The F-analogue of schorl from Grasstein, Trentino - South Tyrol, Italy: Crystal structure and chemistry

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The F-analogue of schorl has been identified in samples from a pegmatite at Grasstein, Trentino-South Tyrol, Italy. The crystal chemistry of this tourmaline has been characterized by a combination of single-crystal structure refinement, chemical analysis, and Mössbauer spectroscopy, yielding the structural formula x(Na0.78K0.01 0.21) y(Fe 1.892+AI0.58Fe0.133+Mn0.132+Ti0.024+Mg 0.02Zn0.020.21Z(Al 5.74)Fe0.263+T(Si5.90 Al0.10O18) (BO 3)3 v(OH)3 wIF0.76(OH) 0.24]; a = 15.997(2), c = 7.179(1) Å,' V= 1591.0(4) Å3, R\{F) = 1.60 %. This F-rich and Fe2+-rich tourmaline, a pneumatolytic phase crystallized in the presence of a F-rich fluid (coexisting with fluorite), is very near the proposed end-member composition of the F-analogue of schorl: NaFe32+Al6Si6O 18(BO0.3)3(OH)3F. The relatively high amount of Fe2+ at the Y site is consistent with the large distance of 2.056 A. Refinement of the F:0 occupancy ratio at the W site yields F0.8O0.2 pfu, consistent with the chemical data (F0.76 apfu). Because of the local bonding of the IF-site anion to three neighbouring y-site cations and the X-site cation, the charge of the A"-site cation should affect the F occupancy at the W site. The cation and anion occupancy of this tourmaline is consistent with observations that tourmalines not dominated by X-site vacancies can have high F concentrations in the IF site if F is present in the coexisting fluid phase. It is thus likely that the occurrence of high amounts of F in Fe-rich tourmalines requires a significant percentage of Fe3+ in the tourmaline structure. © 2006 E. Schweizerbart'sche Verlagsbuchhandlung.

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European Journal of Mineralogy

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