Showing papers on "Silicate minerals published in 1998"

A model of surface site types on oxide and silicate minerals based on crystal chemistry; implications for site types and densities, multi-site adsorption, surface infrared spectroscopy, and dissolution kinetics

Abstract: In this study, a method for estimating site types and ionizable site densities at mineral surfaces by consideration of ideal crystal surfaces has been developed. Periclase, rutile, goethite, hematite, corundum, kaolinite, andalusite, sillimanite, sanidine, albite, anorthite, and quartz surfaces were considered, both because they represent a wide range of crystal structures and because many adsorption and dissolution studies have focused on these minerals. For each mineral, the predominant cleavage or growth faces were studied. To avoid choosing arbitrary slices of the crystal parallel to each cleavage or growth face, the total Brown bond strengths (and the resulting partial charges on coordinatively unsaturated atoms) were calculated for the bonds that must be broken to generate each possible ideal slice on a given plane. The charge-neutral or nearly charge-neutral slices produced with a minimum total strength of bonds severed were examined with the aid of the commercial computer program Crystal-Maker C . Once the ideal mineral surface for a given plane was chosen, the number of sites per unit surface area was calculated using five different methods. For the minerals considered, calculated site densities for a given surface ranged from 0 to 40.8 sites/nm 2 , a considerably larger range than the density of 2.3 to 10 sites/nm 2 that is often cited. The results from each of the methods were compared to available experimental estimates of surface hydroxyl site densities from temperature desorption experiments, infrared data, and tritium exchange methods and to ionizable or reactive site densities from acid-base titrations and chemical reaction methods. Estimates based on the number of broken bonds gave the best agreement with site densities using the tritium exchange method. However, estimates based on the number of coordinatively unsaturated atoms or on the partial charge of coordinatively unsaturated atoms were also consistent with much of the available tritium exchange data. Predicted site types were compared with the available spectroscopic data. Implications of predicted site types with respect to the interpretation of infrared data and for improving adsorption and dissolution rate models are discussed.

Atmosphere-Surface Interactions on Mars: Δ17O Measurements of Carbonate from ALH 84001

Abstract: Oxygen isotope measurements of carbonate from martian meteorite ALH 84001 (δ18O = 18.3 ± 0.4 per mil, δ17O = 10.3 ± 0.2 per mil, and Δ17O = 0.8 ± 0.05 per mil) are fractionated with respect to those of silicate minerals. These measurements support the existence of two oxygen isotope reservoirs (the atmosphere and the silicate planet) on Mars at the time of carbonate growth. The cause of the atmospheric oxygen isotope anomaly may be exchange between CO2 and O(1D) produced by the photodecomposition of ozone. Atmospheric oxygen isotope compositions may be transferred to carbonate minerals by CO2-H2O exchange and mineral growth. A sink of17O-depleted oxygen, as required by mass balance, may exist in the planetary regolith.

Surface Area and Porosity of Primary Silicate Minerals

Abstract: Surface area is important in quantifying mineral-water reaction rates. Specific surface area (SSA) was measured to investigate controls on this parameter for several primary silicate minerals (PSM) used to estimate rates of weathering. The SSA measured by gas adsorption for a given particle size of relatively impurity-free, laboratory-ground samples generally increases in the order: quartz ≈ olivine ≈ albite < oligoclase ≈ bytownite < hornblende ≈ diopside. Reproducibility of BET SSA values range from ±70% (SSA < 1000 cm/g) to ± 5% (SSA > 4000 cm/g) and values measured with N2 were observed to be up to 50% larger than values measured with Kr. For laboratory-ground Amelia albite and San Carlos olivine, SSA can be calculated using log (SSA, cm/g) = b + m log (d), where d = grain diameter (μm), b = 5.2 ± 0.2 and m = –1.0 ± 0.1. A similar equation was previously published for laboratory-ground quartz. Some other samples showed SSA higher than predicted by these equations. In some cases, high SSA is attributed to significant second phase particulate content, but for other laboratory-ground samples, high SSA increased with observed hysteresis in the adsorption-desorption isotherms. Such hysteresis is consistent with the presence of pores with diameters in the range 2 to 50 nm (mesopores). In particular, porosity that contributes to BET-measured SSA is inferred for examples of laboratory-ground diopside, hornblende, and all compositions of plagioclase except albite, plus naturally weathered quartz, plagioclase, and potassium feldspar. Previous workers documented similar porosity in laboratory-ground potassium feldspar. Surface area measured by gas adsorption may not be appropriate for extrapolation of interfacelimited rates of dissolution of many silicates if internal surface is present and if it does not dissolve equivalently to external surface. In addition, the large errors associated in measuring SSA of coarse and/or impurity-containing silicates suggest that surface area-normalized kinetics in both field and laboratory systems will be difficult to estimate precisely. Quantification of the porosity in laboratory-ground and naturally weathered samples may help to alleviate some of the discrepancy between laboratoryand field-based estimates of weathering rate. * E-mail: npm113@psu.edu BRANTLEY AND MELLOTT: SURFACE AREA AND POROSITY OF SILICATES 1768 weathered samples of plagioclase, K-spar, and hornblende based upon microscopic observations and variations in surface area measured by gas adsorption as a function of grain size for soils from Puerto Rico (Schulz et al. 1999) and Merced, California, (White et al. 1996). In addition, porosity in alkali feldspars has been documented by many workers (e.g., Worden et al. 1990; Walker et al. 1995; Lee and Parsons 1995). Pores have also been documented in laboratory-ground silicates: for example, Hodson and coworkers (Hodson et al. 1997; Hodson 1998) documented porosity in potassium feldspar and Brantley et al. (1999) documented mesoporosity in hornblende. A better understanding of the controls on mineral surface area and porosity in surficial environments is important for several reasons. For example, if internal surface area (e.g., Gregg and Sing 1982; Hochella and Banfield 1995) is present in primary silicate minerals, and if internal surface area is more or less reactive than external surface area, then quantification of the ratio of internal vs. external surface area could be important in the extrapolation of mineral-water kinetics from one system to another (see, for example, Lee and Parsons 1995; Hodson 1998). Whereas several authors have suggested that weathered and laboratory-ground samples differ with respect to the relative importance of internal and external area (e.g., Anbeek 1992a, 1992b, 1993; Anbeek et al. 1994), few have quantified the contribution from these two types of surface for minerals which are commonly used in comparisons of field and laboratory weathering rates (e.g., plagioclase, hornblende, olivine, diopside). Specifically, if the ratio of these two types of surface area differ between laboratory-ground and naturally weathered plagioclase, hornblende, olivine, and diopside, then perhaps these differences contribute to the large (up to five orders of magnitude) discrepancies between laboratoryand field-derived dissolution rates for these minerals (White and Brantley 1995). In addition, if porosity is present in silicate grains then the observed aging or sequestration of contaminants in soils and aquifers may be due to sequestration inside these pores (e.g., Farrell and Reinhard 1994; Werth and Reinhard 1997; Luthy et al. 1997). In a series of papers, Mayer (1994, 1999) also proposed that sequestration of organic matter into small pores may allow preservation of the organic matter on continental shelf sediments since hydrolytic enzymes should be excluded from such small pores. Again, however, little is known about mineral controls on the presence of porosity in the silicates making up the sediments. This paper addresses the following questions concerning SSA of primary silicates: What is the range of SSA measured using BET for laboratory-ground primary silicate powders? Can we infer the presence of porosity from adsorption data for laboratory-ground and weathered primary silicates? What do these observations imply for reaction kinetics in the laboratory and in the field? Our approach was to investigate relatively pristine mineral samples that have traditionally been used by geochemists to investigate mineral dissolution (samples largely derived from pegmatite deposits) using standard BET techniques for analysis of gas adsorption-desorption. We also used the same BET technique on seven naturally weathered samples to infer the presence or absence of porosity. BACKGROUND The specific geometric surface area, SSAgeo, can be expressed as a function of grain diameter d:

Adsorption of uranium on clay and the effect of humic substances

Abstract: Adsorption of hexavalent uranium (4.2X10 mol/I) from aqueous solutions on "cypris" clay was studied as a function of pH (1 — 12), concentration of coal humate (0, 0.3 and 3 g/1) and clay concentration (5 and 50 g/1). A batch method was used for the experiments. Uranium remaining in the solution after the adsorption was determined by spectrophotometry of its arsenazo-III complex. Simultaneously, the distribution of coal humate between the solid and liquid phases was measured by UV spectrophotometry. Both the adsorption of uranium and the distribution of coal humate were strongly pH-dependent. The adsorption of uranium passed through two maxima whose form and position on the pH-scale depended on the concentration of clay and on the humate to clay ratio. The precipitation/adsorption of humate on clay was high at low pH values and decreased with increasing pH. The slope of the decrease and its position on the pH-scale depended on the humate to clay ratio. Presence of humate enhanced the adsorption of uranium at pH<4—7.5 (depending on the concentrations of clay and humate) and suppressed it at pH>4—8. The results were compared with literature data and qualitatively interpreted. Conclusions were drawn on probable mechanisms of uranium adsorption in the absence and presence of humate, and on possible applications of cypris clay as barrier or filling material and of the clay-humate system for wastewater treatment.

In-Situ Observation of Oxide and Silicate Mineral Dissolution by Hydrothermal Scanning Force Microscopy: Initial Results for Hematite and Albite

Abstract: The dissolution and growth of oxide and silicate minerals is of interest to the geochemical community in fields ranging from radioactive waste storage, enhanced oil recovery and the influence of weathering on global climate Recently, application of scanning force microscopy (SFM) has revealed new details of dissolution and growth mechanisms for ionic solids such as calcite (Gratz et al, 1993), but the dissolution rates of most oxide and silicate minerals are lower than the range accessible to in situ SFM imaging at room temperature (about 10 6 to 10 -9 moles m -2 s 1; Dove and Chermak, 1994, Stumm and Morgan, 1996) The vapour pressure of water imposes a fundamental temperature limit on currently available SFM fluid cell temperatures Here, we present initial results from an SFM of our design capable of imaging in aqueous solution up to 150~ We report initial observations on the dissolution of hematite and albite in aqueous solution at 100~ ~ and 6 bar Albite dissolution is compared to the behaviour of p e r i c l a s e ( M g O ) at r o o m t e m p e r a t u r e Multicomponent oxides such as albite probably do not dissolve by a step-motion mechanism such as observed for calcite, gypsum, and brucite by SFM (eg Jordan and Rammensee, 1996) Rather, surface alteration and roughening occurs Hydrothermal AFM provides real-time, in situ imaging access to reactions of silicate minerals with hydrothermal solutions thermocouple ports Flow is controlled by a mass flow controller A 600 ml Ti bomb serves as a solution source A Digital Instruments (DI) optical head was mounted on a custom base; electronics were developed to couple our SFM to a DI controller

Vanadium-bearing magnetite from the Matagami and Chibougamau mining districts, Abitibi, Quebec, Canada

Abstract: Vanadium mineralization occurs in oxide-rich horizons within the layered gabbro zones of the upper parts of the Bell River Complex, Matagami, Quebec and the Lac Dore Complex, Chibougamau, Quebec. The vanadium-rich horizons are well defined on the ground and in aeromagnetic surveys by their high magnetic susceptibility; consequently, magnetic susceptibility can be an indicator of vanadium mineralization. The main oxide minerals are ilmenite and titanian magnetite, containing 20% to 70% of volume and the ratio of titanian magnetite to ilmenite is relatively constant, ranging from about 1:1 to 2:1. Their sizes are less than 5 mu m to greater than 1 mm to 2 mm, occurring as coarse- to medium-grained subhedral crystals intergrown with cumulate silicate minerals (plagioclase, pyroxene, etc.). Electron microprobe analyses indicate that the ilmenite grains are mineralogically and compositionally homogeneous and have low V contents (average 0.18% equivalent V 2 O 5 ). In contrast, the titanian magnetite grains are inhomogeneous, consisting of trellisworks of ilmenite lamellae in Ti-poor, V-rich magnetite [less than 2 wt% TiO 2 , and 1.41% equiv. V 2 O 5 (1.16% V 2 O 3 ) for 20 analyses]. Thus, the magnetite is the principal ore mineral of vanadium; it hosts vanadium in the form of V (super 3+) , not V (super 5+) , as is commonly and erroneously reported. We have also devised a flowchart for the beneficiation of vanadium-titanium ores.

Experimental study on the competitive adsorption of metal ions onto minerals

Abstract: The study on the competitive adsorption shows that the magnitude order of metal ions adsorbed onto oxide and silicate minerals in near-neutral solution with low ionic strength is in mole/nm2 as follows: CaCO3 > quarte > hydromuscovite > kaolinite > Ca-montmorillonite > goethite > gibbsite. These minerals can be divided into three groups according to their surface equilibrium constantsK M of the adsorption reactions, which are the function of the dielectric constants e of the absorbent minerals. The relationships between constantsK M and mineral dielectric constants e are described as follows: lgK M 1 = 7.813-26.15/e lgK M 2 = 9.030-26.15/e lgK M 3 =11.63-26.15/e for the adsorption reaction: >SO- + Mn+≥SOMn-1)+ (n = 1, 2, 3)

Surface Thermodynamic Properties of Micas and Related Layer Silicate Minerals

Abstract: The surface tension values for a series of 2:1 phyllosilicate minerals with different structural characteristics and layer charges ranging from 0 to 2 per O10 group have been examined. The average Lifshitz-van der Waals component (ΓLW)value is 40.3 ± 1.9 mJ/m2, and the average Lewis acid (®) parameter is 1.2 ± 0.6 mJ/2 for those minerals with a layer charge greater than zero. In contrast, the Lewis base parameter (γe) varied greatly from a maximum of 59.7 mJ/m to a minimum of 23.7 mJ/m2. Those minerals with a layer charge of zero (three samples) had smaller γLW values of 32.0 ± 1.6 mJ/m2, slightly larger γ® values of 2.0 ± 0.4 mj/m2 and significantly smaller values of γ® of 4.5 ± 1.6 mJ/m2. The hydrophobic versus hydrophilic character of these materials is largely governed by the value of γ° which is strongly related to the value of the layer charge. Thus, the hydrophilicity of a smectite is determined by the charge unbalancing ionic substitutions which attract hydrophilic interlayer cations and ...

Octahedral site Fe2* quadrupole splitting distributions from the Miissbauer spectra of arrojadite

Abstract: The Mijssbauer spectra of arrojadite, (K,Ba)(Na,Ca),(Fe,*,Mn,Mg),oAl(pO*),, (OH,F), at 298 and' 95 K were investigated for the first time The spectra at both temperatures were analyzed in terms of their Fe'?* quadrupole splitting distributions (QSDs) The overall QSDs at both temperatures can be interpreted in terms of five octahedral site Fer* QSD contributions The quadratic elongation, (r), and the variation ofbond angles, o,, for the different sites were calculated on the basis of the structural data obtained by Moore et al (1981) The five QSD contributions are tentatively assigned to Fe2* in the M3, M4, M5, M6, and M7 sites, based on the structural determination and the relation of the quadrupole splitting to the distortion of the octahedra, respectively The Fe,* ions are randomly distributed over the M3, M4, M5, M6, and M7 sites In addition, Mcissbauer data from arrojadite and related phosphate minerals indicate that the mean value of the isomer shift of Fer* in the octahedral sites in phosphate minerals is -007 mm/s larger than that in silicate minerals This difference is explained in terms of electron affinity

Chlorine/Fluid Cycling in Subduction Zones: Evidence from Chloride Concentrations and Chlorine Stable Isotopes

Abstract: Pore waters with chloride concentrations less than seawater are a characteristic feature of accretionary complexes in convergent margins. One well-documented case where such freshened fluids have been documented is in the Nankai Trough, Japan, Ocean Drilling Project (ODP) Site 808. The enigmatic profile of pore water chloride concentration versus depth in these sediments is characterized by an extensive section of lower than seawater chloride concentrations between 560 and -1200 mbsf, with a broad minimum of about 450 mM (~20 % seawater dilution) at ~1,100 mbsf (Fig. 1). Newly obtained chlorine stable isotopic ratios (37C1/35C1) in the pore fluids at the Nankai drill site are highly fractionated (-7.4%0) compared to seawater (0%0), with the most extreme fractionation (Fig. 2) coinciding with the lowest pore water chloride concentration (Fig. 1). Chlorine stable isotopes in subduction zone settings can be fractionated by water-rock exchange when 37C1 is preferentially incorporated over 35C1 into the structural (OH) site of diagenetic or metamorphic hydrous minerals (e.g. smectite, illite, chlorite, serpentine, talc, and amphibole) and by diffusion. In the case of mineral uptake, lower temperature minerals fractionate C1 isotopes more highly than those formed at higher temperatures; but their maximum C1 contents are generally an order of magnitude lower (e.g. < 100 ppm in smectite versus several parts per thousand in amphibole. The source of the freshened fluids at Nankai is presently being debated. On the basis of pore water chloride data alone, previous studies have suggested that clay mineral reactions, in particular those involving smectite, are primarily responsible for the freshening observed. These studies attribute the increase in pore water chloride to ~560 mbsf (Fig. 1) to the in situ hydration of volcanic glass and formation of clays and zeolites. The decrease in chloride between 560 and 1100 mbsf is proposed to result from clay mineral (and zeolite) dehydration reactions. Below 1100 mbsf, the formation of hydrous minerals in the underlying volcaniclastic section is invoked. Data from C1 stable isotopes now permit us to evaluate these hypotheses. In the case of simple loss of H20 from smectite interlayers or zeolites, the CI stable isotope ratios of associated pore waters are unaffected. However, reactions that involve smectite or other Cl-bearing hydrous silicate minerals will affect the C1 stable isotopic signature of the pore fluids. Below -560 mbsf, the two profiles are decoupled. The decrease in pore water chloride to ~1100 mbsf is accompanied by a strong decrease in pore water (~37C1; the opposite of what would be expected if this isotopic signature was the result of clay mineral dehydration that involved the destruction of smectite and/or other low temperature hydrous layer silicates. Such a reaction would tend to release 37C1 into the pore waters, not remove it. Likewise, decoupling is evident between the two profiles below 1100 mbsf where hydration reactions are thought to be occurring. The decoupling of pore water chloride and the chlorine stable isotope data between 560 and 1100 mbsf provides important evidence that H20 in freshened fluids at Nankai are coming from mineral reactions occurring in the seismogenic zone. Calculations demonstrate that contributions to the observed freshening in Nankai, due to the release of H20 from smectite interlayers and zeolites in the section drilled can account for only 4 % freshening of seawater, at best, instead of the 20% observed. Neither is there any physical evidence for recent fluid flow along the drcollement (-960 mbsf). Porosity in sediments below the d+collement is higher (40% vs 30% in the overlying sediments) than in sediments above; and St, He, and O isotope data, as well as pore water chloride and sulphate, suggest that freshened pore waters at Nankai originate from repeated transient episodes of fluid incursion from the high

Hydrogen in α-Alumina and Water Softening of Polycrystals and of Sapphire Between 600°C and 1000°C

Abstract: Trace hydrogen impurities influence the physical and chemical properties of silicate minerals. Hydrogen defects reduce the plastic yield strengths of quartz and olivine (Kronenberg, 1994; Kohlstedt et al., 1996) and they increase rates of oxygen diffusion and A1/Si exchange in feldspars (Farver and Yund, 1990). Hydrogen impurities may similarly affect the properties of oxides and other non-silicate minerals, as suggested by early studies by Heuer et al. (1971), who reported water weakening of fine-grained (2 gm) aluminum oxide at 1200~ and 1400 MPa confining pressure. In this study, we revisit the mechanical properties of ~-A1203 in the presence of water, and examine the uptake of hydrogen impurities by single crystals and polycrystals at temperatures of 600-900~ and PH20 1500 MPa.

Carbonate Versus Silicate Weathering Fluxes of Sr From a High Himalayan Watershed

Abstract: The Himalayan uplift has been implicated as a major factor in the global climatic cooling of the past 40 m.y. because of enhanced silicate weathering and consequent atmospheric CO2 drawdown (Raymo and Ruddiman, 1992). Supporting evidence includes the dramatic increase in marine 87Sr/86Sr ratios at 40 Ma, which appears to be related to Sr inputs from major rivers draining the Himalaya (Palmer and Edmond, 1992). Regional patterns of 87Sr/86Sr ratios in tributaries of the Ganges-Brahmaputra and Indus River systems as well as bedrock 87Sr/86Sr ratios suggest that the major source of the highly radiogenic Sr in Himalayan rivers is weathering of silicate minerals in the High Himalayan Crystalline Series (HHCS) (Krishnaswami et al., 1992; Harris, 1995). Here we report on a geochemical investigation of the ~200 km 2 Raikhot watershed in the western Himalaya on the north side of the Nanga Parbat massif in northern Pakistan, with the aim of elucidating the systematics of Sr release from individual minerals in the HHCS bedrock. The bedrock of the area is predominantly highgrade quartzofeldspathic biotite gneiss and schist with minor anatectic biotite granite and a small amount (-1%) of calc-silicate schist. The bedrock is generally representative of the HHCS, which extends over 2000 km across the length of the Himalaya and dominates the geology of the steep southern slopes that are subjected to rapid erosion during the monsoon season (Harris, 1995). Samples for analysis were taken of rainwater, snow, clear streams, Raikhot River water (which has suspended glacial flour), Raikhot riverbed sand (believed to be representative of unweathered bedrock from the watershed), bedrock from outcrops, and glacial boulders (Blum et al., 1998). Rock samples containing calcite were leached for 1 h in 4 N acetic acid to dissolve carbonate, and silicate rock samples were totally digested for elemental and Sr isotope analysis. Concentrations were measured by ICP-OES and ICP-MS; 878r/86Sr ratios were measured by TIMS. The most abundant minerals in the bedrock of the watershed are quartz, plagioclase, K-feldspar, and biotite; calcite is present in only minor abundance but is important due to its high reactivity. Plagioclase and K-feldspar are assumed to weather to kaolinite, and biotite to vermiculite (or hydrobiotite). A massbalance calculation was performed to quantify the proportions of each of the bedrock minerals that weathered to yield each water composition. This calculation has the following six steps based on the stoichiometry of the weathering reactions. (1) Atmospheric correction. (2) Attributing of Na to the weathering of albite to kaolinite. (3) Attributing of Ca in proportion to the Ca/Na ratio of plagioclase to the weathering of anorthite to kaolinite. (4) Attributing any remaining Si to the weathering of orthoclase to kaolinite. (5) Attributing the remaining K to the weathering of biotite to vermiculite. (6) Attributing excess Ca and Mg to carbonate dissolution. For the Raikhot River samples, the above calculation yields an estimate of the relative amounts of weathering of minerals as follows: 14% plagioclase, 2% orthoclase, 11% biotite, and 73% carbonate. This estimate corresponds to a riverine HCO3 flux that is 18% derived from silicate and 82% derived from carbonate weathering reactions. Analyses of the carbonate and silicate fractions of the riverbed sand indicate that only 1.0 wt.% of the sediment load is carbonate. The calculated percentages of weathering of bedrock minerals for the average quartzofeldspathic gneiss and granite bedrock stream-waters were: 24% plagioclase, <1% orthoclase, 12% biotite, and 64% carbonate. High proportions of riCO;are also derived from carbonate (68-78%) in the stream waters known to drain exclusively quartzofeldspathic gneiss and granite bedrock. The Ca/(1000Sr) ratios of silicate rocks (gneiss and granite) range from 0.07 to 0.4, whereas marble layers range from 1 to 2. 87Sr/86Sr ratios of the silicate rocks (and all their constituent minerals) range between 0.82 and 0.89, whereas marble layers