Tourmaline; structural formula calculations

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Geography and Geology


Chemical analyses for tourmaline are often lacking in values for the light elements (H, Li, Be, B), valence states of the transition metals and problems with the heterogeneity of the crystals. The normalization procedures for tourmaline-group minerals have been compared to determine which schemes provide the most accurate formula in those cases where there is a lack of information. The normalization on the basis of 31 anions, assuming OH values if necessary, produces a more useful formula than schemes that ignore F or B. Iteration of Li may give a first approximation of Li content; Li (sub 2) O may be added to the calculations until Y + Z + T = 15 pfu. Care should be taken with all assumptions; the common assumption that all iron is ferrous and that OH + F = 4 pfu can result in the misidentification of buergerite as "fluor-schorl". Ultimately, as many data should be collected as possible, including information on the structure, Fe (super 3+) /Fe (super 2+) value, and Li and H contents.

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