Occurrences of foitite and rossmanite from the Koktokay No. 3 granitic pegmatite dyke, Altai, northwestern China; a record of hydrothermal fluids

Two X-site-vacant tourmalines, foitite and rossmanite, are recorded from this spodumene-subtype pegmatite. Foitite-schorl crystals develop as fillings in the interstices among Fe-rich dravite crystals in the endocontact zone between the pegmatitic dyke and the metagabbro of the country rock. This tourmaline evolved in two stages, with Fe-rich dravite being followed by foitite-schorl; EPMA results are given. Dravite may develop as a result of pegmatite-derived fluids reacting with fluids from the country rock, whereas the formation of foitite-schorl is attributed mainly to pegmatite-derived fluids, with the occurrence of foitite reflecting the low amounts of Ca and Na in these fluids. In the 'cleavelandite'-spodumene zone, rossmanite occurs as veinlets within the main mass of elbaite crystals and exhibits a preferred orientation; this elbaite has high X-site vacancy perhaps caused by the buffering effect of a Na-rich phase, whereas the formation of rossmanite is more probably controlled by the lack of Na and Ca in the late hydrothermal fluids.