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 end-member formula Ca4Be4Si9O24(OH)(4), that was discovered in the Pilawa Gorna quarry in the eastern part of the Gory Sowie Block, similar to 50 km southwest of Wroclaw, 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 alpha= 1.579, beta= 1.580, gamma= 1.597, all +/- 0.002; 2Vobs = 24(3)degrees, 2Vcalc = 27 degrees; the optic orientation is as follows: X <^> a= 16.1 degrees, Y <^> b = 16.1 degrees, Z // c. Bohseite shows orthorhombic diffraction symmetry, space group Cmcm, a = 23.204(6), b = 4.9442(9), c = 19.418(6) angstrom, V = 2227.7(4) angstrom(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 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 (OH)-O-O(2) + Si-T(4)(4) (broken vertical bar) + Be-T(3)(2) (broken vertical bar) <-> O-O(2)(2) + Al-T(4)(3+) + Si-T(3)(4+), and a general formula for the bavenite-bohseite minerals may be written as Ca4BexSi9 Al4- xO28-x(OH)(x), where x ranges from 2-4 apfu: Ca4Be2Si9Al2O26(OH)(2) (bavenite) to Ca4Be4Si9O24(OH)(4) (bohseite).