Boralsilite (Al16B6Si2O37): a new mineral related to sillimanite from pegmatites in granulite-facies rocks

Abstract

Boralsilite, the first natural anhydrous Al-B-silicate, is a high-temperature phase in pegmatites cutting granulite- facies metapelitic rocks at Larsemann Hills, Prydz Bay, east Antarctica (type locality) and Almgjotheii in the contact aureole of the Rogaland Intrusive Complex, southwestern Norway. Stable assemblages include: (1) quartz-potassium feldspar-boralsilite-schorl/dravite (Larsemann Hills); (2) potassium feldspar-plagioclase(An22)-boralsilite-werdingite-dumoratierite-gra ndidierite (Almgjotheii); (3) quartz-potassium feldspar-boralsilite-dumortierite-andalusite ± sillimanite (Almgjotheii). Boralsilite is estimated to have formed between 600 and 750 °C and 3-5 kbar at conditions where PH2O v extremely weak, and γ∥b. It is monoclinic, space group C2/m with lattice parameters for Larsemann Hills of a = 14.767(1), b = 5.574(1), c = 15.079(1) Å, β = 91.96(1)°, V = 1240.4(1) Å3, Z = 2, and Dcalc = 3.07 g/cm3.