THE CRYSTAL CHEMISTRY OF Si Al SUBSTITUTION IN TOURMALINE
DANIEL J. MacDONALD and FRANK C. HAWTHORNE
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2
Abstract
The structures of nine gem-quality crystals of V-bearing uvite, a 15.95, c 7.17 Å, V 1580 Å3, R3m,
have been refined to R indices of 2.2% using graphite-monochromated MoK X- radiation; the
crystals used for the X-ray data collection were analyzed using an electron microprobe. The
Si content of these crystals is significantly less than 6 atoms per formula unit; assignment
of [4]Al sufficient to fill the Si site results in a linear relationship between and [4]Al content. Examination
of recent results of structure refinement for tourmaline shows no well-defined relationship between and
constituent Z-cation radius. Conversely, there is a well-developed linear relationship between and constituent
Y-cation radius. Site-scattering refinement shows F to be strongly to completely ordered at the
O(1) site. There is no significant positional disorder at the O(1) or O(2) sites.
Keywords: tourmaline, uvite, crystal-structure refinement, site populations,
chemical analysis, Si Al substitution.