Showing papers on "Silicate minerals published in 2007"

Silicon in the Earth's core.

Abstract: The Earth and Moon have iron isotopic abundances that are slightly 'heavy' compared with Mars, the asteroid Vesta and primitive meteorites. The reason for this has been unclear. One suggestion is that there was a large-scale isotopic equilibration during the 'giant impact', the collision between the early Earth and a Mars-sized body that created the Moon, but this has seemed unlikely given that most other elements do not show the effect. An analysis of 44 meteorite and terrestrial samples now shows that the silicon isotopic compositions of basaltic rocks from the Earth and Moon are also distinctly heavy. The finding is consistent with the isotopic equilibration model, and it suggests that silicon was already partitioned into the Earth's core as a light element before the Moon formed. The silicon isotopic compositions of basaltic rocks from the Earth and Moon are distinctly 'heavy'. The similar isotopic composition of the bulk silicate Earth and the Moon are consistent with large-scale isotopic equilibration during the 'giant impact', indicating that Si was already a light element in the Earth's core before the Moon formed. Small isotopic differences between the silicate minerals in planets may have developed as a result of processes associated with core formation, or from evaporative losses during accretion as the planets were built up. Basalts from the Earth and the Moon do indeed appear to have iron isotopic compositions that are slightly heavy relative to those from Mars, Vesta and primitive undifferentiated meteorites1,2,3,4 (chondrites). Explanations for these differences have included evaporation during the ‘giant impact’ that created the Moon (when a Mars-sized body collided with the young Earth). However, lithium5 and magnesium6, lighter elements with comparable volatility7,8,9, reveal no such differences, rendering evaporation unlikely as an explanation. Here we show that the silicon isotopic compositions of basaltic rocks from the Earth and the Moon are also distinctly heavy. A likely cause is that silicon is one of the light elements in the Earth’s core. We show that both the direction and magnitude of the silicon isotopic effect are in accord with current theory10 based on the stiffness of bonding in metal and silicate. The similar isotopic composition of the bulk silicate Earth and the Moon is consistent with the recent proposal11 that there was large-scale isotopic equilibration during the giant impact. We conclude that Si was already incorporated as a light element in the Earth’s core before the Moon formed.

Experimental evaluation of in situ CO2-water-rock reactions during CO2 injection in basaltic rocks: Implications for geological CO2 sequestration

Abstract: [1] Deep aquifers are potential long-term storage sites for anthropogenic CO2 emissions The retention time and environmental safety of the injected CO2 depend on geologic and physical factors and on the chemical reactions between the CO2, the aquifer water, and the host rocks The pH buffer capacity of the aquifer water and the acid neutralization potential of the host rocks are important factors for the permanent stabilization of the injected CO2 Mafic rocks, such as basalt, which primarily consists of Ca, Mg silicate minerals, have a high acid neutralization capacity by providing alkaline earth elements that form stable carbonate minerals The carbonate minerals formed thus sequester CO2 in a chemically stable and environmentally benign form In this study, we present results from a small-scale CO2 injection test in mafic and metasedimentary rocks The injection test was conducted using a single-well push-pull test strategy CO2 saturated water (pH = 35) was injected into a hydraulically isolated and permeable aquifer interval to study the acid neutralization capacity of Ca, Mg silicate rocks and to estimate in situ cation release rates Release rates for Ca, Mg, and Na were calculated by use of solute compositions of water samples retrieved after the CO2 injection, the incubation time of the injected solution within the aquifer, and geometric estimates of the reactive surface area of the host rocks Our results confirm rapid acid neutralization rates and water-rock reactions sufficient for safe and permanent storage of CO2 Carbonic acid was neutralized within hours of injection into a permeable mafic aquifer by two processes: mixing between the injected solution and the aquifer water, and water-rock reactions Calculated cation release rates decrease with increasing pH that is confirmed by laboratory-based experiments Large differences between release rates obtained from the field and laboratory experiments may be mainly due to uncertainties in the estimation of the reactive surface area in the field experiment and in hydrological and geological factors Our results underscore the importance of defining bulk rock dissolution rates under in situ conditions in order to evaluate target formations for permanent mineral sequestration of carbon dioxide

The glauconite–Fe‐illite–Fe‐smectite problem: a critical review

Abstract: Iron silicate minerals are a significant component of sedimentary systems but their modes of formation remain controversial. Our analysis of published data identifies end-member compositions and mixtures and allows us to recognize controls of formation of different mineral species. The compositional fields of glaucony, Fe-illite, Fe–Al smectites are determined in the M+/4Si vs. Fe/Sum of octahedral cations (M+ = interlayer charge). Solid solutions could exist between these phases. The Fe–Al and Fe-rich clay minerals form two distinct solid solutions. The earliest phases to be formed are Fe–Al smectites or berthierine depending on the sedimentation rate. Reductive microsystems appear in the vicinity of organic debris in unconsolidated sediments. The Fe is incorporated first in pyrite and then in silicates after oxidation. Potassium ions diffuse from the sea-water–sediment interface. If not interrupted, the diffusion process is active until reaction completion is reached, i.e. formation of Fe-illite or glauconite or a mineral assemblage (berthierine–nontronite) according to the available Al ion amounts in the microsystem. Mixed-layer minerals are formed when the diffusion process is interrupted because of sedimentation, compaction or cementation. Despite the common belief of their value as palaeoenvironment indicators, these minerals can form in a variety of environments and over a period of millions of years during sediment burial.

Volatile (S, Cl and F) and fluid mobile trace element compositions in melt inclusions: implications for variable fluid sources across the Kamchatka arc

Abstract: Volatile element, major and trace element compositions were measured in glass inclusions in olivine from samples across the Kamchatka arc. Glasses were analyzed in reheated melt inclusions by electron microprobe for major elements, S and Cl, trace elements and F were determined by SIMS. Volatile element–trace element ratios correlated with fluid-mobile elements (B, Li) suggesting successive changes and three distinct fluid compositions with increasing slab depth. The Eastern Volcanic arc Front (EVF) was dominated by fluid highly enriched in B, Cl and chalcophile elements and also LILE (U, Th, Ba, Pb), F, S and LREE (La, Ce). This arc-front fluid contributed less to magmas from the central volcanic zone and was not involved in back arc magmatism. The Central Kamchatka Depression (CKD) was dominated by a second fluid enriched in S and U, showing the highest S/K2O and U/Th ratios. Additionally this fluid was unusually enriched in 87Sr and 18O. In the back arc Sredinny Ridge (SR) a third fluid was observed, highly enriched in F, Li, and Be as well as LILE and LREE. We argue from the decoupling of B and Li that dehydration of different water-rich minerals at different depths explains the presence of different fluids across the Kamchatka arc. In the arc front, fluids were derived from amphibole and serpentine dehydration and probably were water-rich, low in silica and high in B, LILE, sulfur and chlorine. Large amounts of water produced high degrees of melting below the EVF and CKD. Fluids below the CKD were released at a depth between 100 and 200 km due to dehydration of lawsonite and phengite and probably were poorer in water and richer in silica. Fluids released at high pressure conditions below the back arc (SR) probably were much denser and dissolved significant amounts of silicate minerals, and potentially carried high amounts of LILE and HFSE.

Metasomatism of sub-arc mantle peridotites below southernmost South America: reduction of fO2 by slab-melt

Abstract: Quaternary basalts in the Cerro del Fraile area contain two types of mantle xenoliths; coarse-grained (2–5 mm) C-type spinel harzburgites and lherzolites, and fine-grained (0.5–2 mm) intensely metasomatized F-type spinel lherzolites. C-type xenoliths have high Mg in olivine (Fo = 90–91) and a range in Cr# [Cr/ (Cr + Al) = 0.17–0.34] in spinel. Two C-type samples contain websterite veinlets and solidified patches of melt that is now composed of minute quenched grains of plagioclase + Cr-spinel + clinopyroxene + olivine. These patches of quenched melts are formed by decompression melting of pargasitic amphibole. High Ti contents and common occurrence of relic Cr-spinel in the quenched melts indicate that the amphibole is formed from spinel by interaction with the Ti-rich parental magma of the websterite veinlets. The fO2 values of these two C-type xenoliths range from ΔFMQ −0.2 to −0.4, which is consistent with their metasomatism by an asthenospheric mantle-derived melt. The rest of the C-type samples are free of “melt,” but show cryptic metasomatism by slab-derived aqueous fluids, which produced high concentrations of fluid-mobile elements in clinopyroxenes, and higher fO2 ranging from ΔFMQ +0.1 to +0.3. F-type lherzolites are intensely metasomatized to form spinel with low Cr# (∼0.13) and silicate minerals with low MgO, olivine (Fo = ∼84), orthpyroxene [Mg# = Mg/(Mg + ΣFe) = ∼0.86] and clinopyroxene (Mg# = ∼0.88). Patches of “melt” are common in all F-type samples and their compositions are similar to pargasitic amphibole with low TiO2 (<0.56 wt%), Cr2O3 (<0.55 wt%) and MgO (<16.3 wt%). Low Mg# values of silicate minerals, including the amphibole, suggest that the metasomatic agent is most likely a slab melt. This is supported by high ratios of Sr/Y and light rare earth elements (REE)/heavy REE in clinopyroxenes. F-type xenoliths show relatively low fO2 (ΔFMQ −0.9 to −1.1) compared to C-type xenoliths and this is explained by the fusion of organic-rich sediments overlying the slab during the slab melt. Trench-fill sediments in the area are high in organic matter. The fusion of such wet sediments likely produced CH4-rich fluids and reduced melts that mixed with the slab melt. High U and Th in bulk rocks and clinopyroxene in F-type xenoliths support the proposed interpretation.

Forsterite Dissolution Kinetics: Applications and Implications for Chemical Weathering

Abstract: Silicate minerals are the most common mineral group in the earth’s crust so it is not surprising that their weathering reactions dominate the chemistry of many earth surface processes. This project used forsterite as a model system to identify the important factors that affect silicate mineral dissolution rates and grain lifetimes in the weathering environment. I determined an empirical rate law for forsterite dissolution of forsterite in oxalic

Fluid evolution and kinetics of metamorphic reactions in calc-silicate contact aureoles—From H2O to CO2 and back

Abstract: Mineral assemblages in contact-metamorphic aureoles are the products of the interplay between heat transfer and fluid flow induced by intrusion of magma. In wall rocks containing carbonate and silicate minerals, metamorphic reactions produce CO 2 , which then becomes part of the hydrodynamic system. Although observed assemblages are the ultimate products of T-X CO 2 fluid -t paths in aureole rocks, the complexities of the paths, and hence the evolution of a hydrodynamic system, are difficult to decipher from them. Numerical simulations were conducted to model the mineralogical evolution in a hydrodynamic system and to evaluate the extent and effects of reaction retardation. Simulations reveal that fluid composition in the inner aureole evolves rapidly toward high X CO 2 fluid as the rocks heat up before an appreciable amount of water is exsolved out of the pluton. After local fluid pressure drops when early reactions are coming to completion, infiltration of magmatic water becomes significant and can drive production of typical inner aureole minerals such as wollastonite. Fluid compositions in the outer aureole reflect largely (1) the initial CO 2 production, as fluids are driven down the pressure gradients from the inner aureole, and (2) the subsequent infiltration of magmatic H 2 O. Simulations also suggest temperature overstepping of the onset of reactions and retarded consumption of reactant minerals, which leads to coeval metastable reactions. However, the final simulated mineral assemblage in the inner aureole reflects equilibration with H 2 O-rich fluids that is usually seen in the field, although evidence for kinetic retardation may be preserved in some rocks, especially in the outer aureole.

Effect of quaternary ammonium salts on flotation behavior of aluminosilicate minerals

Abstract: The electrokinetic properties and flotation of diaspore, kaolinite, pyrophyllite and illite with quaternary ammonium salts collectors were studied. The results of flotation tests show that the collecting ability of quaternary ammonium salts for the four minerals is in the order (from strong to weak) of octadecyl dimethyl benzyl ammonium chloride(ODBA), cetyl trimethyl ammonium bromide(CTAB), dodecyl trimethyl ammonium chloride(DTAC). Under the condition of alkalescence, it is possible to separate the diaspore from the silicate minerals such as kaolinite, illite and pyrophyllite using quaternary ammonium salts as collector. Isoelectric points (IEP) of diaspore, kaolinite, pyrophyllite and illite are pH=6.0, 3.4, 2.3 and 3.2, respectively. Quaternary ammonium salts can change ζ-potential of the aluminosilicate minerals obviously. The flotation mechanisms were explained by ζ-potential and Fourier transform infrared spectrum (FT-IR) measurements. The results demonstrate that only electrostatic interaction takes place between aluminosilicate minerals (diaspore, kaolinite, pyrophyllite and illite) and quaternary ammonium salts.

A Study on Variation in Ionic Composition of Aqueous System in Different Lithounits around Perambalur Region, Tamil Nadu

Abstract: Hydrogeochemistry of a region is a reflection of the hydrodynamic processes, Lithological composition and physical constraints. Case study has been carried out in a varied lithologica] terrain with Archaean, Gondwana and Cretaceous rocks to unravel the hydrogeochemical process. The study area is around Perambalur region lying between Latitudes 11°O8OO" - ll°31OO" and Longitudes 78°35'00" - 78°5950" P co 2 values of water samples show wide variation in lithology, reflecting higher saturation index in carbonate minerals Mixing of water was witnessed in the Cretaceous formation Release of bicarbonate ions into the system is mainly derived from weathering of carbonate minerals reflecting lesser significance of silicate weathering. Thermodynamic equilibrium plot shows grouping of samples in the Kaolinite field with seasonal variation Saturation index (SI) for carbonate minerals is higher than silicate minerals Dilutions of samples are noted in the Archaean formation during Post-Monsoon season and migration of SI plume of carbonate minerals is towards southeast during Post-Monsoon season Geochemical reactions determining the water chemistry of the study area are also obtained by statistical analysis.

K2[(UO2)As2O7] – the First Uranium Polyarsenate

Abstract: Yellowish crystals of K-2[(UO2)AS(2)O(7)] (t) have been synthesized by solid-state reactions method. The structure of 1 [orthorhombic, Pmtnn, a = 12.601(2), b = 13.242(2), c = 5.621 (1) angstrom, V = 937.9(3)angstrom(3) Z = 4] has been solved by direct methods and refined to R-1 = 0.049, wR(2) = 0.1060 for 1059 observed reflections. The structure of I is based upon [(UO2)AS(2)O(7)](2-) sheets formed by corner sharing between [UO6](6-) distorted octahedra and [AS(2)O(7)](4-) polyarsenate groups. The K+ cations are either in eightfold or tenfold coordination and are located between the sheets. The topology of the uranyl arsenate sheet is related to silicate minerals of the melilite group and related synthetic silicate, aluminate and germanate compounds.

Investigation of lichens using molecular techniques and associated mineral accumulations on a basaltic flow in a Mediterranean environment

Abstract: The role of lichens in the breakdown of rocks in various environments is well documented. We investigated the formation of secondary minerals under 13 different fungal species growing on a basaltic flow in Sanliurfa (Turkey) to understand the influence of lichen species on the transformation of minerals in a Mediterranean environment. We used molecular technique (rDNA sequence) to identify 13 different species of lichens (7 crustose, 5 foliose and 1 pathogenic). X-ray diffraction and scanning electron microscopy were used to determine the composition of mineral accumulations. The formation of quartz and 2:1 phyllosilicates in various layers (top, brown and white) of the weathered basaltic flows under all the lichen colonies may be the result of precipitated silica alone (quartz) or in combination with aluminum (2:1 clays) released as a by-product during the breakdown/weathering of primary silicate minerals present in the basalt. However, aeolian deposition may also be a possible source of these mineral species. Whewellite, a calcium oxalate mineral, accumulates in the weathered basalt underneath all the species of lichens. We believe that the formation of whewellite was due to organic acids excreted by fungal hyphae to dissolve primary minerals (e.g., olivine and feldspars); this lichen-mediated process released enough calcium and generated oxalate necessary for the formation of whewellite.

Flotation Separation on Rare Earth Minerals and Gangues

Abstract: The flotation process of native rare earth minerals such as bastnasite, monazite, mixed minerals of bastnasite and monazite, using the new effective collector Dh was studied, respectively, and the flotation properties were described. The good qualities of the new collector Dh were revealed through comparing with other collector of rare earth minerals. The test results of different ore samples showed that at moderate pulp pH (8.5～9.5), rare earth minerals could be effectively separated from barium, calcium and silicon bearing intergrowth minerals (barite, calcite and silicate minerals) and high quality rare earth concentrates could be obtained successfully by the new collector Dh, acid silica gel, turpentine and reagents fitting together rationally. In order to determine optimum technical conditions, the effect of pulp pH, pulp temperature, pulp density and the effect of dosage of reagents (Dh and acid silica gel) on the flotation were investigated in the test. Simultaneously, the mechanism of the flotation of rare earth minerals from intergrowth minerals was explored. The infrared spectra for Dh and rare earth cation by analysis in theory showed that Dh formed chelate complex with rare earth cation and were adsorbed on the surfaces of rare earth minerals. The mechanism of the intergrowth minerals depressed by acid silica gel can be explained as gummy colloid hydrolyzed from acid silica gel which were selectively absorbed on the gangue minerals, making them hydrophilic and depressed, with pulp pH value of alkalescent.

Nutrients enhancing the bacterial iron dissolution in the processing of feldspar raw materials

Abstract: Naturally occurring iron oxides are often coated on silicate grains or are impregnated in the matrix in silicates of industrial importance. These Fe-rich impurities can be removed from industrial minerals such as granite through bioleaching. Heterotrophic bioleaching may substantially reduce the need of aggressive chemical bleaches treat industrial silicate minerals. This process involves a siliceous matrix, which is why silicate heterotrophic bacteria of the genus Bacillus are of potential use. These organisms are noted for their ability to reduce ferric iron coupled with dissolution. Solution phase assays were used to monitor iron reduction activity by a Bacillus sp. during the bioleaching of natural feldspar raw materials under various experimental conditions. The rate of reductive iron dissolution was dependent on the presence of yeast extract, nitrate and sulphate in the medium. Quinone stimulated the Fe(III) reduction in feldspars raw materials in cell suspensions. The dissolution of iron was enhanced in the presence of technical-grade sucrose and molasses. The amendment of the medium with river water and indigenous bacteria increased the bacterial reduction of iron. The Fe content in granite samples treated by bioleaching decreased by about 60 %. The process needs controlled conditions for the bacterial iron reduction and is dependent on the mineralogical composition of non-metallic ores.

Magnesium-rich minerals in sediment and suspended particulates of South Florida water bodies : Implications for turbidity

Abstract: Fine sediments in shallow water bodies such as Lake Okeechobee are prone to resuspension. Predominantly inorganic "mud" sediment that covers approximately 670 km2 of the lake has been recognized as a persistent source of turbidity. The objective of this study was to determine if mineral components of sediments in Lake Okeechobee and water conveyances of the northern Everglades also occur as suspended sediment and hence constitute a potential abiotic contributor to turbidity. Sediment samples were collected from nine stations within the lake and eight locations north of Water Conservation Area 2A in the Everglades. Water samples were also collected at selected locations. The silt and clay mineralogy of sediment and suspended particles was determined using X-ray diffraction, thermogravimetry, scanning-electron microscopy, energy-dispersive X-ray elemental microanalysis, and high-resolution transmission-electron microscopy. Clay fractions of the lake sediment contained the Mg silicate minerals sepiolite and palygorskite, along with smectite, dolomite, calcite, and kaolinite. Sediment silt fractions were dominated by carbonates and/or quartz, with smaller amounts of Ca phosphates and sepiolite. Mineralogy of the mud sediment was similar to that reported for geologic phosphate deposits. This suggests that the mud sediment might have accumulated by stream transport of minerals from these deposits. Suspended solids and mud-sediment mineralogy were similar, except that smectite was more abundant in suspended solids. Everglade samples also contained Mg-rich minerals. The small size, low density, and fibrous or platy nature of the prevalent mud sediment minerals make them an abiotic, hydrodynamically sensitive source of persistent turbidity in a shallow lake. Mitigation efforts focused exclusively on P-induced biogeochemical processes do not address the origin or effects of these minerals. Ecological management issues such as turbidity control, P retention, geologic P input, and suitability of dredging are related to mud-sediment properties and provenance.

Influences of phosphates on dispersion of fine alumin-silicate minerals

Abstract: Influences of phosphates on the dispersion of diaspore,kaolinite,illite and pyrophyllite were systematically investigated by sedimentation experiments.The dispersion mechanisms of sodium hexametaphosphate on the four aluminum-silicate minerals were confirmed by measurements of zeta potentials and theoretical calculations of DLVO.The results show that the dispersity order of phosphates from strong to weak is as follows: sodium hexametaphosphate,sodium pyrophosphate,trisodium phosphate.The absolute value of zeta potentials of aluminum-silicate minerals and the electrostatic repulsion among mineral particles increase with addition of sodium hexametaphosphate.The space steric effects among mineral particles are also intensified due to the adsorption of sodium hexametaphosphate on the surface of aluminum-silicate minerals,then the steric repulsion among particles is enhanced.

Kinetic experiments of water rock interactions at high temperatures and high pressures corresponding to the middle crust conditions.

Abstract: In order to simulate the water-rock interactions in the middle crust,the dissolution kinetic experiments of minerals (albite,actinolite,diopside)and basalt were carried out by using packed bed reactor,in the temperature range of 25 to 400℃and at 22MPa.Experiments found that dissolution rates behave a fluctuation in dissolution rates,in the case of crossing the critical state of water,i.e.over the temperature range of 300 to 400℃.When a multi-oxide silicate mineral dissolved in water,different dissolved aqueous metals behave different release rates.Experiments found that the maximum dissolution rates of silicate minerals always occurred at 300℃,such as the maximum release rate of Si was at 300℃.As temperature less than 300℃,the release rates of Na,K, Mg,Ca,Fe,Al were higher than Si.Oppositely,the release rates of Si of silicate minerals were higher than those of other metals while temperature above 300℃.Authors also performed dissolution kinetic experiments of basalts and granodiorite in water in the temperature range of 25 to 400℃and 23MPa.The maximum release rates of Si in those experiments are also found at 300℃.The fluids distributed in the middle crust present in a sub-critical state to the critical state region,behaving a strong variation of water properties. The variations of water properties in the middle crust will induce the fluctuation in kinetics of water-rock interaction.Thus,the variations of water properties and the kinetic behavior of water-rock interactions affect the features in the middle crust,such as strong release of Silica,breaking of silicate framework of minerals,and following rock layer collapse.