Mn-rich tourmaline and fluorapatite in a Variscan pegmatite from Eibenstein an der Thaya, Bohemian massif, Lower Austria

Abstract

Mn-rich tourmaline (up to 8.89 wt% MnO) and fluorapatite (up to 9.15 wt% MnO) occur in an elbaite-subtype pegmatite from the Drosendorf Complex (near Eibenstein an der Thaya, Bohemian massif, Austria). Based on geological and radiometrical data (337 ± 5 Ma from Sm/Nd garnet-albite isochron) the pegmatite might be related to the Variscan Rastenberg type granodiorites or Weinsberg type granites. Coexisting minerals with Mn-rich, pink-brown to yellow-brown tourmaline and bluish, Mn-rich fluorapatite are quartz, feldspars, Mn-rich garnet (63 mol% spessartine), pink Mn-bearing muscovite (up to ∼0.3 wt% MnO), topaz, cassiterite and rarely, beryl and bertrandite. Tourmaline composition evolves from Mn-poor (0.09 apfu ) schorl through Al-rich Mn-bearing (0.15–0.33 apfu ) schorl, Mn-rich (0.58 apfu ) Fe-bearing olenite-elbaite to Mn-rich (0.86–1.27 apfu ) olenite-elbaite, which reflects the Mn-enrichment of the pegmatite system during the late stages of its evolution. Mn-rich Li-bearing olenite has the formulae x (Na 0.71 Ca 0.01 □ 0.28 ) Y (Al 1.36 Mn 2+ 1.27 Li 0.34) Z Al 6.00 T Si 6 B 3 O 27 [(OH) 3 (F 0.44 ,OH,O)] (8.89 wt% MnO) and X (Na 0.84 Ca 0.03 K 0.01 □ 0.12 ) Y (Al 1.33 Mn 2+ 0.86 Li 0.51 Fe 2+ 0.06 Ti 0.02 □ 0.22 ) Z Al 6.00 T Al(Si 5.74 Al 0.26 ) B 3 O 27 [(OH) 3 ) (F 0.54 OH 0.37 O 0.09 ] (6.22 wt% MnO), with lattice parameters a = 15.9158(4) A, c = 7.1201(2) A ( R = 0.021). This pegmatite has two different zones: a Mn-enriched zone and a Mg-enriched zone. The younger, fine-grained, Mn-enriched zone contains the Mn-bearing minerals described above which have low Mg contents. The coarse-grained, Mg-enriched zone contains Mg-bearing muscovite, tourmaline from the dravite-schorl series, and pale-green fluorapatite but essentially contains no Mn-bearing minerals. Interestingly, albite has a pale bluish-greenish color in the Mn-enriched zone while it has a white color in the Mg-enriched zone.