Silicate minerals

About: Silicate minerals is a research topic. Over the lifetime, 1794 publications have been published within this topic receiving 67064 citations.

Microanalysis of Fe 3+ /ΣFe in oxide and silicate minerals by investigation of electron energy-loss near-edge structures (ELNES) at the Fe M 2,3 edge

Abstract: The Fe M2,3-edge spectra of solid solutions of garnets (almandine-skiagite Fe3(Al1–xFex)2[SiO4]3 and andradite-skiagite (Fe1–xCax)3Fe2[SiO4]3), pyroxenes (acmite-hedenbergite (Ca1–xNax)(Fe2+1−xFe3+x)Si2O6), and spinels (magnetite-hercynite Fe(Al1–xFex)2O4) have been measured using the technique of parallel electron energy-loss spectroscopy (EELS) conducted in a transmission electron microscope (TEM). The Fe M2,3 electron energy-loss near-edge structures (ELNES) of the minerals exhibit a characteristic peak located at 4.2 eV and 2.2 eV for trivalent and divalent iron, respectively, prior to the main maximum at about 57 eV. The intensity and energy of the pre-edge feature varies depending on Fe3+/ΣFe. We demonstrate a new quantitative method to extract the ferrous/ferric ratio in minerals. A systematic relationship between Fe3+/ΣFe and the integral intensity ratio of the main maximum and the pre-edge peak of the Fe M2,3 edge is observed. Since the partial cross sections of the Fe M2,3 edges are some orders of magnitude higher than those of the Fe L2,3 edges, the Fe M2,3 edges are interesting for valence-specific imaging of Fe. The possibility of iron valence-specific imaging is illustrated by Fe M2,3-ELNES investigations with high lateral resolution from a sample of ilmenite containing hematite exsolution lamellae that shows different edge shapes consistent with variations in the Fe3+/ΣFe ratio over distances on the order of 100 nm.

Satellite-derived mineral mapping and monitoring of weathering, deposition and erosion

Abstract: The Earth’s surface comprises minerals diagnostic of weathering, deposition and erosion. The first continental-scale mineral maps generated from an imaging satellite with spectral bands designed to measure clays, quartz and other minerals were released in 2012 for Australia. Here we show how these satellite mineral maps improve our understanding of weathering, erosional and depositional processes in the context of changing weather, climate and tectonics. The clay composition map shows how kaolinite has developed over tectonically stable continental crust in response to deep weathering during northwardly migrating tropical conditions from 45 to 10 Ma. The same clay composition map, in combination with one sensitive to water content, enables the discrimination of illite from montmorillonite clays that typically develop in large depositional environments over thin (sinking) continental crust such as the Lake Eyre Basin. Cutting across these clay patterns are sandy deserts that developed <10 Ma and are well mapped using another satellite product sensitive to the particle size of silicate minerals. This product can also be used to measure temporal gains/losses of surface clay caused by periodic wind erosion (dust) and rainfall inundation (flood) events. The accuracy and information content of these satellite mineral maps are validated using published data.

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

Oxygen isotope fractionation between oxygen of different sites in illite minerals: a potential single-mineral thermometer

Abstract: The experimental results of Hamza and Epstein mark internal oxygen isotope fractionations of hydrosilicates as potential single-mineral thermometers. In this study methodical investigations were made to determine the oxygen isotope ratios of hydroxyl groups in silicate minerals. As a reference material a commercial kaolinite was examined by vacuum extraction and by use of a modified partial fluorination technique first deseribed by Hamza and Epstein. The concordance of the results argue against oxygen isotope fractionation during dehydroxylation. Consequently, vacuum extraction can be used to determine the internal fractionation of minerals, which contain no ferrous iron. For calibration of the internal oxygen isotope fractionation, hydrothermally formed illites from the “Lone Gull” uranium deposit in Canada and from the “Leuggern” exploration drill site in Switzerland were investigated. Formation temperatures of the hydrothermal mineralization were estimated by mineral paragenesis, illite crystallinity and by oxygen isotope fractionations on coexisting mineral phases. the oxygen isotope fractionation between oxygen of different sites in several selected illites from both regions has been analysed. The results indicate a linear correlation between the illite-OH oxygen isotope fractionation and temperature. The fractionation can be expressed by the following equation: $$1000\ln \alpha _{\left( {ill - OH} \right)} = - 0.076t + 30.42$$ (i equals temperature in °C).