Bohseite, ideally Ca 4 Be 4 Si 9 O 24 (OH 4 , from the Piława Górna quarry, the Góry Sowie Block, SW Poland

Bohseite is an orthorhombic calcium beryllium aluminosilicate with variable Al content and an endmember formula Ca 4 Be 4 Si 9 O 24 (OH 4 ), that was discovered in the Piława Górna quarry in the eastern part of the Góry Sowie Block, ∼50 km southwest of Wrocław, SW Poland. It occurs in a zoned anatectic pegmatite dyke in close association with microcline, Cs-rich beryl, phenakite, helvite, 'lepidolite', probably bertrandite and unidentified Be-containing mica as alteration products after a primary Be mineral, probably beryl. Bohseite forms fan-like or parallel aggregates (up to 0.7 cm) of white, platy crystals (up to 2 mm long) with characteristic striations. It is white with a white streak, is translucent and has a vitreous lustre; it does not fluoresce under ultraviolet light. The cleavage is perfect on {001} and fair on {010}, and neither parting nor twinning was observed. Bohseite is brittle with a splintery fracture and Mohs hardness is 5–6. The calculated density is 2.719 g cm –3 . The indices of refraction are α= 1.579, β = 1.580,γ = 1.597, all ±0.002; 2Vobs = 24(3)°, 2Vcalc = 27°; the optic orientation is as follows: X ^ a = 16.1°, Y ^ b = 16.1°, Z // c Bohseite shows orthorhombic diffraction symmetry, space group Cmcm , a = 23.204(6), b = 4.9442(9), c = 19.418(6) Å, V = 2227.7(4) Å 3 , Z = 4. The crystal structure was refined to an R 1 value of 2.17% based on single-crystal data, and the chemical composition was determined by electron-microprobe analysis. Bohseite is isostructural with bavenite. Bohseite was originally approved with an end-member composition of Ca 4 Be 3 AlSi 9 O 25 (OH) 3 , but subsequent discovery of compositions with Be > 3.0 apfu led to redefinition of its end-member composition, holotype sample and locality, as reported here. There is extensive solid solution in bavenite–bohseite according to the scheme O(2) OH – + T (4) Si 4+ + T (3) Be 2+ ↔ O(2) O 2– + T (4) Al 3+ + T (3) Si 4+ , and a general formula for the bavenite–bohseite minerals may be written as Ca 4 Be x Si 9 Al 4–x O 28–x (OH) x , where x ranges from 2–4 apfu: Ca 4 Be 2 Si 9 Al 2 O 26 (OH) 2 (bavenite) to Ca 4 Be 4 Si 9 O 24 (OH) 4 (bohseite).