Mössbauer and optical spectrometry of selected schörl-dravite tourmalines

Abstract

Spectrometric studies were carried out on samples of tourmaline (schorl-dravite series) from geological environments where first-phase-formed tourmaline underwent influence of geochemically different fluids. Samples are from a differentiated magmatic complex of Trento-Alto Adige, Italy, and from hydrothermal gold and silver deposits of the Humboldt Range, Nevada, USA. Chemical data were obtained from electron microprobes. The results of Mossbauer measurements suggest three to five doublets. Fe occurs in two valence states. The Z-site, usually fulfilled with Al3+ and Fe3+, is assigned only to Al3+ and Fe2+. This location was found in nearly all samples studied. In the Y-site Fe2+ and Fe3+ are obviously present. Isomer shifts with intermediate values can be assumed to be related to intervalence charge transfer (IVCT). Optical spectroscopy reveals absorption bands at 9 000 and 14000 cm−1, which are assigned to a charge transfer between Fe2+ and Fe3+, the 23 000 cm−1 absorption band is supposed to be due to Fe2+ → Ti4+ charge transfer. The occupation of the Z-site only by Fe2+ and the coexistence of divalent and trivalent Fe in the Y-site could be explained by selective oxidation in Y-site through a late process.