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 Mo X-radiation. Electron microprobe analysis, site-scattering refinement, electronic absorption (published), stereochemical analysis, and Mcissbauer spectra show the structural formula to be x(Na0.814Ca0.009K0.0140.163)y(Mg1.301Mn0.003Fe0.0512+Fe0.5603+Cr0.006Ti0.030Al0.985)z(Al5.090Mg0.910)(BO3)3Si6O18(O,OH)4. 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.

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