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Bohseite, ideally Ca4Be4Si9O24(OH4, from the Piława Górna quarry, the Góry Sowie Block, SW Poland

Published online by Cambridge University Press:  02 January 2018

E. Szełęg
Affiliation:
Department of Geochemistry, Mineralogy and Petrography, University of Silesia, Będzińska 60, 41-200 Sosnowiec, Poland
B. Zuzens
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
F. C. Hawthorne*
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
A. Pieczka
Affiliation:
Department of Mineralogy, Petrography and Geochemistry, AGH University of Science and Technology, Mickiewicza 30, 30-059 Kraków, Poland
A. Szuszkiewicz
Affiliation:
University of Wrocław, Institute of Geological Sciences, Cybulskiego 30, 50-205 Wrocław, Poland
K. Turniak
Affiliation:
University of Wrocław, Institute of Geological Sciences, Cybulskiego 30, 50-205 Wrocław, Poland
K. Nejbert
Affiliation:
Institute of Geochemistry, Mineralogy and Petrology, University of Warsaw, Żwirki and Wigury 93, 02-089 Warszawa, Poland
S. S. Ilnicki
Affiliation:
Institute of Geochemistry, Mineralogy and Petrology, University of Warsaw, Żwirki and Wigury 93, 02-089 Warszawa, Poland
H. Friis
Affiliation:
Natural History Museum, University of Oslo, Postboks 1172, Blindern, 0318 Oslo, Norway
E. Makovicky
Affiliation:
Department of Geosciences and Natural Resources, University of Copenhagen, Rolighedsvej 23, 1958 Frederiksberg, Denmark
M. T. Weller
Affiliation:
Department of Chemistry, University of Bath, Bath BA2 7AY, UK
M.-H. Lemée-Cailleau
Affiliation:
Institut Laue-Langevin, B.P.156, 6 rue Jules Horowitz, 38042 Grenoble Cedex 9, France

Abstract

Bohseite is an orthorhombic calcium beryllium aluminosilicate with variable Al content and an endmember formula Ca4Be4Si9O24(OH4), 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) Å3, 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).

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2017

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