Potassium-bearing, fluorine-rich tourmaline from metamorphosed fluorite layer in leucocratic orthogneiss at Nedvědice, Svratka Unit, western Moravia
Three tourmaline parageneses occur in orthogneiss and associated rocks at Nedvědice: i) columns of black tourmaline, locally corroded, show patchy zoning with schorl > schorl cores, and occur in a metamorphosed fluorite layer; ii) black columnar crystals of accessory to subordinate homogeneous schorl from leucratic orthogneiss are locally concentrated in thin bands or in elongated clusters with quartz and muscovite; iii) black columnar tourmaline crystals with schorl > dravite rims and dravite > schorl cores are heterogenous in thin section and BSE images, and occur in muscovite >> biotite schists. Tourmaline from all three parageneses is characterized by high F (up to 0.77 apfu) and K (up to 0.09 apfu) in tourmaline from the metamorphosed fluorite layer. Two dominant substitutions were recognized in tourmaline: 1) Fe <–> Mg substitution in schorl-dravite from mica schists and metamorphosed fluoroite layer; and 2) OH <–> F substitution tyopical for tourmaline from all rock types. Potassium rarely substitutes for Na at the X-site in the tourmaline structure. The chemical compositions suggest that K <–> Na, KFe3+ <–> NaAl and/or NaFe2+OH <–> KFe3+O substitutions may be responsible for K in tourmaline from Nedvědice.
SNIP (Scopus, 2015): 0.700
IF (ISI, 2015): 1.326
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