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

The F-analogue of schorl has been identified in a pegmatite at Grasstein, Trentino. The crystals have a maximum size of approximately 1 mm on quartz crystals and are pale brownish to pale greyish blue in colour; they have been characterized by single-crystal structure refinement (R 1.6%), Moessbauer spectroscopy and EPMA, yielding the structural formula (super X) (Na (sub 1.78) K (sub 0.01) [] (sub 0.21) ) (super Y) (Fe (super 2+) (sub 1.89) Al (sub 0.58) Fe (super 3+) (sub 0.13) Mn (super 2+) (sub 0.13) Ti (sub 0.02) Mg (sub 0.02) Zn (sub 0.02) [] (sub 0.21) ) (super Z) (Al (sub 5.74) Fe (super 3+) (sub 0.26) ) (super T) (Si (sub 5.90) Al (sub 0.10) O (sub 18) )(BO (sub 3) ) (sub 3) (super V) (OH) (sub 3) (super W) [F (sub 0.76) (OH) (sub 0.24) ], with a 15.997, c 7.179 Aa, V 1591.0 Aa (super 3) . This tourmaline, rich in F and Fe (super 2+) , is a pneumatolytic phase crystallized in the presence of a F-rich fluid (coexisting with fluorite). It is very near to the proposed end-member composition (super ) for the F-analogue of schorl NaFe (super 2+) (sub 3) Al (sub 6) Si (sub 6) O (sub 18) (BO (sub 3) ) (sub 3) (OH) (sub 3) F. It is likely that the occurrence of high amounts of F in Fe-rich tourmalines requires a significant percentage of Fe (super 3+) in the tourmaline structure.