Abstract
Boralsilite, the first natural anhydrous Al-B-silicate, is a high-temperature phase in pegmatites cutting granulite-facies metapelitic rocks at Larsemami 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 feld- spar-boralsilite-schorl/dravite (Larsemaim Hills); (2) potassium feldspar-plagioclasefAn22)- boralsilite-werdingite-dunioitierite-grandidierite (Almgjotheii); (3) quartz-potassium feld- spar-boralsilite-dumortierite-andalusite ± sillimanite (Ahngjotheii). Boralsilite is estimated to have formed between 600 and 750 °C and 3-5 kbar at conditions where PH₂O < Ptot. The name is from the composition, boron, aluminum, and silicon. Representative electron and ion microprobe (SIMS) analyses of Larsemaim Hills are: SiO2 10.05 [12.67]; Al2O3 71.23 [69.15]; FeO 0.48 [1.10]; MgO below detection [0.23]; BeO 0.004 [0.094]; B2O3 19.63 [18.11] wt%, totals 101.39 [101.35] wt% where the numbers in brackets were determined from Almgjotheii material. However, the SIMS B2O3 values appear to be systematically too high; boron contents calculated assuming B + Si = 8 and O = 37 atoms per formula unit (apfu) yield B2O3 18.53 wt% corresponding to Fe0.08Al15.98B6.09Si1.91O37 ideally Al16B6Si2O37 for Larsemaim Hills. The analogous composition of Mg0.07Fe0.18- Al15. 66Be0.04B5.565Si2.435037 for Almgjotheii appears to result from solid solution of boralsilite with sillimanite (or Al8B2Si2O19) and subordinate werdingite. Boralsilite forms prisms up to 2 nun 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 λ = 589 nm, the Larsemaim Hills material has α = 1.629(1), β = 1.640(1), γ= 1.654(1), 2Vmeas = 81.8 (6), r > ν extremely weak, and γ||b. It is monoclinic, space group C2/m with lattice parameters for Larsemaim Hills, of a = 14.767(1), b= 5.574(1), c = 15.079 (1) Å, β= 91.96(1)°.F = 1240.4 (2) Å3, Z= 2. and Pcalo = 3.07 g/cm3.
© 2015 by Walter de Gruyter Berlin/Boston
