Showing papers on "Silicate minerals published in 1984"

Mössbauer Spectroscopy of Silicate Minerals

Abstract: Silicate minerals are the commonest accessible materials on Earth. Thanks to the strong affinity of silicon for oxygen and the low density of the resulting oxides (3 g/cm3) compared with the average global density (5.5 g/cm3). Well over 90% of the crust is made up of silicates. Other elements are concentrated in the upper layers of the Earth when they form particularly stable simple oxides or complex oxides with Si02, and just eight elements together account for nearly 99% of the crustal mass. After oxygen and silicon itself come aluminum, iron, and four alkali or alkaline earth elements. Aluminosilicates are therefore the principal terrestrial minerals. Iron is believed from the evidence of meteoritic and stellar abundances, and the Earth’s density profile, to be the most abundant element in the whole Earth (35 wt. %), but it represents only 5% of the mass, and the less than 2% of the atoms in the crust. No other useful Mossbauer element is sufficiently plentiful for it to appear as a significant constituent of common natural minerals (e.g., Sn 2 ppm; Sb 0.2 ppm; Dy 3 ppm). The striking fact that 94% of the crust, by volume, is made of oxygen reflects the large ionic radius of O2−, 1.40 A, and the elementary principle of crystal chemistry that the structures of oxides, silicates included, are basically closely packed arrays of oxygen anions with small metallic cations in the interstices. Figure 1 summarizes the composition of the crust by mass, number of atoms, and volume, thereby situating in a global context the subject matter of this chapter—silicate minerals containing iron along with varying amounts of other major elements.

A preliminary description of complex graphite, sulphide, arsenide, and platinum group element mineralization in a pegmatoid pyroxenite of the Stillwater complex, Montana, U.S.A.

Abstract: We describe here for the first time a major occurrence of graphite along with sulphide, arsenide, and platinum group element mineralization in the Stillwater Layered Anorthosite Complex, Montana. The graphite occurs in a pegmatoid pyroxenite stockwork intruding between major stratigraphic units and also cross-cutting them. The major silicate minerals are highly altered with graphite blades frequently replacing plagioclase and pyroxene. Graphite also occurs as globules and massive fine-grained aggregates frequently forming concentric pipe-like structures. Sulphides and arsenides including platinum group element minerals and a new rhenium sulphide are associated. We point out the proximity of this and other possibly similar deposits in the Stillwater to dunite intrusions and also the similarity to mineralization observed in dunite pipes and potholes of the Merensky Reef, Bushveld Complex. The fact that major graphite-sulphide mineralization occurs stratigraphically under the main palladium-platinum zone of the Stillwater intrusion may have bearing on the genesis and nature of the “J-M Reef”. The association of major graphite with sulphides and significant Ni, Cu, Pd, As, Re, Ag, Te, Bi as well as with carbonate veins and Rare Earth minerals, probably represents a new type of ore.

Silicate dissolution influence on Filson Creek chemistry, northeastern Minnesota

Abstract: The chemistry of surface water, precipitation, and ground water in Filson Creek watershed was studied to evaluate the kinetics of dissolution of silicate minerals in a natural watershed underlain by troctolite. Annual chemical budgets in 1977 showed net losses of dissolved silica, bicarbonate, calcium, magnesium, and sodium of about 31.0, 19.6, 4.8, 3.8, and 2.1 kg per hectare per year, respectively, and net gains of dissolved potassium and sulfate of 0.4 and 8.9 kg/ha/yr. On a molar basis, about 1.5 times as much magnesium was lost as was calcium, even though the amount of mafic minerals in till and bedrock underlying the watershed is about 10 times less than the amount of calcic plagioclase. Molar concentrations of magnesium in base flow and ground water were about the same as concentrations of calcium and sodium. These field data indicate that magnesium loss from the mafic minerals may be faster than either calcium or sodium loss from plagioclase. Stoichiometric “reconstruction” of primary minerals from base-flow water chemistry and kaolinite was done as a first approximation to evaluate the rates of loss. The calculations indicated that the release of magnesium from mafic minerals would have to be more than ten times faster than the release of calcium and sodium from plagioclase to account for the amount of magnesium in base flow. These calculations compare favorably with experimental linear rates of cation and silica release from forsterite, enstatite, augite, and labradorite.

Reactions between technetium in solution and iron-containing minerals under oxic and anoxic conditions

Abstract: The behavior of technetium in the geosphere is of particular importance in nuclear fuel waste management studies because this man-made element has a long half-life and, under ambient conditions in the laboratory, is not readily sorbed on geologic materials. Autoradiographic analyses of rock and mineral thin sections contacted with /sup 95/TcO/sub 4//sup -/-containing solutions, under oxic and anoxic conditions, have confirmed that virtually no sorption takes place in the presence of oxygen. However, under anoxic conditions (< 0.2 ..mu..g/g oxygen in the atmosphere), sorption of technetium was observed on iron-oxide inclusions in ferrous-iron-containing minerals (biotite, and olivine, pyroxene, hornblende) and on iron-oxide coatings on microfractures in granite, but not on the ferrous-iron minerals within the granite themselves. Subsequent static sorption tests with crushed magnetite showed that sorption is a function of the composition of the solution and of the radionuclide concentration, and again occurred only in the absence of oxygen. This behavior is in contrast with that observed with metallic iron, which sorbs technetium strongly, even in the presence of air. These results show that technetium can be contained by magnetite in the geosphere, provided reducing conditions can be maintained. This can be aided, for example, by the incorporation ofmore » iron or iron oxides in the buffer and backfill materials in the waste disposal vault. 11 references, 8 figures, 6 tables.« less

Trace-metal geochemistry of sediments from Baffin Bay

Abstract: The major- (Si, Al, Ti, Fe, Mn, Ca, Mg) and trace-metal (Zn, Cu, Pb, Co, Ni, Cr, V, Li, Cd, Hg) compositions of surficial sediments from Baffin Bay and the sounds (Lancaster, Jones, Smith) leading into the Arctic Ocean have been determined. The sediment composition varies regionally and in response to textural variations and the heavy-metal concentrations are at or near natural levels in relation to their source rocks and other uncontaminated shelf and oceanic sediments.Chemical partition indicates that most of the heavy-metal concentrations (80–99%) except for Cd (27–71%) are derived from various sulphide, oxide, and silicate minerals. The host minerals are predominantly fine grained and have accumulated at the same rate as other detrital clastic material of comparable grain size. As a result, the highest trace-metal concentrations occur in the fine-grained sediments occupying the deepest parts of the sounds and Baffin Bay. Although most of the metals have reached their depositional site by water transpo...

Carbon abundances in mantle minerals determined by nuclear reaction analysis

Abstract: Profiles of C concentration versus depth were determined in a spinel megacryst and two olivine crystals of a mantle xenolith from Kilbourne Hole, New Mexico, using the 12C(d,p)13C nuclear reaction technique. The 0-1400 A layers of all crystals are characterized by C concentrations of ∼1000 wt. ppm. This C is interpreted to be a contaminant acquired during sample preparation or ion bombardment. Below the surface, measurable C concentrations were not found in the spinel and one of the olivine crystals, but a bulk C content of 425 ppm was measured for a second crystal. The latter is interpreted to be due to the presence of discrete C-rich phase(s) in submicroscopic inclusions or microcracks rather than C dissolved in the crystal lattice. The detection limit potentially obtainable by the (d,p) reaction technique for bulk C in minerals is several 10's of ppm.

New Ways of Characterizing Layered Silicates and their Intercalates

Abstract: Four new methods of probing the atomic and microstructural characteristics of the clay minerals are described: solid-state, magic-angle-spinning 27 Al and 29 Si n.m.r. (along with 18 C n.m.r. of mobile, intercalated organic species); X-ray induced photoelectron studies encompassing photoelectron diffraction as a complement to conventional photoelectron spectroscopy; high-resolution electron microscopy either alone or in association with electron-stimulated X-ray emission microanalysis; and neutron scattering techniques. In reviewing the principles, scope and application of these methods specific case histories are selected from representative minerals belonging to the serpentines, kandites, smectites, micas, vermiculites, chloritoids, zeolites and intergrowths of these with one another or with other silicate minerals. Emphasis is placed on problems not readily amenable to solution by traditional, X-ray based procedures.

Diagenetic Matrix in Proterozoic Graywackes from Sweden

Abstract: Four diagenetic mechanisms of matrix formation were recognized in graywacke sandstones from the Visingso Group (Upper Proterozoic, southern Sweden). These mechanisms are 1) chemical alteration of detrital silicate minerals, 2) authigenesis (crystallization from pore solutions), 3) pseudoplastic flow of soft argillaceous fragments, forming pseudomatrix, and 4) alteration of pseudomatrix either through fixation or leaching of cations. Compared to the previously published studies dealing with graywacke matrix, the new contribution in the present study is 1) alteration of detrital unstable particles is shown to occur through dissolution and replacement by clay minerals, and 2) the microprobe and X-ray diffraction analyses, scanning electron and petrographic microscopes were used together in the mineralogical and textural investigation.

Partitioning of Heavy Metals into Mineral and Organic Fractions in Sediments from a Reservoir

Abstract: A sediment core was collected from S Reservoir, which is located in the western part of Okayama Prefecture. The vertical distribution of heavy metals (Fe, Mn, Zn, Ni, Cu and Cd) and fine particles (<37μm) was measured. The heavy metal contents of the sediment core are relatively high in small grain layers. Partitioning of heavy metals into NH2OH HCl soluble, H2O2 soluble and HF soluble fractions has been determined with a selective chemical leaching technique on the sediment samples. The greatest parts of Mn and Cd are contained in the NHOH HCl soluble fraction (mostly iron and manganese oxide and hydroxide coatings on silicate minerals), whereas more than 50% of Fe, Zn, Ni and Cu is present in the HF soluble fraction (silicate lattice). The variations of heavy metal contents in the sediment core are chiefly due to those in NH2OH HCl soluble and HF soluble fractions. Only manganese is significantly enriched in the NH2OH HCl soluble fraction of the settling material collected by the sediment trap near the bottom, probably due to the release of manganese from bottom sediment into overlying water. Manganese oxide and hydroxide reprecipitated under oxic conditions of the bottom water after the release might have been effectively collected by the sediment trap.

Occurrence of detrital, authigenic and adsorbed inorganic constituents in lignite from the beulah mine, north dakota

Abstract: Inorganic constituents in the lignite of the Beulah- Zap Bed consist primarily of: 1) detrital and authigenic quartz, clay minerals and pyrite with lesser silicates, carbonates, and accessory minerals; and 2) organically adsorbed and bound S, Ca, Na, Al, Mg and other elements. Samples from the Beulah Mine were studied by scanning electron microscope (SEM) and microprobe (EMPA) techniques. Mineral constituents were investigated in polished sections by SEM-EPMA study of samples from nine seam locations. Abundant quartz was observed as 20-30 micron diameter subangular grains. Clay minerals occurred as fine-grained masses in specific mineralized areas 30-400 microns in diameter. Members of the kaolinite, smectite and mica/illite groups were observed. Pyrite occurred as rounded blebs or framboids 20-30 microns across. Samples at the base of the seam contained massive pyritic replacements of wood. Carbonates included small amounts of calcite, magnesite, and dolomite predominantly in fractures. Minor minerals included hematite, feldspars, rutile, zircon, barite and gypsum. Adsorbed constituents were investigated by SEM-EPMA techniques in the study of fractured surfaces. Some correlation of element distribution and SEM appearance was noted. For example, some obviously woody fragments had high Na values with relatively lower Ca and S when compared to other morphological forms of organic material. Many elements showed distinctly polymodal distributions when results from all morphological types were grouped. Results of the studies reported here show that major types of inorganic constituents are highly variable in the coal in both mode of occurrence and distribution within the seam. This suggests a complex depositional, diagenetic and post-diagenetic geologic history for the Beulah-Zap Bed.