THE INCORPORATION OF BORON INTO THE VESUVIANITE STRUCTURE

Vesuvianite can contain significant amounts of boron (up to-4 wtTo B2O). The crystal stnrctures of five boron-bearing vesuvianite crystals have been refined using MoKo X-ray data. Although the crystals are significantly biaxiat (2y < 20o), very careful examination of the diffraction data shows no deviation from 4lmmm Laue symmetry. All crystals show many reflec-tions apparently violating the extinction critelrafor PLlwec symmetry, but these are due to double diffraction, not true Bragg diffraction; hence the space group PLlnnc was used for all of the refinements. The structure of all specimens of boron-bearing vesuvianite-(-[ca,Ln3+,Na]re[Al,Fe3+,Mg,Fe2+,Mn,Ti]13*r[Si118_yo68[o,oH,F]rc,)c> y,Z=2, a- 15.70, c- 11.75 A, V = 2895 Ar) was refined to an R index in the range 34c/o. Two n6w sites were identified and are common to all the refined structures; tlese are designated as Z sites. Z(1) is at the special position 8, (-0.05,-0.05, r/q), and is surrounded by a disordered anay of anions that can adopt a tetrahedral configuration around the central position. Z(2) is at the special position?a (V+, Va,7a) and is surrounded by an array of disordered anions that can adopt a triangular configuation about the cenfra