Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original Paper

Pavel Uher, Davidová, Vikár

Schorl composition from the barren granitic pegmatites in the Western Carpathians: Two examples from the Nízké Tatry mountains, Central Slovakia

Journal of the Czech Geological Society, volume 46 (2001), issue 1-2, 21 - 26

Tourmaline group minerals belong to rather uncommon accessory minerals of the Hercynian granitic pegmatites of the Central Western Carpathians, Slovakia. Black tourmaline from two barren quartz-feldspar-(biotite-muscovite) pegmatites of the Nízké Tatry Mts.: Bystrá Valley - Srdiečko (TBV) and Magurka - Adolf adit (TMA), were studied by EMPA. Both tourmalines show schorl compositions with intermediate X-site vacanies: 0.25-0.33 pfu, slight compositional zonality with increasing Fe/(Fe+Mg) ratio from center to rim of crystals, and low Ca and K contents. The most striking feature of the TMA schorl is a notably high Mn content: 0.98-1.56 wt. % MnO, 0.14-0.23 apfu, exceptional for schorl from poorly fractionated granite-pegmatite environment. Nevertheless, the TMA schorl also reveals higher Fe/(Fe+Mg) ratio in comparison to TBV: 0.69-0.83 and 0.54-0.60, respectively, and Mn values correlate positively with Fe, Fe/(Fe+Mg) and X-site vacancies. Thus, the elevated Mn content in TMA schorl is probably a consequence of local Mn-rich environment, probably due to absence of garnet as well as a relatively higher overall fractionation level.