TOURMALINE: STRUCTURAL FORMULA CALCULATIONS

The results of chemical analyses of tourmaline are usually incomplete owing to difficulties in dealing with the light elements, valence states of transition metals, and problems with the heterogeneity of crystals. Where there are undetermined components, assumptions must be made in order to develop a structural formula. The normalization procedures for tourmaline-group minerals have been compared to determine which schemes provide the most accurate formula in cases where there is a lack of information. The normalization on 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; Li2O may be added to the calculations until Y + Z + T = 15 apfu . Care should be taken with all assumptions; the common assumption that all iron is ferrous and OH + F = 4 apfu can result in the misidentification of buergerite as “fluor-schorl”. Ultimately, as many data should be collected as possible, including information on the structure, Fe3+/Fe2+ value, and Li and H contents.