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 Mg-Y for Fe-Y(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 Fe2+ and Fe3+ populate both Y and Z sites, but show a strong preference for Y. Specific mean bond distances (A) optimised for major elements are: Al-Y-O = 1.908, Mg-Y-O = 2.084, Fe-Y(2+)-O = 2.139, Al-Z-O = 1.900, Mg-Z-O = 2.077 and Fe-Z(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-O7D. 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.
Crystal chemistry of the schorl-dravite series / Bosi, Ferdinando; Lucchesi, Sergio. - In: EUROPEAN JOURNAL OF MINERALOGY. - ISSN 0935-1221. - STAMPA. - 16:2(2004), pp. 335-344. [10.1127/0935-1221/2004/0016-0335]