Earth Sciences, Geochemistry & Petrology; tourmaline, fluor-elbaite, elbaite, Zn-enrichment, electron microprobe, Raman spectroscopy, granitic pegmatites, Piława Górna, Sudetes
Tourmalines are a group of minerals which may concentrate various accessory components, e.g. Cu, Ni, Zn, Bi, Ti, Sn. The paper presents fluor-elbaite and elbaite from a dyke of the Julianna pegmatitic system at Piława Górna, at the NE margin of the Bohemian Massif, SW Poland, containing up to 6.32 and 7.37 wt.% ZnO, respectively. Such high amounts of ZnO are almost two times higher than in the second most Zn-enriched tourmaline known to date. The compositions of the Zn-rich tourmalines from Piława Górna, studied by electron microprobe and Raman spectroscopy, correspond to the formulae: (Na0.73Ca0.01•0.25)Σ1(Al1.03Li0.79Zn0.76Fe2+0.33Mn0.09)Σ3Al6B3Si6O27(OH)3(F0.66OH0.34), and (Na0.78Ca0.01•0.21)Σ1(Al1.06Li0.87Zn0.88Fe2+0.10Mn0.09)Σ3Al6B3Si6O27(OH)3(OH0.84F0.16), respectively, with Zn as one of the main octahedral occupants. A comparison with other tourmalines and associated Zn-rich fluor-elbaite and elbaite from the pegmatite indicates that atypically high Zn-enrichment is not a result of Zn-Fe fractionation, but dissolution and reprecipitation induced by a late (Na,Li,B,F)-bearing fluid within the assemblage of gahnite spinel and primary schorl-type tourmaline. This strongly suggests Na-Li-B-F metasomatism of gahnite-bearing mineral assemblages as that is the only environment that can promote crystallization of a hypothetical Zn-dominant tourmaline. The compositions of the Zn-rich fluor-elbaite and elbaite suggest three possible end-members for such a hypothetical tourmaline species: NaZn3Al6B3Si6O27(OH)3(OH), •(Zn2Al)Al6B3Si6O27(OH)3(OH) and Na(Zn2Al)Al6B3Si6O27(OH)3O.