Tourmalines from the Koktokay No.3 pegmatite, Altai, NW China: spectroscopic characterization and relationships with the pegmatite evolution

Abstract

Mossbauer spectra (MS), infrared spectra (IR) in the OH stretching region, and reflectance spectra (RS) were obtained for natural tourmalines from the Koktokay No.3 pegmatite, the contact zone, and the altered country rock, Altai, NW China. Black Fe-rich dravite in the altered country rock and black Fe-rich dravite + schorl-foitite have the lowest degree of Fe 2+ ordering at Y-site and a high Fe 3+ /Fe 2+ ratio (0.35 and 0.30, respectively). Black elbaite-schorl in zones I to IV has a moderate degree of Fe 2+ ordering at Y-site and a low Fe 3+ /Fe 2+ ratio (0.11–0.18). Green elbaite in the inner pegmatite has the highest degree of Fe 2+ ordering at Y-site and contains no ferric iron. Tourmalines from the altered country rock, the contact zone, and zones I to IV of the pegmatite contain only OH − and no molecular water, whereas those from zones V to VIII of the pegmatite contain both OH − and molecular water. Variations of degrees of Fe 2+ ordering at Y-site of tourmaline crystal structure are generally consistent with the magmatic to hydrothermal evolution of the Koktokay No.3 pegmatite. However, the lowest degrees of Fe 2+ ordering at Y-site of tourmalines from the altered country rock and the contact zone are probably caused by rapid consolidation rates. The presence of molecular water in tourmalines from zones V to VIII reflects an increase of water fugacity during fractionation evolution of the pegmatite. The fact that molecular water is absent in tourmaline from the altered country rock and the contact zone suggests that there was a very low water fugacity during the interaction between fluids from the pegmatite and the country rock. The variation of Fe 3+ /Fe 2+ ratio suggests a decreasing oxidation state from the altered country rock across the contact zone through zones I–IV to zones V–VIII of the pegmatite, and generally consistent with previous investigations.