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Crystal chemistry of the schorl-dravite series

Dipartimento di Scienze della Terra, Università di Roma "La Sapienza", P.le A. Moro 5, 1–00186 Roma, Italy

^* E-mail: sergio.lucchesi{at}uniromal.it

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 ^YMg for ^YFe²⁺, 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 Fe²⁺ and Fe³⁺ populate both Y and ^Z sites, but show a strong preference for Y. Specific mean bond distances (Å) optimised for major elements are: ^YAl-O = 1.908, ^YMg-O = 2.084, ^YFe²⁺-O = 2.139, ^zAl-O = 1.900, ^zMg-O = 2.077 and ^zFe²⁺-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.

Key-words: Crystal structure, crystal-chemistry, disorder, schorl, dravite, tourmaline.

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