- © 2016 E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers
Axinite is an exotic accessory mineral in granitic pegmatites because conditions favorable for its formation are seldom reached. The compositional ranges of axinite from granitic pegmatites are close to the axinite-(Mn) and the axinite-(Fe) end-members. We report data on the composition of axinite-(Mn) and the formation conditions of axinite-containing mineral assemblages in miarolitic granitic pegmatites of the Malkhan gem-tourmaline deposit. Axinite-(Mn) occurs only in the pockets in which Li-Al tourmaline (rubellite) is either absent or transformed into an aggregate of asbestiform tourmaline. Axinite-(Mn) is associated with quartz, orthoclase, adularia and laumontite. The pockets may also contain dark, corroded Mn-Li-Al tourmaline and colorless or pale-pink beryl. The mineral association with axinite-(Mn) was formed from low-salinity (1.6–10.5 wt% NaCl-eq.), B-rich fluids (up to 13 wt% H3BO3) of high alkalinity in the temperature interval from < 415°С (maximum temperature of adularia formation) to <250–230°C (upper limit of laumontite stability) and at less than 1.5 kbar fluid pressure. The axinite-(Mn) contains high concentrations of BeO (0.58–0.69 wt%) and the main trace elements are REE, Sn, Nb, Ta, Hf, Th and U. The axinite-(Mn) + adularia ± laumontite association is an indicator of unfavorable conditions for the occurrence of gem tourmalines in miarolitic granitic pegmatites.