Abstract An experimental study of cadmium substitution into Ag X S 2 phases (X = As, Bi, Sb) indicates significant amounts of cadmium can be incorporated in the structure by the mechanism 2Cd ⇌ Ag + X . The limit of substitution of Cd in the high-temperature polymorph of miargyrite, β-AgSbS 2 is 6.2 at.%, whereas the low-temperature polymorph, α-AgSbS 2 , can accommodate at least 12 at.% cadmium. Substitution of Cd into the cubic β-AgSbS 2 induces a small monoclinic distortion and the unit cell becomes pseudo-cubic. The α ⇌ β transition in Cd-substituted miargyrites is modified by the solute ions, with both a reduction of T c and transition smearing evident. Similar effects are also recorded in Cd-substituted AgAsS 2 and AgBiS 2 .
Abstract Combined microfocus XAS and XRD analysis of α-particle radiation damage haloes around thorium-containing monazite in Fe-rich biotite reveals changes in both short- and long-range order. The total α-particles flux derived from the Th and U in the monazite over 1.8 Ga was 0.022 α particles per atomic component of the monazite and this caused increasing amounts of structural damage as the monazite emitter is approached. Short-range order disruption revealed by Fe K -edge EXAFS is manifest by a high variability in Fe–Fe bond lengths and a marked decrease in coordination number. XANES examination of the Fe K -edge shows a decrease in energy of the main absorption by up to 1 eV, revealing reduction of the Fe 3+ components of the biotite by interaction with the 2 4 He 2+ , the result of low and thermal energy electrons produced by the cascade of electron collisions. Changes in d spacings in the XRD patterns reveal the development of polycrystallinity and new domains of damaged biotite structure with evidence of displaced atoms due to ionization interactions and nuclear collisions. The damage in biotite is considered to have been facilitated by destruction of OH groups by radiolysis and the development of Frenkel pairs causing an increase in the trioctahedral layer distances and contraction within the trioctahedral layers. The large amount of radiation damage close to the monazite can be explained by examining the electronic stopping flux.
We investigated structural changes during precipitation of HgS from sulfidic solutions using Xray absorption and X-ray diffraction techniques. The results show that initially an unstable low Hg coordination complex forms that is probably chain-like in structure, with one sulfide atom at 2.35 Å and one at 2.97 Å. This is rapidly transformed to a four-coordinate mercury sulfide compound that initially forms as clusters with the local ordering characteristics of cubic metacinnabar. However, during aggregation the black Hg-S precipitate loses its initial longer-range ordering and becomes pseudocubic. As it ages, the pseudocubic structure transforms to a cubic structure, and then to stable crystalline metacinnabar. This study provides clear evidence that the precipitation and formation of metal sulfides is a complex multistage process
Abstract The oxidation of chalcopyrite and pyrite was examined using Fe- K - and Cu- K -edge REFLEXAFS spectroscopy. The Fe XANES of the pyrite proved to be a very sensitive indicator of oxidation, revealing the development of a goethite-like surface species; the EXAFS data showed an increasing O:S ratio with the degree of oxidation and gave Fe–O distances of c . 1.9 Å. On the oxidized chalcopyrite surfaces, the development of Fe-O and Cu-O species was observed, with both the XANES and EXAFS revealing the progressive development of these species with oxidation. Differences in the sensitivity of the XANES and EXAFS to the degree of oxidation can be related to the degree of long range order and changes in the intensity of the pre-edge feature of the Fe are a function of its oxidation state and coordination geometry in the surface species.
Abstract Tiny green crystals from Kabwe, Zambia, associated with hopeite and tarbuttite (and probably first recorded in 1908 but never adequately characterized because of their scarcity) have been studied by X-ray diffraction, microchemical and electron probe microanalysis, infrared spectroscopy, and synthesis experiments. They are shown to be orthorhombic, stoichiometric CuZnPO 4 OH, of species rank, forming the end-member of a solid-solution series to libethenite, Cu 2 PO 4 OH, and are named zincolibethenite. The libethenite structure is unwilling to accommodate any more Zn substituting for Cu at atmospheric pressure, syntheses using Zn-rich solutions precipitating a mixture of zincolibethenite with hopeite, Zn 3 (PO 4 ) 2 .4H 2 O. Single-crystal X-ray data confirm that the Cu(II) occupies the Jahn-Teller distorted 6-coordinate cation site in the libethenite lattice, and the Zn(II) occupies the 5-coordinate site. The space group of zincolibethenite is Pnnm , the same as that of libethenite, with unit-cell parameters a = 8.326, b = 8.260, c = 5.877 Å , V = 404.5 Å 3 , Z = 4, calculated density = 3.972 g/cm 3 (libethenite has a = 8.076, b = 8.407, c = 5.898 Å , V = 400.44 Å 3 , Z = 4, calculated density = 3.965 g/cm 3 ). Zincolibethenite is biaxial negative, with 2V α (calc.) of 49°, r < v , and α = 1.660, β = 1.705, and γ = 1.715 The mineral is named for its relationship to libethenite.
Economic Mn-oxide ore deposits of commercial grade occur in the Rhodope massif near Kato Nevrokopi in the Drama region, Northern Greece. The Mn-oxide mineralization has developed by weathering of continental hypogene rhodochrosite-sulphide veins. The vein mineralization is confined by tectonic shear zones between marble and metapelites, extending laterally into the marble as tabular, pod or lenticular oreshoots (up to 50 m x 20 m x 5-10 m). Supergene oxidation of the hypogene mineralization led to the formation of in-situ residual Mn-oxide ore deposits, and secondary infills of Mn-oxide ore in embryonic and well developed karst cavities. Whole rock geochemical profiles across mineralized zones confirm the role of thrusts and faults as solution passageways and stress the importance of these structures in the development of hydrothermal and supergene mineralization at Kato Nevrokopi. Three zones an recognized in the insitu supergene veins: (A) a stable zone of oxidation, where immobile elements form (or substitute in) stable oxide mineral phases, and mobile elements are leached; (B) a transitional (active) zone in which element behavior is strongly influenced by seasonal fluctuations of the groundwater table and variations in pH-Eh conditions; and (C) a zone of permanent flooding, where variations in pH-Eh conditions are minimal. Zone (B) is considered as the source zone for the karst cavity mineralization. During weathering, meteoric waters, which were CO2-rich (P-CO2 similar to 10(-3.8) to 10(-1.4)) and oxygenated (fO(2) -10(-17) for malachite), percolated downward within the veins, causing breakdown and dissolution of sulfides and marble, and oxidation of rhodochrosite to Mn-oxides. Karat cavity formation was favored by the high permeability along thrust zones. Dissolved Mn2+ was transported into karst cavities in reduced meteoric waters at the beginning of weathering (pH similar to 4-5), and as Mn(HCO3)(2) in slightly alkaline groundwaters during advanced weathering (pH similar to 6-8). Mn4+-oxide precipitation took place by fO(2) increase in ground waters, or pH increase by continuous hydrolysis and carbonate dissolution. In the well developed karst setting, some mobility of elements occurred during and after karst ore formation in the order Na>K>Mg>Sr>Mn>As>Zn>Ba>Al>Fe>Cu>Cd>Pb. (C) 1998 Canadian Institute of Mining, Metallurgy and Petrolem. Published by Elsevier Science Ltd. All rights reserved.
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