Schorl-oxy-schorl to dravite-oxy-dravite tourmaline from granitic pegmatites;: examples from the Moldanubicum, Czech Republic

Wet-chemical analyses (41) of tourmaline from granitic pegmatites (barren, barren-pocket, beryl, lepidolite types) in the Moldanubicum, Czech Republic revealed that members of the oxy-subgroup-common oxy-schorl, minor oxy-dravite and rare oxy-fioitite are more abundant relative to the relevant members of the hydroxy-subgroup. The primary substitution mechanisms in tourmaline show combination of heterovalent substitutions: (AlO)-Al-Y-O-W R-Y(-1)2+ (W)(OH)(-1), (X)boxed dot(Y)Al(2)(W)O (Na-1R)-Na-X-R-Y-(2+)(2) (W)(OH)(-1), (X)boxed dot(Y)Al (Na-1R-12+)-Na-X-R-Y and (X)boxed dot(W)(OH)(NaO-1)-Na-X-O-W, and homovalent substitutions: Fe2+Mg-1 and (OH)F-1. Tourmalines with the chemistry expressed by the general formula (X)(Na(0.5)boxed dot(0.5))(Y)(R22+Al)Al-6(BO3)(3)Si6O18 (V)(OH)(3)(W)(O0.5OH0.5) crystallized in very similar PT conditions in granitic systems saturated on Na, Al, Si and H2O, it indicates the importance of short-range order requirements on tourmaline chemical composition. Abundance of heterovalent substitutions involving the W-site requires determination of light elements (H, B, F, Li) and Fe2+/Fe3+ in tourmalines to specify substitution mechanisms with certainty. Normalization of electron-microprobe data of (Fe,Mg)-rich, (Ca,Li,F)-poor tourmalines from granitic pegmatites on (OH,F)(3.5)O-0.5, which is more probable than (OH,F)(4), seems to be suitable.