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

Abstract

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 Mossbauer spectroscopy, yielding the structural formula X (Na 0.78 K 0.01 □ 0.2l ) Y (Fe 2+ 1.89 Al 0.58 Fe 3+ 0.13 Mn 2+ 0.13 Ti 4+ 0.02 Mg 0.02 Zn 0.02 □ 0.21 ) Z (Al 5.74 Fe 3+ 0.26 ) T (Si 5.90 Al 0.10 O 18 ) (BO 3 ) 3 V (OH) 3 W [F 0.76 (OH) 0.24 ]; a = 15.997(2), c = 7.179(1) A, V = 1591.0(4) A 3 , R 1( F ) = 1.60 %. This F-rich and Fe 2+ -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: NaFe 3 2+ Al 6 Si 6 O 18 (BO 3 ) 3 (OH) 3 F. The relatively high amount of Fe 2+ at the Y site is consistent with the large distance of 2.056 A. Refinement ofthe F:O occupancy ratio at the W site yields F 0.8 O 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 Fe 3+ in the tourmaline structure.