Bohseite, ideally Ca4Be4Si9O24(OH)4; from the Pilawa Gorna Quarry, the Gory Sowie Block, SW Poland

• Published in: Mineralogical magazine 2017-02, Vol.81 (1), p.35-46
• Main Authors: Szeleg, E, Zuzens, B, Hawthorne, Frank C, Pieczka, A, Szuszkiewicz, A, Turniak, K, Nejbert, K, Ilnicki, S. S, Friis, H, Makovicky, E, Weller, M. T, Lemee-Cailleau, M. H
• Format: Article
• Language: English
• Subjects: alkali feldspar; alkaline earth metals; aluminosilicates; anatexite; bavenite; bertrandite; beryllium; bohseite; Central Europe; chain silicates; chemical composition; coexisting minerals; crystal structure; crystal-structure refinement; dikes; Dolnoslaskie Poland; electron probe data; electron-microprobe analysis; end-member; Europe; feldspar group; formula; framework silicates; granites; helvite; igneous rocks; infrared spectra; infrared spectrum; intrusions; lattice parameters; lepidolite; metals; metamorphic rocks; mica group; microcline; migmatites; Mineralogy; nesosilicates; orthosilicates; pegmatite; phenakite; phenakite group; physical properties; Pilawa Gorna Quarry; plutonic rocks; Poland; refinement; sheet silicates; silicates; sodalite group; solid solution; sorosilicates; Sowie Mountains; spectra; Sudeten Mountains; sulfides; Wroclaw Poland; X-ray diffraction data
• ISSN: 0026-461X; 1471-8022
• DOI: 10.1180/minmag.2016.080.066

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, ∼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 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) A, V = 2227.7(4) A3, 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)O2- + T(4)Al3+ + T(3)Si4+, and a general formula for the bavenite-bohseite minerals may be written as Ca4BexSi9Al4-xO28-x(OH)x, where x ranges from 2-4 apfu: Ca4Be2Si9Al2O26(OH)2 (bavenite) to Ca4Be4Si9O24(OH)4 (bohseite).