| Tourmaline is a complex borosilicate with the general formula: [9]X [6]Y3 [6]Z6 [4]T 6 O18 ([3]BO3) 3 V3 W; it is common as an accessory mineral throughout the rock cycle. Due to complexities in the chemical composition, including the presence of the light elements B, H and (often) Li, transition-metals in multiple valence states, and chemical heterogeneity, tourmaline is not as well understood as other common accessory minerals.;Tourmaline normalization procedures have been compared to determine which schemes provide the most accurate mineral formula when there is undetermined information. 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.;A number of potential new tourmaline species have been described, including "fluordravite", ideally Na Mg3 Al6 (Si 6O18) (BO3)3 (OH)3 F; "fluorschorl", ideally Na Fe2+3 Al6 (Si6O 18) (BO3)3 (OH)3 F; "fluorelbaite", ideally Na (Li1.5Al1.5) Al6 (Si6O 18) (BO3)3 (OH)3 F; "hydroxyuvite", ideally Ca Mg3 (Al5Mg) (Si6O18) (BO3)3 (OH)3 (OH); and "cralpoite", ideally Na Cr3+3 (Al4Mg2) (Si6O18) (BO3)3 (OH)3 O.;A suite of chromian tourmalines were investigated, with Cr2O 3 values ranging up to ∼35 wt%. Comparison of SREF and EMPA data show details of Cr site assignment. Included in this suite are "cralpoite" and chromdravite, whose approved formula may need to be revised to show Cr dominant at both the Y- and Z-sites. Problems remain with some of the chromian tourmaline analyses, which may not be resolved until accurate H-values can be determined.;Comparison of SREF and EMPA data for povondraite show the best method for distributing species between the octahedral sites involves filling the Z-site in the order Al, Mg, Fe3+; there is indication that minor amounts of Fe2+ also occur. Povondraite has the most K reported for any species of tourmaline, and in some analyses (Na + K) exceeds the structural maximum of 1 apfu. The nature of the excess alkalis has not yet been resolved, but cannot be real as no additional electron-density is seen in the refined crystal structures.;The triangular (BO3) groups in tourmaline have thus far been assumed to be relatively constant in dimensions. SREF and EMPA data for a suite of 99 tourmalines indicate that the B-O2 and B-O8 bond lengths are not constant, but vary according to the chemical composition of the tourmaline. Statistical analysis of the bond-valence contributions for the X-O2, Y-O2, Z-O8 and Z' -O8 bonds as they relate to the B-O2/B-O8 bond-valence indicate that the geometry of the (BO3) triangle is most influenced by the Z'-O8 bond. |
