Polyhedron Distortions in Tourmaline

Distortion parameters [� , � 2 , DI(Y‐O), DI(O‐ Y‐O), DI(O‐O)] have been calculated for cation polyhedra in tourmalines of different chemical compositions. Tourmalines with greater amounts of small, highly charged ions in the YO6 octahedron display greater bond-length distortion in Y. The size and charge of the occupants at the Y site have been included in an empirically determined formula intended to evaluate the bond-length distortion of the YO6 octahedron. This equation demonstrates that the bond-length distortion in the YO6 octahedron is a function of the size and charge of the Y-site occupants, and is predictable from those occupants (r = 0.972 for observed versus calculated distortion). The distortion of the Z octahedron in a tourmaline is largely a function of the of that tourmaline [r = ‐0.985 for all tourmalines where O3 is occupied by 3 (OH) apfu], although the occupant of the O3 site also affects that distortion. The bond-length distortion of the TO4 tetrahedron is small, but examination of the angle distortion of the tetrahedron demonstrates a strong covariance with the charge at the X site (r = 0.892). Finally, distortions in the XO9 polyhedron were found to correlate with the charge of the occupants at the Y site and the fluorine content ( r = 0.881).