THE INCORPORATION OF BORON INTO THE VESUVIANITE STRUCTURE

Vesuvianite can contain significant amounts of boron (up to similar to 4 wt% B2O3). The crystal structures of five boron-bearing vesuvianite crystals have been refined using MoK alpha X-ray data. Although the c rystals are significantly biaxial (2V < 20 degrees), Very careful exam ination of the diffraction data shows no deviation from 4/mmm Laue sym metry. All crystals show many reflections apparently violating the ext inction criteria for P4/nnc symmetry, but these are due to double diff raction, not true Bragg diffraction; hence the space group P4/nnc was used for all of the refinements. The structure of all specimens of bor on-bearing vesuvianite (similar 3+,Mg,Fe2+,Mn,Ti](13+x)[Si]O-18-y(68)[ O,OH,F](10), x > y, Z = 2, a approximate to 15.70, c approximate to 11 .75 Angstrom, V approximate to 2895 Angstrom(3)) was refined to an R i ndex in the range 3-4%. Two new sites were identified and are common t o all the refined structures; these are designated as T sites. T(1) is at the special position 8h (similar to 0.05, similar to 0.05, 1/4), a nd is surrounded by a disordered array of anions that can adopt a tetr ahedral configuration around the central position. T(2) is at the spec ial position 2a (1/4, 1/4, 1/4) and is surrounded by an array of disor dered anions that can adopt a triangular configuration about the centr al position. The T(1) position is very close to the H(1) position in b oron-free vesuvianite, and the T(2) position is very close to the H(2) position in boron-free vesuvianite; hence occupancy of T(1) and T(2) precludes occupancy of two H(1) and the H(2) positions, in agreement w ith the substitution B + Mg reversible arrow 2H + Al proposed by Great et al. (1992) for boron-bearing vesuvianite.