Co-ordination of boron in sillimanite

Edward S. Grew
Institut für Mineralogie, Ruhr-Universität Bochum
Bochum 1, West Germany

George R. Rossman
Division of Geological and Planetary Sciences, 170-25, California Institute of Technology,
Pasadena, California 91125, U.S.A.

ABSTRACT

Previous ion-microprobe analysis of six sillimanites associated with kornerupins show that the sillimanite can incoroprate from 0.035 to 0.43 wr% B₂O₃. Boron appears to substitute for silicon concomitantly with Mg substitution for Al such that the Mg/B ration is close to 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. Infrared spectra provide features between 1372 and 1128 cm^-1 that are consistent with boron in threefold co-ordination.