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

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(Na_0.78K_0.01□_0.2l) ^Y(Fe²⁺_1.89Al_0.58Fe³⁺_0.13Mn²⁺_0.13Ti⁴⁺_0.02Mg_0.02Zn_0.02□_0.21) ^Z(Al_5.74Fe³⁺_0.26) ^T(Si_5.90Al_0.10O₁₈) (BO₃)₃^V(OH)₃W[F_0.76(OH)_0.24]; a = 15.997(2), c = 7.179(1) Å, V = 1591.0(4) ų, R1(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⁺Al₆Si₆O₁₈(BO₃)₃(OH)₃F. The relatively high amount of Fe²⁺ at the Y site is consistent with the large <Y-O> distance of 2.056 Å. Refinement ofthe F:O occupancy ratio at the W site yields F_0.8O_0.2 pfu, consistent with the chemical data (F_0.76 apfu). Because ofthe local bonding ofthe W-site anion to three neighbouring Y-site cations and the X-site cation, the charge of the X-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 W 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.