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

¹ Institute of Geological Sciences, Masaryk University, Kotláská 2,611 37 Brno, Czech Republic
² Institute of Geochemistry, Mineralogy and Mineral Resources, Charles University, Albertov 6, 128 43 Praha, Czech Republic
³ Department of Geological Sciences, University of Manitoba, Winnipeg R3T 2N2, Canada

^* E-mail: mnovak{at}sci.muni.cz

^** Present address: Precambrian Geoscience Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, Ontario, Canada P3E 6B5.

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-foitite are more abundant relative to the relevant members of the hydroxy-subgroup. The primary substitution mechanisms in tourmaline show combination of heterovalent substitutions: ^YAl^WO ^YR²⁺_–1^W(OH)_–1, ^XYAl₂ ^WO ^XNa_–1 ^YR²⁺_–2 ^W(OH)_–1, ^XYAl ^XNa_–1 ^YR²⁺_–1 and ^XW(OH) ^XNa_–1 ^WO_–1, and homovalent substitutions: Fe²⁺Mg_–1 and (OH)F_–1. Tourmalines with the chemistry expressed by the general formula ^X(Na_0.50.5)^Y(R²⁺₂Al)^ZAl₆(BO₃)₃Si₆O₁₈ ^V(OH)₃ ^W(O_0.5OH_0.5) 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)_3.5O_0.5, which is more probable than (OH, F)₄, seems to be suitable.

Key-words: tourmaline, schorl, oxy-schorl, dravite, oxy-dravite, oxy-foitite, wet-chemical analyses, substitution mechanisms, granitic pegmatites, Moldanubicum, Czech Republic.

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