The B2Si8O30 groups of tetrahedra in axinite and comments on the deformation of Si tetrahedra in silicates

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Abstract
The crystal structure of axinite, H(Fe,Mn)Ca2Al2BSi4O16, has been redetermined and refined from three-dimensional, equi-inclination diffractometer intensity data. The crystal used in this study came from Woodlake, California, and has an Fe:Mn ratio close to 1:1. Its unit-cell constants are: a = 7.1566(15), b = 9.1995(20), c = 8.959(2) Å, α = 91.8(8)°, β = 98.14(02)°, γ = 77.30(02)° and its space group is P[unk]. The final R value is 0.042. The most noteworthy features of the axinite structure include: (1) the B2Si8O30 group in which four pairs of Si tetrahedra are connected by two B tetrahedra into a novel group of tetrahedra; (2) the finite octahedral chains of four Al and two Fe(Mn) which are, in turn, connected by elongated Ca antiprisms into sheets; (3) the B2Si8O30 tetrahedral groups which link these octahedral sheets together; and (4) the presence of a hydrogen bond between two oxygen atoms, one of which is bonded to a silicon atom. The distortions of the Si tetrahedra in axinite are examined by plots of tetrahedral-edge length versus the mean Si–O distances of the two oxygen atoms of the edge. The edges between bridge and nonbridge oxygen atoms were found to be invariably the shortest. Similar charts were prepared for several pyrosilicates, pyroxenoids, clinopyroxenes and clinoamphiboles. The relationship between the ratios of (O–O):(Si–O) length and types of oxygen is discussed as measures of tetrahedral distortion.

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