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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-plagioclasefAn 22 )- 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 P H₂O  &lt; P tot . The name is from the composition, boron, aluminum, and silicon. Representative electron and ion microprobe (SIMS) analyses of Larsemaim 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 0.08 Al 15.98 B 6.09 Si 1.91 O 37  ideally Al 16 B 6 Si 2 O 37  for Larsemaim Hills. The analogous composition of Mg 0.07 Fe 0.18 - Al 15. 66 Be 0.04 B 5.565 Si 2.435 0 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 nun long || b&gt; 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), 2V meas  = 81.8 (6), r &gt; 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) Å 3 , Z= 2. and P calo  = 3.07 g/cm 3 .

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