enhancing reagent and buffer (maintaining the optimum pH of 7.1 ± 0.1). The fused melt of the flux mixture, when disintegrated in water, gives clear and stable solution and has high tolerance for most of inorganic quenchers compared to reported phosphate buffers. Also just by dilution (due to high sensitivity of LIF), the concentration of quenchers could be brought down well within the tolerance limit. The accuracy and precision of the method was evaluated by analyzing Certified Reference Materials (IGS-33 and IGS-34 of Institute of Geological Sciences, UK) and Synthetic Minerals. The accuracy of the data is further evaluated by comparing with standard decomposition methods. The results are well within the experimental error. The RSD of the method is ±10% (n = 6) at 10 ppm level for Ilmenite and for other minerals the RSD of the method is ±5% (n = 6) at 50 ppm level. The method is being routinely applied to various refractory samples received from Rare Metal and Rare Earth Investigations for determination of uranium by laser fluorimetry.

Applied Mineralogical Studies on Iranian Hard Rock Titanium Deposit

Abstract: The Qara-aghaj hard rock titanium deposit has been located in the 36 Km at the North-West of Euromieh, Iran. Mineralogical studies performed by XRD, XRF, Optical microscopy and SEM studies indicated that ilmenite, magnetite and apatite are main valuable minerals. The gangue minerals consist of the silicate minerals such as pyroxene, olivine, plagioclase and some secondary minerals. Ilmenite in Qara-aghaj ore occurs in three forms: ilmenite grains, exsolved ilmenite lamellae in magnetite and ilmenite particles disseminated in silicate minerals. The grain forms liberated in 150µm are only recoverable by physical methods. The maximum content of TiO2 in ilmenite lattice is determined 48% by EDX. Although the ore has 8.8% average grade of TiO2, the recoverable TiO2 is only about 6.72% the studied sample contained 18.3% ilmenite and the amount of recoverable ilmenite is only about 14% (6.72% TiO2). This is due to the ilmenite exsolutions and inclusions in the magnetite and silicate minerals, and the TiO2 in solid solution in the lattices of these minerals. In fact, about 77% of whole ilmenite content of the ore will be recoverable. EDX analysis showed that Fe is substituted partially in ilmenite by Mn and Mg. Some narrow lamellae of hematite are formed inside ilmenite. By analyzing of magnetite, it was found that the V2O5 content is up to 1%. V 3+ is found in magnetite lattices by replacing Fe 3+ . Analyzing of clinopyroxenes indicated that augite, containing Ti, is the main form of this group. Ilmenite, apatite and magnetite are valuable minerals for production of TiO2, P2O5 and Fe, respectively and the V2O5 can be extracted from magnetite as a by-product.

Authigenic Silicate Minerals – Sepiolite-Palygorskite, Zeolites and Sodium Silicates

Abstract: Publisher Summary This chapter deals with micromorphological features of silicate neoformation in soil environments. Micromorphological techniques have been widely applied to the study of silicate neoformation, both in the field of soil science and for the study of diagenetic processes in sedimentary geology. Their use has greatly contributed to the recognition of silicate mineral authigenesis in earth surface conditions. Micromorphology also provides information about the transformation of authigenic silicates and their inherited counterparts. Electron microscopy has been most commonly used, as a technique that is ideally suited for the study of submicroscopic particles. For palygorskite and sepiolite, this has led to an over-representation in the literature of SEM and TEM images that merely illustrate a fibrous particle morphology rather than textural features with genetic implications. Electron microscopy studies of zeolites and sodium silicates show a highly characteristic and invariable crystal morphology, which makes SEM and TEM important tools for their identification.

Navajo Sandstone-brine-CO 2 interaction: implications for geological carbon sequestration

Abstract: The injection of CO2 into deep saline aquifers is being considered as an option for greenhouse gas miti- gation. However, the response of an aquifer to the injected CO2 is largely unknown. Experiments involving the reac- tion of Navajo Sandstone with acidic brine were conducted at 200C and 25 or 30 MPa to evaluate the extent of fluid- rock interactions. The first experiment examined sandstone interaction with CO2-impregnated brine; the second experiment examined sandstone dissolution in CO2-free acidic brine; the third one is carried out in a mixed-flow reactor and designed to measure sandstone dissolution rates based on time-series Si concentrations. The solution chemistry data indicate that the SiO2(aq) increases gradu- ally and pH increases slowly with reaction progress. Sili- cate minerals in the sandstone display textures (dissolution features, secondary mineralization), indicating that these phases are reacting strongly with the fluid. Dissolution of feldspars and conversion of smectite to illite are likely to be the two reactions that contribute to the release of SiO2(aq). The product minerals present at the end of the experiments are illite, illite/smectite, allophane, and car- bonate minerals (for the CO2-charged system). Dissolved CO2 is likely to acidify the brine and to provide a source of carbon for the precipitation of carbonate minerals. Mineral trapping through the precipitation of carbonate minerals is favored thermodynamically and was observed in the experiments. The chemical reactions likely increase the bulk porosity of the sandstone due to dissolution of silicate minerals. However, allophane and illite/smectite fill voids in sandstone grains. There is no evidence for the removal of clay coatings due to chemical reactions. It is uncertain whether the mechanical forces near an injection well would mobilize the smectite and allophane and clog pore throats. Trace amounts of metals, including Cu, Zn, and Ba, were mobilized.

Pore Size Distribution And Ultrasonic Velocities of Compacted Na-montmorillonite Clays

Abstract: Shale exhibits dual-porosity structure and has more complex pore-structure than that of sandstones and limestones. The predominant gas flow occurs through the interconnected fracture network system and this system is recharged by the gas flowing through the matrix. Shale matrix has predominantly microto meso-pores (pore-size below 50 nm as per IUPAC classification). Another unique aspect of these sediments is the high pore surface area to pore volume ratio. Clays are layered–sheet silicate minerals of extremely small grain size which contributes to the large surface areas. The gas molecules attach themselves to these large surface areas by adsorption.

Surface modification of phyllosilicate minerals by fluorination methods

Abstract: The effect of fluorination on various types of phyllosilicate minerals has been investigated. Two different fluorination techniques have been used: direct F2 gas and cold radio frequency plasma involving c-C4F8 or O2/CF4 mixtures. The modifications of the surface composition and properties have been followed mostly by x-ray photoelectron spectroscopy (XPS). Depending of the fluorination reagents, a reactive etching process involving M-F bonding occurs (direct F2 gas; O2–CF4 rf plasma) or a carbon fluoride deposition takes place (c-C4F8 rf plasma). In the case of F2-gas treated minerals, Si 2p XPS signal accounts for the presence of fluorinated Si environments.