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Hydrogen bonding in borcarite, an unusual borate-carbonate mineral

Published online by Cambridge University Press:  05 July 2018

P. C. Burns  and
F. C. Hawthorne
P. C. Burns
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
F. C. Hawthorne
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
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Abstract

The crystal structure, including hydrogen positions, of borcarite, Ca4Mg[B4O6(OH)6](CO3)2, monoclinic, a = 17.840(4), b = 8.380(2), c = 4.445(1) Å, β = 102.04(3)°, V = 649.9(3) Å3, space group C2/m, has been refined by full-matrix least-squares methods to R = 2.5% and wR = 3.0% for 1020 unique observed [|F| ≥ 5σ(F)] reflections collected using Mo-Kα X-radiation. The H positions were located on difference-Fourier maps and were refined using the ‘soft’ constraint that O-H distances are ∼ 0.96 Å. The 4:4T FBB (fundamental building block) of the borcarite structure contains four Bϕ4 tetrahedra (ϕ = unspecified ligand) which share corners to form a four-membered polyhedral ring. Borcarite is the only mineral known to contain this FBB. The FBBs do not polymerize, but each shares three anions with an Mgϕ6 octahedron on either side, forming rods of composition [MgB4O6(OH)6]4- along the c-axis. The rigidity of these rods is enhanced by hydrogen bonding, and individual rods are connected through Caϕ8 polyhedra, Cϕ3 triangles and hydrogen bonds.

Keywords

borcaritecrystal structurehydrogen bondborateRussia
Type
Mineralogy
Information
Mineralogical Magazine , Volume 59 , Issue 395 , June 1995 , pp. 297 - 304
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1995

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Footnotes

*

Present address: Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK

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