Showing papers on "Silicate minerals published in 1986"

Aluminium mobilization in soils and waters

Abstract: The toxicity of dissolved aluminium to many plant and animal species is one of the major deleterious effects of the acidification of the environment. Al mobilization from insoluble forms in minerals and rocks is governed mainly by the pH of the solution surrounding weathering minerals. To a first approximation, the solubility product of gibbsite gives an indication of Al concentrations expected in solution. Secondary silicate minerals such as kaolinite, imogolite and allophane, and sulphate minerals such as alunite, maintain lower Al concentrations in solution than does gibbsite, particularly at pH 5. The cation exchange system of acid soils provides a large reserve of ionic Al, which can be brought into solution when soluble salts percolate through soils. Ligands such as fluoride and organic anions, which form soluble Al complexes, combine with much of the Al in solution when they are present in suitable concentrations, and therefore maintain higher concentrations of Al than might be expected from mineral solubility equilibria, particularly at pH 5–7. Slow reactions involving solid phases, and uncertain data for possible Al species present, limit the rigorous application of equilibrium thermodynamics to these systems, but the factors mentioned provide a reasonable understanding of the major features of aluminium mobilization.

Dielectric and polarization behavior of forsterite at elevated temperatures

Abstract: The determination of the dielectric and polarization behavior of silicate minerals is important in the understanding of chemical bonding and the evaluation of interionic potentials in model calculations. The analysis of crystal-defect energetics and the atomistic theory of diffusion processes rely heavily upon the polarizability of component ions in the silicate. In this effort, dielectric constant (relative dielectric permittivity) and dielectric loss data have been obtained from capacitance measurements of single crystals of synthetic forsterite (MgrSiOo) for orientations parallel to the a, b, and c crystallographic axes. Measurements were obtained as a function of temperature (25C,4C-1000'C) and electric-field frequency (20 kHz-l MHz). The static polarizabilities of forsterite at these temperatures are derived from the dielectric constants by utilizing the Clausius-Mosotti relation. The use of the polarizability additivity rule is confirmed for predicting the polarization of forsterite from the component oxides at high temperatures. Dielectric conductivities are determined from the dielectric loss values at 40G900'C and are in agreement with previous studies of single-crystal forsterite. INrnooucrroN In recent years there has developed an interest in quantiffing the kinetics ofgeochemical and mineralogical processes. The comprehension of reaction mechanisms and of the approach of a mineral assemblage to equilibrium is vital for the application of geothermometry and geochronology, as well as for the analysis of mineral alteration, order-disorder status in minerals, crystal-growth and nucleation processes, and others. A useful and powerful technique in such an efort is the theoretical analysis of the basic atomic structure and energetics of the phases of concern. Although application of solid-state physics to ionic solids and other simple compounds has generally been met with success (see Kittel, 1976), there are limitations in analyzing the more complex atomistic nature of silicate minerals. The major hindrance in such an effort is evaluating the degree ofcovalency associated with the silicate mineral bonding, in particular the Si4 bond (Pauling, I 980), and the related polarization properties of the crystal (Lasaga and Cygan, 1982; Cygan and Lasaga, in prep.). The recognition of the influential role of polarization in crystal bonding and lattice eneryies lies in the classic work of Mott and Littleton (1938). They determined the theoretical polarization energy about a lattice point that contains an excess ofcharge (i.e., a point defect) and ap

Lysis of erythrocytes by silicate minerals

Abstract: In vitro studies of the destruction (lysis) of bovine red blood cells (erythrocytes) by some silicate minerals showed the reaction to be complete in less than 1 hr and very destructive to the cell membrane. The activity as lysing agents was found to be in the order smectites > silica > palygorskite -~ sepiolite > chrysotile > kaolinite. Different compositions (Fe, A1, Mg, Li, vacancy) of the octahedral sheet of the smectite and fibrous clay minerals did not appreciably alter their hemolytic activity. The most active particle size range for kaolinite and montmoriUonite was 0.2-2 ~m. Structural folding of palygorskite reduced lysis suggesting that edge surfaces and silanol groups are important in this process. Aluminum oxides and hydroxides caused no lysis, and coatings of positively charged aluminum-hydroxy polymers on montmorillonite, silica, palygorskite, and kaolinite significantly reduced lysis.

Problems in the dissolution of silicates by acid mixtures

Abstract: The decomposition of silicate rocks using mixtures of mineral acids has been studied. The conventional acid method of dissolving rocks with hydrofluoric-perchloric-nitric acid mixtures is unsatisfactory when pyrophyllite is present. This mineral is not destroyed by acid attack, whereas other silicates such as mica, kaolinite, quartz, feldspars, smectite and interstratified illite-smectite are decomposed. If the acid attack is carried out in a pressure vessel, pyrophyllite is dissolved completely, but with more difficulty than the other silicate minerals present.

The Solubility Relationships of Calcium and Magnesium Minerals in Processed Oil Shales

Abstract: One of the major problems occurring with the oil shale industry is the production of large amounts of solid wastes (spent shale). The high alkalinity of these waste materials often presents problems for successful management practices to minimize overall pollution impact on the environment. The objective of this study was to determine solubility of calcium and magnesium minerals to explain high alkalinity of spent shales. Two oil shales, one processed by Los Alamos National Laboratory (LANL) and another processed by Lurgi Ruhrgas (Lurgi) were examined in this study. The x-ray diffraction analysis was used to identify different mineral phases of spent oil shales. The results indicate that processing oil shales at high temperature destroyed carbonate minerals and formed silicate minerals including wollastonite (αCaSiO₃), clinoenstatite (MgSiO₃), or diopside (CaMg(SiO₃)₂). These minerals buffered pH above 11.00 and controlled Ca²⁺ and Mg²⁺ activities in solution. The experimental findings in this study further suggest that oxides or hydroxides of Ca and Mg produced initially from processing oil shales did not persist after moisture was allowed to contact the spent shale.

Characterization of pyrite in products from the reverse flotation of coal

Abstract: The effectiveness of two-stage reverse flotation for desulfurization of coal is significantly dependent on the microstructural features of the heavy mineral fractions in the coal seam. This dependence has been evaluated by a systematic mineralogical analysis of the flotation products obtained from both Eastern (Lower Freeport) and Midwestern (Illinois No. 6) coals by SEM/EDAX and TGA. Comparison of the results from this detailed analysis suggests that the effectiveness of desulfurization during reverse flotation is determined by the relative degree of pyrite association with silicate minerals and carbonaceous material. When locked pyrite contains a higher relative percentage of silicate minerals desulfurization by reverse flotation is more effective.

Thermodynamics applied to potassium exchange in aluminosilicate minerals and soils.

Abstract: The enthalpies of Ca-»K exchange in 2:1 layer silicate minerals and in soils identify six types of cation exchange site, with up to four in any one soil or clay, which control ion exchange and the extent to which clays swell on adsorption of glycol. It is suggested that these sites are on the planar surfaces of mica, illite, vermiculite and montmorillonite, with a particular type of strongly K selective site at steps, cracks and expansion boundaries (wedge zones) of micaceous clays. Such thermodynamic measurements permit the quantitative determination of very small amounts of 2:1 layer silicate minerals and their K contents in clays and soils with mainly 1:1 or amorphous minerals, and thus a more precise determination of K exchange behaviour, especially selectivity and fixation.

Solubility products and stoichiometric solubilities of minerals in hydrothermal systems.

Abstract: Solubility products of minerals in terms of molality of the metal (i.e., total molality of the metal ion) and stoichiometric solubilities of them expressed as amount of mineral (moles and grams) dissolved in 1000 grams of water for some oxide, hydroxide and silicate minerals were calculated using thermodynamic data at elevated temperatures and at various pH, PO2, PCO2 and mC1.coefficient of each species was assumed to be unity for the calculation. The solubility products, “solubility products” in this paper, and the stoichiometric solubilities of minerals in hydrothermal systems can be used for interpreting formation of gangue minerals in veins and hydrothermal alteration of rocks. Quartz veins are distributed widely and amounts of quartz in pottery stones are usually larger than the original rocks. Such field occurrences are attributed to the result that the “solubility product” and the stoichiometric solubility of quartz increase with temperature, while those of silicates usually decrease. Amounts of solution which flowed through a vein are estimated from those of gangue minerals, if temperature, pH etc. are known. The pattern of water flow in a geothermal area is suggested by the compositional change of rocks.

Source of silicate and carbonate cements during deep burial diagenesis

Abstract: Detrital silicate minerals and silicate cements (formed during shallow burial) of siliciclastic sandstones commonly dissolve during deep burial diagenesis. Quartz, feldspars, mica, and garnet among detrital silicate minerals, and quartz and kaolinite among authigenic silicate minerals show extensive dissolution features during deep burial diagenesis of siliciclastic sandstones of the Gondwana Supergroup, India. No dissolution features were observed in zircon, tourmalene, and rutile among detrital minerals or in chlorite and smectite among early formed authigenic minerals. Dissolution enriched the pore fluids in silica, potassium, sodium, calcium, magnesium, iron, and aluminum. Authigenic cements formed during this stage are illite, quartz, feldspar, iron oxide, and carbonates of calcium, magnesium, and iron. Mass-balance calculations show that the source of all silicate cements formed during deep burial diagenesis was internally derived from the dissolution of both detrital and early formed authigenic cements. However, a considerable gap exists between the amounts of cations (calcium, magnesium, and iron) derived internally and the respective amounts of these cations needed to form the various carbonate cements at this stage. Therefore, an outside source for these cations is needed to explain the formation of carbonate cements. A large mass transfer of cations from outside the sediment source seems remote sincemore » ground-water movement, which probably carried cement from an external source, is extremely restricted at great burial depths. Therefore, carbonate cements may have been major constituents during shallow burial diagenesis in Gondwana sandstones. Subsequently, these early formed carbonates were completely dissolved and remobilized as late-stage carbonate cement.« less