Showing papers on "Silicate minerals published in 2013"

The oxidation state of the mantle and the extraction of carbon from Earth’s interior

Abstract: Determining the oxygen fugacity of Earth's silicate mantle is of prime importance because it affects the speciation and mobility of volatile elements in the interior and has controlled the character of degassing species from the Earth since the planet's formation. Oxygen fugacities recorded by garnet-bearing peridotite xenoliths from Archaean lithosphere are of particular interest, because they provide constraints on the nature of volatile-bearing metasomatic fluids and melts active in the oldest mantle samples, including those in which diamonds are found. Here we report the results of experiments to test garnet oxythermobarometry equilibria under high-pressure conditions relevant to the deepest mantle xenoliths. We present a formulation for the most successful equilibrium and use it to determine an accurate picture of the oxygen fugacity through cratonic lithosphere. The oxygen fugacity of the deepest rocks is found to be at least one order of magnitude more oxidized than previously estimated. At depths where diamonds can form, the oxygen fugacity is not compatible with the stability of either carbonate- or methane-rich liquid but is instead compatible with a metasomatic liquid poor in carbonate and dominated by either water or silicate melt. The equilibrium also indicates that the relative oxygen fugacity of garnet-bearing rocks will increase with decreasing depth during adiabatic decompression. This implies that carbon in the asthenospheric mantle will be hosted as graphite or diamond but will be oxidized to produce carbonate melt through the reduction of Fe(3+) in silicate minerals during upwelling. The depth of carbonate melt formation will depend on the ratio of Fe(3+) to total iron in the bulk rock. This 'redox melting' relationship has important implications for the onset of geophysically detectable incipient melting and for the extraction of carbon dioxide from the mantle through decompressive melting.

Nominally Anhydrous Minerals and Earth's Deep Water Cycle

Abstract: Deep reservoirs of water incorporated as hydroxyl into solid silicate minerals of the Earth's interior may contain the majority of the planet's hydrogen and have acted as buffers to maintain ocean volume and continental freeboard over geologic time Two tenths of one weight percent H 2 O in subducted oceanic crustal material and subsequently released to the hydrosphere from mid-ocean ridge basalt is sufficient to recycle the total ocean volume once over 45 billion years It is possible that actual fluxes are several times this amount The nominally anhydrous minerals of the transition zone (410-660 km depth) may serve as a large internal reservoir New and recent data on molar volumes and elastic properties indicate that hydration has a larger effect on shear velocities than does temperature within their respective uncertainties Based on these new data, seismic velocities ill this region are consistent with significant hydration (one-half percent or more H 2 O by weight in a pyrolite-composition mantle) The data indicate that lateral velocity variations in the Transition Zone (TZ) may reflect variations in hydration rather than variations in temperature, at least in regions distant from subduction zones

Iron silicate microgranules as precursor sediments to 2.5-billion-year-old banded iron formations

Abstract: Banded iron formations (BIFs) are chemical sedimentary rocks comprising alternating layers of iron-rich and silica-rich minerals that have been used to infer the composition of the early Precambrian ocean and ancient microbial processes. However, the identity of the original sediments and their formation is a contentious issue due to postdepositional overprinting and the absence of modern analogues. Petrographic examination of the ca. 2.5 Ga Dales Gorge Member of the Brockman Iron Formation (Hamersley Group), Western Australia, reveals the presence of abundant silt-sized microgranules composed of stilpnomelane. The microgranules are most common in the least-altered BIF where they define sedimentary laminations, implying a depositional origin. We suggest that the precursor mineral was an iron-rich silicate that formed either in the water column or on the seafloor. The microgranular texture may have developed due to clumping of amorphous mud, forming silt-sized floccules. The microgranules were resedimented by dilute density currents and deposited in lamina sets comprising a basal microgranular-rich lamina overlain by amorphous mud with dispersed microgranules. The lamina sets collectively define plane-lamination structure, probably of the lower flow regime. The microgranular textures are preserved only where early diagenetic silica prevented the compaction of lamina sets. Episodic resedimentation of iron silicates alternating with periods of nondeposition and seafloor silicification provides an explanation for some of the characteristic banding in BIF. We propose that for most of the early Precambrian, the persistence of ferruginous oceans with elevated silica concentrations favored the widespread growth of iron silicate minerals, which in environments starved of continental sediments formed extensive deposits of the precursor sediment to iron formation.

Direct electrolytic dissolution of silicate minerals for air CO2 mitigation and carbon-negative H2 production

Abstract: We experimentally demonstrate the direct coupling of silicate mineral dissolution with saline water electrolysis and H2 production to effect significant air CO2 absorption, chemical conversion, and storage in solution. In particular, we observed as much as a 10(5)-fold increase in OH(-) concentration (pH increase of up to 5.3 units) relative to experimental controls following the electrolysis of 0.25 M Na2SO4 solutions when the anode was encased in powdered silicate mineral, either wollastonite or an ultramafic mineral. After electrolysis, full equilibration of the alkalized solution with air led to a significant pH reduction and as much as a 45-fold increase in dissolved inorganic carbon concentration. This demonstrated significant spontaneous air CO2 capture, chemical conversion, and storage as a bicarbonate, predominantly as NaHCO3. The excess OH(-) initially formed in these experiments apparently resulted via neutralization of the anolyte acid, H2SO4, by reaction with the base mineral silicate at the anode, producing mineral sulfate and silica. This allowed the NaOH, normally generated at the cathode, to go unneutralized and to accumulate in the bulk electrolyte, ultimately reacting with atmospheric CO2 to form dissolved bicarbonate. Using nongrid or nonpeak renewable electricity, optimized systems at large scale might allow relatively high-capacity, energy-efficient (<300 kJ/mol of CO2 captured), and inexpensive (<$100 per tonne of CO2 mitigated) removal of excess air CO2 with production of carbon-negative H2. Furthermore, when added to the ocean, the produced hydroxide and/or (bi)carbonate could be useful in reducing sea-to-air CO2 emissions and in neutralizing or offsetting the effects of ongoing ocean acidification.

Sequestration of Martian CO2 by mineral carbonation

Abstract: Carbonation is the water-mediated replacement of silicate minerals, such as olivine, by carbonate, and is commonplace in the Earth’s crust. This reaction can remove significant quantities of CO2 from the atmosphere and store it over geological timescales. Here we present the first direct evidence for CO2 sequestration and storage on Mars by mineral carbonation. Electron beam imaging and analysis show that olivine and a plagioclase feldspar-rich mesostasis in the Lafayette meteorite have been replaced by carbonate. The susceptibility of olivine to replacement was enhanced by the presence of smectite veins along which CO2-rich fluids gained access to grain interiors. Lafayette was partially carbonated during the Amazonian, when liquid water was available intermittently and atmospheric CO2 concentrations were close to their present-day values. Earlier in Mars’ history, when the planet had a much thicker atmosphere and an active hydrosphere, carbonation is likely to have been an effective mechanism for sequestration of CO2.

Reduction of gold(III) chloride to gold(0) on silicate surfaces.

Abstract: The mechanism of adsorption and reduction of the gold chloride complex on silicate minerals is investigated. Gold chloride, supplied as HAuCl(4) solution, is rapidly adsorbed on the silicate surfaces, the Au(III) is reduced to metallic gold, and gold particles grow on the surface. SEM images show agglomerates of gold unevenly distributed on the surface of the silicates, including in some areas forming agglomerates, especially on quartz and feldspar. Silica gel forms via dissolution of silicates in acidic conditions and also has strong adsorption/reduction potential for gold. A mechanism for the adsorption and reduction is proposed, involving ligand substitution between gold chloride and OH() groups on defect sites in silicate surfaces. Consequently, gold can be reduced by hydrogen or silicon radicals at the defect sites. Adsorption of Au(III) by silicate minerals, followed by reduction, could play an important role in the deposition of gold in natural systems, as well as causing loss of gold from leaching processes during hydrometallurgical gold recovery.

Geochemical evolution and quality assessment of water resources in the Sarcheshmeh copper mine area (Iran) using multivariate statistical techniques

Abstract: Hydrochemical data were gathered throughout the last 12 years from 57 sampling stations in the drainage basin of the Sarcheshmeh copper mine, Kerman Province, Iran. The mean values of these data for each sampling station were used to evaluate water quality and to determine processes that control water chemistry. Principal component analyses specified the oxidation of sulfide minerals, dissolution of carbonate and sulfate minerals and weathering of silicate minerals as the principal processes responsible for the chemical composition of water in the study area. Q-mode cluster analysis revealed three main water groups. The first group had a Ca-HCO3–SO4 composition whereas the second and third groups had Ca–SO4 and Ca–Mg–SO4 composition, respectively. The results of this study clearly indicated the role of sulfide minerals oxidation and the buffering processes in the geochemical evolution of water in the Sarcheshmeh area. Due to these processes, extensive changes occurred in the chemical composition of water by passage through the mining area or waste and low-grade dumps, so that the fresh water of the peripheral area of the pit evolved to an acid water rich in sulfate and heavy metals at the outlet of the pit and in the seepages of waste and low-grade dumps.

Melting of carbonate wall rocks and formation of the heterogeneous aureole of the Panzhihua intrusion, China

Abstract: The Panzhihua gabbroic intrusion, part of the plumbing system of the Emeishan large igneous province, intruded late-Proterozoic dolomites and marls about ∼263 Ma ago. The dolomites in the contact aureole were converted to brucite marbles and a diverse suite of forsterite, diopside and garnet skarns. The variation in mineralogy is explained in part by differences in the composition of the protolith, particularly the proportion of silica minerals and clay, and in part by transfer of elements from intruding magmas. The trace element compositions of most marbles and skarns are very similar to those of unmetamorphosed dolomites and marls, but some contain high Si, Ti, and Fe contents that are interpreted to have come from a magmatic source. Three brucite marbles sampled ∼10 m from the contact of the intrusion and named “enriched brucite marble” have trace element compositions very different from their dolomitic protolith: their rare earth elements are strongly enriched whereas levels of Nb-Ta, Zr-Hf and Ti are very low. These characteristics resemble those of carbonate liquid in equilibrium with silicate liquid or more probably with silicate minerals in the case of Panzhihua, a similarity we take to indicate that the sample underwent partial melting. Samples taken up to 300 m from the contact contain brucite indicating that high temperatures persisted well into the country rocks. However, other samples collected only tens of metres from the contact are only slightly recrystallized indicating that conditions in the aureole were highly variable. We suggest that temperatures within the aureole were controlled by conduction of heat from the main intrusion and by supply of additional heat from abundant small dykes within the aureole. Circulation of fluids derived from deeper levels in the aureole flushed the carbon dioxide from the dolomite, lowering temperature needed to partially melt carbonate to the temperatures attained near the intrusion. Irregular but extensive heating destabilized the carbonates of the aureole and decarbonation reactions associated with carbonate breakdown and melting emitted a large volume of CO2, with potential impact on global climate.

Experimental study on the effects of temperature cycling on coarsening of plagioclase and olivine in an alkali basalt

Abstract: We conducted experiments on an alkali basalt at 1-atmosphere in order to investigate the effects of temperature cycling on crystal coarsening. Experiments at 1,150 °C near the Ni–NiO buffer indicate that coarsening of plagioclase and olivine crystals is greatly accentuated by temperature cycling. For a given experiment duration, crystal number density decreases with temperature cycle amplitude and average crystal size increases with increasing amplitude. We observed little correlation between cycle period and crystal number density or average crystal size. We suggest that dissolution and size-proportional crystal growth during repeated heating and cooling decrease crystal number density and increase average crystal size. These experiments indicate that the texture of silicate minerals can be modified by temperature cycling and that phenocrysts may develop quicker in silicate magmas when the magma temperature is cycled.

Coupling Mineral Carbonation and Ocean Liming

Abstract: The process by which basic/ultrabasic silicate minerals (e.g., olivine) are reacted with CO2 to produce solid carbonate minerals (“mineral carbonation”) has been suggested as a method to sequester carbon dioxide from point sources into stable carbonate minerals. Alternatively, the addition of lime (produced from calcining carbonate minerals) to the surface ocean (“ocean liming”), which results in an increase in ocean pH and a draw-down of atmospheric CO2 has been proposed as a “geoengineering” technology, which stores carbon as dissolved alkalinity in the surface ocean. Combining these approaches, in which the magnesium carbonate minerals produced from mineral carbonation are used as a feedstock for ocean liming (mineral carbonation-ocean liming; MC–OL), may reduce the limitations of individual technologies while maximizing the benefits. Approximately 1.9 metric tons of magnesium silicate (producing 0.7 ton of magnesium oxide) are required for every net ton of CO2 sequestered. A total of 0.7 ton of CO2 is...

A closer look at supercritical water

Abstract: Water, the fluid of life at ambient pressure (P) and temperature (T), is mostly present under supercritical conditions in the Earth’s crust and mantle (1): that is, above the vapor-liquid critical point (647 K and 221 MPa). As a free fluid or dissolved in silicate minerals, supercritical water greatly influences the structure and dynamics of our planet. From a technological standpoint, there are several, new emerging applications for supercritical water as a green solvent (2), including the catalytic conversion of biomass into fuels and the oxidation of hazardous materials.

Multivariate statistical analyzing of chemical parameters of groundwater in Vojvodina

Abstract: The geothermal waters of the Province of Vojvodina in northern Serbia, south part of the Pannonian Basin, are characterized by temperatures between 24 and 75 °C. Dominant ions in these waters are sodium and bicarbonate, but the waters also contain significant amounts of calcium, magnesium, potassium, chlorides, and ammonium. Components that are of balneological significance include iodine, bromine, fluorine, strontium, lithium and barium, metasilicic and metaboric acids. Reported are the physicochemical parameters of this groundwater drilled from 200 to 1200 m depth in Vojvodina's territory. Hydrogeochemical studies were carried out in this area with the objective of identifying the geochemical processes and their relation to groundwater quality. Weathering of silicate minerals controls the concentration of major ions such as sodium, calcium, magnesium and potassium in the groundwater of this area. The reverse ion exchange process controls the concentration of calcium, magnesium and sodium in hard rock formations, and dissolution of carbonate minerals and accessory minerals is the source of Ca and Mg, in addition to cation exchange in the sedimentary formations. In general, the chemical composition of the groundwater in this area is influenced by rock–water interaction, dissolution and deposition of carbonate and silicate minerals and ion exchange. An attempt has been made to study quality of groundwater using multivariate statistical technique such as cluster analyses. Hydrogeochemical data for 13 groundwater samples were subjected to correlation and R- and Q-mode cluster analysis, where R-mode analysis reveals the inter-relations among the variables studied, and Q-mode analysis reveals the inter-relations among the samples studied.

Solubilization of Magnesium-Bearing Silicate Minerals and the Subsequent Formation of Glushinskite by Aspergillus niger

Abstract: Microbes may play a substantial role in the weathering and alteration of minerals. However, not enough concerns have been realized about the complexity of microbe-mineral interactions. The present work reports the interactions between fungi and minerals with emphasis on the role of silicate minerals as the metal donor for the precipitation of secondary mineral. Herein, two magnesium-bearing silicate minerals with different structures, forsterite and talc, were added to the submerged cultures of Aspergillus niger (A. niger). It is shown that forsterite exhibits a better solubilization effect than talc, and the secondary mineral glushinskite only precipitates in the presence of forsterite substrates. Oxalic acid excreted by A. niger plays a key role in the biological leaching and mineralization processes. Moreover, the forsterite particles with certain size added to the cultures tend to inhibit the aggregation of fungal biomass, and therefore affect the morphology of A. niger aggregates in the submerged cul...

The first report of platinum-group minerals in magnetite-bearing gabbro, freetown layered complex, Sierra Leone: Occurrences and genesis

Abstract: Platinum-group minerals (PGM) have been located in magnetite-bearing gabbroic rocks of the Freetown Layered Complex, Sierra Leone for the first time. The PGM occurrences described here are from four stratiform layers characterized by a local, more primitive, composition of the associated silicate minerals and elevated platinum-group elements (PGE) and base metal assays. The uppermost layer (Horizon B) is the best exposed; the three lower layers are currently known only from single outcrops. Platinum-group minerals are irregularly distributed through these layers. Altered PGE arsenides and antimonides, possibly derived from sperrylite (PtAs2) and “mertieite” [Pd8(Sb,As)3 to Pd11(Sb,As)4], occur in the lowest known PGE-enriched layer (Horizon M). Cooperite (PtS) and Pt-Fe alloys were observed in a higher layer (Horizon D). Pd-Cu alloys including nielsenite (PdCu3) are unique to the next stratigraphically higher PGM-bearing layer (Horizon C). The uppermost layer (Horizon B) contains the most PGM. Pt-Fe alloy (usually Pt3Fe) is by far the most abundant PGM and cooperite is a significant component of the assemblage. Bowieite [(Pt,Ir,Rh)2S3], tulameenite (Pt2CuFe), and laurite (RuS2) have also been found in this horizon. Cooperite occurs in a late magmatic, high Ca-amphibole (cannilloite) which has formed interstitially to plagioclase and between plagioclase and olivine at a depth of about 20 km. The cooperite is often attached to rounded magnetite grains within the amphibole. Where cooperite is located at the edge of the amphibole or in interstitial locations it has been altered to Pt-Fe alloy. Platinum-group minerals sometimes occur near the margin of sulfides, mainly chalcopyrite and minor pentlandite. The entire PGM assemblage is found mainly interstitial to olivine, pyroxene, and plagioclase, often at the edge of amphibole or in interstitial sites associated with amphibole, chlorite, or quartz. Many of the PGM grains are less than 1 micron in size, with a few in the 2–4 micron range; the largest example is a grain of Pt-Fe alloy that measures 9 by 6 microns. The PGM described here from the rocks are three orders of magnitude smaller than the eluvial and alluvial PGM of the Freetown complex, which are predominantly Pt-Fe alloys, laurite, erlichmanite (OsS2), and Os-Ir alloys. The large eluvial and alluvial PGM might be derived from a horizon containing coarse-grained mineralization, but no vestige of such a horizon has yet been discovered. Previous studies of this area have suggested that secondary remobilization of the PGE has occurred, leading to the growth of large secondary PGM. The primary PGE-mineralization associated with magnetite and Cu-rich sulfide mineralization maybe due to late sulfur saturation in the magma after Ni has been removed by being incorporated into earlier olivine. The presence of digenite, native copper, and nielsenite indicates a low sulfur fugacity, possibly due to oxidation. The four distinctive PGM-bearing horizons are the result of separate injections of magma. The silicate compositions, the minor element content, the PGM mineral assemblage, and the Cu/Pd ratios of the rocks of the PGM-bearing horizons demonstrate that these four magma pulses are compositionally distinct.

Effect of Sulfate Evaporate Salt Layer over the Formation of Skarn-Type Iron Ores

Abstract: The skarn type iron deposit is one of the most important types of rich iron deposits,accounting for about 60% of total rich iron ore reserves in China.The ore-controlling mechanism of sulfate evaporate salt layers is unclear,however,the relationship between layers and skarn iron deposits have been attracting widespread attention of geoscientists recently.In this paper,the effect of sulfate evaporate salt layers on the skarn type iron mineralization has been discussed,through the case studies of Daye type and Handan type deposits which are the important iron types in China.The sulfate evaporate salt layers are rich in carbonates,gypsum and halite,and they can provide a large number of agents of mineralization for the sodium alteration,scapolitization,skarn alteration,and the Fe~(2+) transport as the complex(e.g.Na-Fe-Cl),like Na~+,Cl~-,CO_3~(2+) and so on.On the other hand,the sulfate evaporate salt layer is the most important oxidation barrier in the depth crust,which could oxidize the Fe~(2+) into Fe~(3+) in the silicate magma and hydrothermal solution and enrich the iron to be the iron deposit.It is a key factor of the ore-forming of the skarn type iron deposit.In Daye district,the salt layers belong to the Jialingjiang Group in the lower part of Middle Triassic,and in Handan-Xingtai district,the salt layers belong to the Majiagou Group and Fengfeng Group in Middle Ordovician.The high δ~(34) S values of sulfide in both districts indicate that most of sulfur of the skarn iron deposit is derived from sulfates in the salt layers,and the proportion is approximately 80%.The reduction temperature is more than 500℃.The temperature of the sulfide precipitation was lower and the time of the precipitation was relatively later than reduction.The higher the δ~(34) S value of sulfate or the reduced temperature is,the higher the δ~(34) S value of sulfide will be.Otherwise the more the proportion of the original magma sulfur is,the lower the δ~(34) S value of sulfide is.While the magma is assimilating salt layers,SO_4~(2-) oxidize Fe~(2+) into Fe~(3+) in the silicate melt,which prevent Fe~(2+) to enter the lattice silicate minerals,with forming Fe_3O_4 or Fe_2O_3 to enter melt.The immiscibility occurs between iron oxide and silicate melt in magma chamber,by the effects of P,NaCl,water,and so on,forming the iron ore magma which eventually precipitated in a favorable tectonic area as the magma type iron deposit.In skarn iron deposits,ore magma filling type and hydrothermal-metasomatic type ore bodies coexist,with a certain zonation or gradual transition in space.The ore magma filling type ore bodies are usually located deeply near the intrusive rocks related mineralization,while the hydrothermal-metasomatic ore bodies are located in front of ore fluid migration.In addition,SO_4~(2-) itself is reduced into S~(2-),while it is oxidizing the Fe~(2+),and S~(2-) combine with Fe~(2+) to form pyrite which is located in top or side of iron deposits.

Mineral inclusions in corundum from chromitites in the Kangjinla chromite deposit,Tibet

Abstract: Large amounts of microdiamond and moissanite have been recovered from the Kangjinla chromitite,Luobusa ophiolite, Tibet,indicating a high reduced and high pressure environment they formed. In this paper many corundum grains and their mineral inclusions have been studied by EPMA and EDS. The inclusions contain simple oxides ( such as rutile) ; nature titanium; alloys such as Ti-N,Ti-Si,Ti-C,Ti-Si-P,Ti-B and so on; REE-bearing silicate minerals,as well as some unknown minerals. Combined the previous study,we conclude that the corundum and their inclusions formed in a high reduced and high pressure environment atdeep mantle,and the corundum in the Kangjinla chromitite might be regarded as a new mineral index of a high-pressure environment.