Co-ordination of boron in sillimanite

Abstract

Ion-Microprobe analyses of six sillimanites associated with kornerupine show that the sillimanite can incorporate from 0.035 to 0.43 wt. % B_2O_3 (Grew and Hinthorne, 1983). Boron appears to substitute for silicon concomitantly with Mg substitution for Al such that the atomic Mg/B ratio is close to 0.5. This substitution results in a deficiency of cationic charge, which Grew and Hinthorne (1983) attributed to a submicroscopic rearrangement of the sillimanite structure involving loss of oxygen. A possible substitution scheme is 2(B + xMg) → 2(Si + xAl) + (1 + x)O, where x ≃ 0.5. In the present study, we have addressed the question of co-ordination of boron in sillimanite. As boron can occur in trigonal or tetrahedral coordination with oxygen, there is no compelling reason that B substitution for Si implies tetrahedral co-ordination for B.

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