ORDERING OF FE AND MG IN THE TOURMALINE CRYSTAL-STRUCTURE - THE CORRECT FORMULA

Twelve tourmaline specimens spanning the entire range of Fe - Mg subst itution were analyzed by electron microprobe and crystal-structure ref inement to determine partitioning of these elements into the 2 octahed ral sites, Y and Z. For each specimen the empirical formula, cell para meters, atomic coordinates, mean bond lengths, atomic site occupancies and bond-valence sums are given. Disorder of Mg between the Y and Z s ites of the tourmaline structure is relatively common for many composi tions and significantly complicates formula calculation. To enable for mula calculation in the absence of detailed structural information, st ructural-compositional correlations were sought. A good correlation ex ists between the partitioning behaviour of Mg and total Fe/(Fe+Mg), i. e. Mg(Y) congruent-to 3 [1 - Fe/(Fe+Mg)], but the Y-site Mg content ca n only be reliably calculated with this equation for compositions with FeO(total) greater than 7 wt.%. However, the Mg content at the Z site (and by deduction, at the Y site) can be calculated for all compositi ons from the chemical composition and unit cell volume (V) by: Z[Mg/(M g + Al)] = 0.209 [exp[(V-1540)/40] - 1]. The substitutions of Ti4+ and B for Si and of OH and F for 0 are discussed. The method of calculati on for the correct empirical formula of a tourmaline is given.