The F-analogue of schorl from Grasstein, Trentino South Tyrol, Italy: crystal structure and chemistry
Ertl, Andreas; Kolitsch, Uwe; Prowatke, Stefan; Dyar, M. Darby; Henry, Darrell J.
European Journal of Mineralogy Volume 18 Number 5 (2006), p. 583 - 588
published: Sep 1, 2006
ArtNo. ESP147051805006, Price: 29.00 €
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(Fe2+1.89Al0.58Fe3+0.13Mn2+0.13Ti4+0.02Mg0.02Zn0.02□0.21) Z(Al5.74Fe3+0.26) T(Si5.90Al0.10O18) (BO3)3 V(OH)3 W[F0.76(OH)0.24]; a = 15.997(2), c = 7.179(1) Å, V = 1591.0(4) Å3, 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: NaFe2+3Al6Si6O18 (BO3)3(OH)3F. The relatively high amount of Fe2+ at the Y site is consistent with the large distance of 2.056 Å. Refinement of the F:O 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 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 Fe 3+ in the tourmaline structure.