BORALSILITE (AL16B6SI2O37) - A NEW MINERAL RELATED TO SILLIMANITE FROM PEGMATITES IN GRANULITE-FACIES ROCKS

Boralsilite, the first natural anhydrous Al-B-silicate, is a high-temp erature phase in pegmatites cutting granulite-facies metapelitic rocks at Larsemann Hills, Prydz Bay, east Antarctica (type locality) and Al mgjotheii in the contact aureole of the Rogaland Intrusive Complex, so uthwestern Norway. Stable assemblages include: (1) quartz-potassium fe ldspar-boralsilite-schorl/dravite (Larsemann Hills); (2) potassium )bo ralsilite-werdingite-dumortierite-grandidierite (Almgjotheii); (3) qua rtz-potassium feldspar-boralsilite-dumortierite-andalusite +/- sillima nite (Almgjotheii). Boralsilite is estimated to have formed between 60 0 and 750 degrees C and 3-5 kbar at conditions where P-H2O < P-tot. Th e name is from the composition, boron, aluminum, and silicon. Represen tative electron and ion microprobe (SIMS) analyses of Larsemann Hills are: SiO2 10.05 [12.67]; Al2O3 71.23 [69.15]; FeO 0.48 [1.10]; MgO bel ow 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 fro m Almgjotheii material. However, the SIMS B2O3 values appear to be sys tematically too high; boron contents calculated assuming B + Si = 8 an d O = 37 atoms per formula unit (apfu) yield B2O3 18.53 wt% correspond ing to Fe0.08Al15.98B6.09Si1.91O37 ideally Al16B6Si2O37 for Larsemann Hills. The analogous composition of Mg0.07Fe0.18Al15.66Be0.04B5.565Si2 .435O37 for Almgjotheii appears to result from solid solution of boral silite with sillimanite (or Al8B2Si2O19) and subordinate werdingite. B oralsilite forms prisms up to 2 mm long parallel to b and 0.25 mm acro ss and is commonly euhedral in cross section. It is colorless and pris matic 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 > v extremely weak, and gam ma parallel to b. It is monoclinic, space group C2/m with lattice para meters for Larsemann Hills of a = 14.767(1), b = 5.574(1), c = 15.079 (1) Angstrom, beta = 91.96(1)degrees, V = 1240.4 (2) Angstrom(3), Z = 2, and D-calc = 3.07 g/cm(3).