Bohseite, ideally Ca₄Be₄Si₉O₂₄(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 Ca₄Be₄Si₉O₂₄(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 mmlong) 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) ų, Z = 4. The crystal structure was refined to an R₁ 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 Ca₄Be₃AlSi₉O₂₅(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)Si⁴⁺ + ^T(3)Be²⁺→ ^O(2)O^2- + ^T(4)Al³⁺ + ^T(3)Si⁴⁺, and a general formula for the bavenite-bohseite minerals may be written as Ca₄Be_xSi₉Al_4-xO_28-x(OH)_x, where x ranges from 2-4 apfu: Ca₄Be₂Si₉Al₂O₂₆(OH)₂ (bavenite) to Ca₄Be₄Si₉O₂₄(OH)₄ (bohseite).