Abstract
Nineteen tourmaline samples of various provenances and geological settings were studied by EMPA, SREF and MS to represent the schorl-dravite compositional field. All samples belong to the Alkali group (except one with an X -site vacancy content of 0.53 apfu) and to the Oxy- and Hydroxy-subgroups. Among divalent cations, the main substitution involves Y Mg for Y Fe 2+ , to produce the two end-members dravite and schorl. Site populations were determined by a new minimization procedure that simultaneously accounts for both structural and chemical data. Results show that the crystals are characterized by disordered cation distribution between Y and Z sites: Al populates both sites, with a marked preference for the smaller Z octahedron; Mg is often equally distributed between Y and Z . Both Fe 2+ and Fe 3+ populate both Y and Z sites, but show a strong preference for Y . Specific mean bond distances (A) optimised for major elements are: Y Al-O = 1.908, Y Mg-O = 2.084, Y Fe 2+ -O = 2.139, z Al-O = 1.900, z Mg-O = 2.077 and z Fe 2+ -O = 2.131. In the schorl-dravite solid solution, structural variations appear to be primarily due to Y and Z interactions. These effects are conspicuous over the entire structure, as Y dimensions directly affect the a cell parameter, while Z is similarly correlated with c. The dimensions of Y and Z octahedra are determined by Al contents. Dimensional variations of Z are well described by its bond-distance variations, except for Z -07D. Both octahedra reciprocally interact, influencing their distortions: inverse correlations exist between Y dimension vs. Z quadratic elongation and Z dimension vs. Y quadratic elongation. As a common feature, the effects of the octahedral second coordination sphere are only confined to polyhedral distortions instead of dimensional variations, which only depend on site populations.
Published Version
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