| Popis |
Tourmaline supergroup is regarded as a robust and versatile group of minerals, useful as an important tool to record the evolution of magmatic and metamorphic systems. Elbaite subtype of rare-element granitic pegmatites was originally defined from the Moldanubian Zone of the Bohemian Massif. Tourmaline is the dominant Li-bearing mineral; it is typically associated by (typically late-stage) borate minerals and B/beraing silicates, including hambergite, datolite, danburite, boromuscovite, tusionite also B-bearing polylithionite. Early tourmaline commonly belongs to the schorl-elbaite series with typical strong enrichment in Mn (and in some cases F), low vacancy and dominant Na. Schorl occurs in various forms in pegmatites border, wall and intermediate zones; however, elbaite is restricted to irregularly distributed pockets (from few mm to several dm in size) and late veinlets. Elbaite subtype pegmatites exhibits low activity of P, reduced F and high B, documented by presence of late/stage boron-rich minerals: Recent data show that some elbaite subtype pegmatites can feature exceptional variability in chemical evolution of primary and secondary tourmalines, especially in composition of the late-stage, most fractionated generations. Tourmalines of the elbaite subtype pegmatite typically belongs to the alkali group, with minor importance of vacancy and rather scarce Ca, however some examples show exceptional enrichment in vacancy in primary and secondary tourmalines [5]. Influence of Ca is typical in latest stages of tourmaline formation (e. g. Řečice, Rudolfov, Dolní Rožínka), locally reaching composition of fluor-liddicoatite. Late hydrothermal tourmalines may also form in outer pegmatite zones, as recently documented by occurrence of Al – deficit, Fe3+, Ti – rich tourmaline replacing primary biotite. In the Řečice pegmatite, it shows composition close to bosiite, in Dolní Rožínka it is Al-deficient schorl-dravite. Besides the usual evolution of the primary tourmaline (from Mg-, Fe-, to Li,Al,Mn- and Li,Al-rich compositions), the elbaite-subtype pegmatites are therefore characterized by significant B(±F)-rich late-magmatic to hydrothermal stage.
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