Crystal Structure of KBSi3O8 Isostructural with Danburite

Mitsuyoshi Kimata
Institute of Geoscience, University of Tsukuba, Ibaraki 305, Japan

Abstract: The crystal structure of KBSi3O8 (orthorhombic, Pnam, with a = 8.683(1), b = 9.253(1), c = 8.272(1) Å, V = 664.4(1) Å3, Z = 4) has been determined by the direct method applied to 3-dimensional reflection data. The structure of a microcrystal with the dimensions 20 × 29 × 37 µm was refined to an unweighted residual of R = 0.031 using 386 non-zero structure amplitudes. KBSi3O8 adopts a structure essentially different from reedmergneritc NaBSi3O8, with the low albite (NaAlSi3O8) structure, and isotypic with danburite CaB2Si2O8 which has the same topology as paracelsian BaAl2Si2O8. The chemical relationship between this sample and danburite gives insight into a new coupled substitution; K+ + Si4+ = Ca2+ + B3+ in the extraframework and tetrahedral sites. The present occupancy refinement revealed partial disordering of B and Si atoms which jointly reside in two kinds of general equivalent points, T(1) and T(2) sites. Thus the expanded crystal-chemical formula can be written in the form K(B0.44Si0.56)2(B0.06Si0.94)2O8.

The systematic trend among crystalline compounds with the M+T3+T4+3O8 formula suggests that they exist in one of four structural types; the feldspar structures with T3+/T4+ ordered and/or disordered forms, and the paracelsian and the hollandite structures.

Keywords: danburite • crystal structure • feldspar • paracelsian • hollandite • reedmergnerite

Mineralogical Magazine; March 1993 v. 57; no. 386; p. 157-164; DOI: 10.1180/minmag.1993.057.386.15
© 1993, The Mineralogical Society
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