Crystal-chemical effects of heat treatment on Cr- and V-bearing Ca-dominant green tourmalines (fluor-uvite, magnesio-lucchesiite)

We present a detailed study of thermally driven changes in Ca-dominant tourmaline using Electron Probe Microanalysis (EPMA), powder X-ray diffraction (PXRD), Raman and optical absorption spectroscopy (OAS). The KEN samples from Kenya (5 crystal fragments) can be classified as fluor-uvite, while TAN from Tanzania (5 fragments) is dominantly magnesio-lucchesiite. Tourmaline samples were thermally treated in air at 700, 800, 900, and 1000 °C. Both sets were green with different hues, TAN with a bluer hue than KEN. After heating, no significant visual changes were observed in TAN samples until breakdown at 1000 °C. In KEN, after heating at 700 °C, the yellowish tint disappeared. Raman spectroscopy showed no significant changes after heating. Raman luminescence spectra revealed the sharp intensive band located at 680-684 nm, accompanied by a broad band between 690 and 900 nm in all samples heated up to 900 °C. OAS spectra look similar, with two major bands in blue and yellow to red regions. In TAN, the bands shift with the increasing Cr/V from 608 to 604 nm. In KEN with low Cr/V, the band is located between 613-617 nm. After heating at 1000 °C, both samples broke down with different associations: cordierite/indialite and glass from magnesio-lucchesiite, and cordierite/indialite, mullite-like phase, spinel and glass from fluor-uvite. There was a difference in volume between the samples; both samples expanded in volume, but the KEN sample expanded more with larger pores likely resulting from a higher proportion of volatile phase in this sample.