Crystal structure of KBSi30g isostructural with danburite
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چکیده
The crystal structure of KBSi,Os (orthorhombic, Pnam, with a = 8.683(1), b = 9.253(1), c = 8.272(1) A, V = 664.4(1) A3:Z = 4) has been determined by the direct method applied to 3dimensional reflection data. The structure of a microcrystal with the dimensions 20 x 29 x 37 ~m was refined to an unweighted residual of R = 0.031 using 386 non-zero structure amplitudes. KBSi30s adopts a structure essentially different from reedmergnerite NaBSi,Os, with the low albite (NaAlSi30s) structure, and isotypic with danburite CaB2Si20s which has the same topology as paracelsian BaAI2Si20s. 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 Band Si atoms which jointly reside in two kinds of general equivalent points, T(I) and T(2) sites. Thus the expanded crystal-chemical formula can be written in the form K(Bo.44Sio.s6h(Bo.o6Sio.94hOs. The systematic trend among crystalline compounds with the M+ T3+r+ 30S 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,
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