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„Oxy-dravit“ z turmalinitů krkonošsko-jizerského krystalinika

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Title in English „Oxy-dravite“ from tourmalinites of the Krkonoše-Jizera Crystalline Massif


Year of publication 2012
Type Article in Periodical
Magazine / Source Bulletin mineralogicko-petrologického oddělení Národního muzea v Praze
MU Faculty or unit

Faculty of Science

Field Geology and mineralogy
Keywords „oxy-dravite“; dravite; chemical composition; tourmalinite; Krkonoše-Jizera Crystalline Massif; Northern Bohemia
Description Tourmalinites (Tu + Qtz + Ms + Chl +/- Grt) from the Krkonoše-Jizera Unit form stratiform layers hosted in mica schists (Ms + Qtz + Grt + Chl +/- Tu +/- Bt). The tourmaline usually exhibits three compositional domains. The oldest Mg-rich tourmaline (Mg/(Mg+Fe) = 0.81-0.99) with fine-grade zoning volumetrically predominates. Mg-rich tourmaline is replaced from rim by schorl-dravite (Mg/(Mg+Fe) = 0.51-0.69), and locally in muscovite rich layers is partly to fully replaced by tourmaline following trend between schorl-dravite and foitite-magnesio-foitite (Mg/(Mg+Fe) = 0.42-0.65). The volumetrically dominant Mg-rich tourmaline is alkali (0.59-0.85 apfu Na; 0.04-0.39 pfu Xvac; 0.03-0.19 apfu Ca), enriched in Al (6.14-7.09 apfu), and calculated WO2- (0.25-0.87 apfu O). It typically follow trend along a join between “oxy-dravite” and dravite. Mg-rich tourmaline is an early formed, premetamorphic, and reflects important contribution of seawater in the oxy-dravite forming fluids. It is probably genetically related to sulfide deposit formations in closely associated amphibolites. In the contrary, both generations of later schorl-dravite are interpreted to be metamorphic.
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