Abstract
The crystal structure of a dravite (tourmaline) sample from Osarara, Narok District, Kenya, a = 15.947(2), c = 7.214(1) Å, V = 1589.0(6) Å3, R3m, has been refined to an R index of 2.0% based on 1136 reflections measured with MoKα X-radiation. Electron microprobe analysis, site-scattering refinement, electronic absorption (published), stereochemical analysis, and Mcissbauer spectra show the structural formula to be . Of particular interest is the assignment of significant Mg to the Z site and Al to the Y site. For published structural refinements of tourmaline, the grand mean size of the Y and Z polyhedra is a linear function of the constituent cation radii. That is not the case for the Y and Z sites individually. However, greatly improved linearity occurs if significant Mg is assigned to the Z site for some compositions when Al > 6.0 atoms pfu; this indicates that the usual assumption that Al completely occupies the Z site before occupying any other site is not universally correct.