gsminmag Mineralogical Magazine Mineralogical Magazine 0026-461X Mineralogical Society 10.1180/minmag.1965.035.269.22 The Crystal Structure of Sinhalite1 Fang J. H. Newnham R. E. Laboratory for Insulation Research, Massachusetts Institute of Technology, Cambridge, Massachusetts, U.S.A.

Supported by the U.S. Air Force, Aeronautical Systems Division, under Contract AF 33 (616)-8353, and by Advanced Research Projects Agency, Department of Defense, through Contract SD-90.

 3 1965 35 269 196 199 5 6 1964 Copyright © 1965, The Mineralogical Society 1965 Summary

The Ceylonese gemstone sinhalite, AlMgBO4, is isomorphous with olivine and chrysoberyl. Aluminium and magnesium are oetahedrally coordinated and boron occupies slightly distorted tetrahedra. Because of size differences, the smaller Al ion can be assigned to the inversion symmetry sites, with Mg in mirror plane positions. Least-squares refinement based on single crystal X-ray data gave an accurate set of atomic coordinates with mean interatomic distances: Al-O 1·90, Mg-O 2·10, B-O 1·49 Å.