Search

Se-bearing polybasite- Tac is associated with galena, chalcopyrite, sphalerite, tetrahedrite, electrum and quartz at the Martha mine, an epithermal silver–gold deposit located adjacent to the Deseado Massif, Santa Cruz province, Argentina. Three samples, with variable chemical composition and showing the 111 unit-cell type, were studied by means of X-ray single crystal diffraction and electron microprobe. The unit-cell parameters were modeled using a multiple regression method as a function of the Ag, Sb, and Se contents. The predicted values resulted in excellent agreement with experimental unit-cell parameters. We observed that high contents of selenium in polybasite are associated with relatively low copper contents. This finding corroborates previous studies that the copper content of pearceite–polybasite group minerals can be very low if selenium is present. [ABSTRACT FROM AUTHOR]

Coarse-grained xenoliths of hibonite + grossite + Mg-Al-V spinel from Cretaceous pyroclastic rocks on Mt. Carmel, N. Israel, and from Sierra de Comechingones, Argentina, include spherules, rods and dense branching structures of native vanadium and V Al alloys. Microstructures suggest that vanadium melts became immiscible with the host Ca-Al-Mg-Si-O melt, and nucleated as droplets on the surfaces of the oxide phases, principally hibonite. Many extended outward as rods or branching structures as the host oxide crystal grew. The stability of V0 implies oxygen fugacities ≥9 log units below the Iron-Wustite buffer, suggesting a hydrogen-dominated atmosphere. This is supported by wt%-levels of hydrogen in gasses released by crushing, by Raman spectroscopy, and by the presence of VH 2 among the vanadium balls. The oxide assemblage formed at 1400–1200 °C; the solution of hydrogen in the metal could lower the melting point of vanadium to these temperatures. These assemblages probably resulted from reaction between differentiated mafic melts and mantle-derived CH 4 + H 2 fluids near the crust-mantle boundary, and they record the most reducing magmatic conditions yet documented on Earth. • Hibonite-grossite-vanadium xenoliths in basalt record Earth's most reducing conditions. • Vanadium (V0) melts were immiscible with Ca-Al-Mg-Si oxide melts at T > 1200 °C. • Melting T of V0 was lowered by solution of H 2 ; VH 2 phase coexists with V0. • V0 "dendrites" contain ca 3% void spaces, developed by expulsion of H 2 on cooling. • Raman spectra suggest that some H 2 diffused into host hibonite. [ABSTRACT FROM AUTHOR]