Boralsilite (Al ₁₆ B ₆ Si ₂ O ₃₇ ); a new mineral related to sillimanite from pegmatites in granulite-facies rocks

University of Maine, Department of Geological Sciences, Orono, ME, United States

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 (An ₂₂ )-boralsilite-werdingite-dumortierite -grandidierite (Almgjotheii); (3) quartz-potassium feldspar-boralsilite-dumortierite-andalusite+ or -sillimanite (Almgjotheii). Boralsilite is estimated to have formed between 600 and 750 degrees C and 3-5 kbar at conditions where P _H2O

tot . The name is from the composition, boron, aluminum, and silicon. Representative electron and ion microprobe (SIMS) analyses of Larsemann Hills are: SiO ₂ 10.05 [12.67]; Al ₂ O ₃ 71.23 [69.15]; FeO 0.48 [1.10]; MgO below detection [0.23]; BeO 0.004 [0.094]; B ₂ O ₃ 19.63 [18.11] wt%, totals 101.39 [101.35] wt% where the numbers in brackets were determined from Almgjotheii material. However, the SIMS B ₂ O ₃ values appear to be systematically too high; boron contents calculated assuming B+Si = 8 and O = 37 atoms per formula unit (apfu) yield B ₂ O ₃ 18.53 wt% corresponding to Fe (sub 0.08) Al (sub 15.98) B (sub 6.09) Si (sub 1.91) O ₃₇ ideally Al ₁₆ B ₆ Si ₂ O ₃₇ for Larsemann Hills. The analogous composition of Mg (sub 0.07) Fe (sub 0.18) -Al (sub 15.66) Be (sub 0.04) B (sub 5.565) Si (sub 2.435) O ₃₇ for Almgjotheii appears to result from solid solution of boralsilite with sillimanite (or Al ₈ B ₂ Si ₂ O ₁₉ ) and subordinate werdingite. Boralsilite forms prisms up to 2 mm long b and 0.25 mm across and is commonly euhedral in cross section. It is colorless and prismatic cleavage is fair. Optically, it is biaxial (+); at lambda = 589 nm, the Larsemann Hills material has alpha = 1.629(1), beta = 1.640(1), gamma = 1.654(1), 2V _meas = 81.8(6), r>nu extremely weak, and gamma 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) Aa, beta = 91.96(1) degrees , V = 1240.4 (2) Aa ³ , Z = 2, and D _calc = 3.07 g/cm ³ .

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