Hydrogen Bonding in Borcarite, an Unusual Borate-Carbonate Mineral

P. C. Burns* and F. C. Hawthorne
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
*Present address: Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK

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- 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: borcarite • crystal structure • hydrogen bond • borate • Russia

Mineralogical Magazine; June 1995 v. 59; no. 395; p. 297-304; DOI: 10.1180/minmag.1995.059.395.13
© 1995, The Mineralogical Society
Mineralogical Society (www.minersoc.org)