Oxy-dravite from Wołowa Góra Mountain, Karkonosze massif, SW Poland: Crystallochemical and structural studies

Abstract

Yellowish dravitic tourmaline (dominated by the oxy-dravite component) associated with secondary fluor-dravite/fluor-schorl and dravite/schorl tourmalines was found in a quartz vein cropping out in the eastern part of the Karkonosze Mountains range, SW Poland. The crystal structure of this tourmaline was refined to an R1 value of 1.85% based on single-crystal data, and the chemical composition was determined by electron-microprobe analysis. The tourmaline, a representative of the alkali-tourmaline group, has the structural formula: (Na0.75Ca0.12□0.13)Σ1(Mg1.93Al0.95Ti0.06${\rm Fe}_{{\rm 0}{\rm. 04}}^{{\rm 2 +}} $ V0.01)Σ3(Al5.38Mg0.62)Σ6B3Si6O27(OH)3(O0.46OH0.33F0.21)Σ1, and is characterized by an extremely high Mg/(Mg + Fe) ratio of 0.97–0.99, the WO2– content that reaches 0.59 apfu resulting in a local predominance of the oxy-dravitic component and Mg–Al disorder on the octahedral Y and Z sites of the order of 0.64 apfu. This disordering results in an increasing <Z–O> distance with ~1.925 Å, and unit-cell parameters a = 15.916(1) Å and c = 7.180(1) Å. The tourmaline formed during Variscan prograde metamorphism under the influence of a released (H2O,B,F)-bearing fluid. The fluid mobilized the most soluble components of partly altered silicic volcaniclastic material of the Late Cambrian to Early Ordovician bimodal volcanism to become the protolith for adjacent quartzo-feldspathic schists and amphibolites, and propagated them into the surrounding granitic gneisses of the Kowary unit in the eastern metamorphic cover of the Karkonosze granite.

Physical properties and crystal structure of near end-member oxy-dravite from the Beluga occurrence, Nunavut territory, Canada

“…In comparison with other data on oxy-dravite (Ertl et al, 2003;Bosi and Skogby, 2013;Pieczka et al, 2018;Sȩk et al, 2022), the sample studied shows similar chemical composition and structural parameters (Table 7). Oxy-dravite from the quartz vein located in a granitic gneiss of the Kowary gneiss series (Wołowa Góra Mountain; Pieczka et al, 2018), has comparable values of Si 4+ , Ti 4+ and F -. However, the Beluga oxy-dravite has lower contents of Ca 2+ (0.08 vs. 0.15 apfu) and Mg 2+ (1.89 vs. 2.53 apfu) but higher contents of Fe 2+ (0.15 vs. 0.04 apfu), Na + (0.88 vs. 0.80 apfu) and Al 3+ tot (7.06 vs. 6.36 apfu).…”

“…The Y-site is occupied mainly by Mg and Al, in a lesser amount also by Fe and Ti. The mean bond length for <Y-O> is 1.999 Å, only slightly lower than in the Y Fe 3+ -rich oxy-dravites studied by Bosi and Skogby (2013) and Pieczka et al (2018). The ionic radius value of Fe 3+ in tourmaline (Grice and Ercit, 1993;Hawthorne, 1996 and2002;Bosi and Luccesi, 2007;Bosi and Skogby, 2013;Pieczka et al, 2018).…”

“…Occupancies of octahedral sites were refined using bond-valence optimisation (Table 3) which confirmed disorder of Mg and Al between the Y-and Z-sites, typical for Mg-rich tourmalines (e.g. Grice and Ercit, 1993;Hawthorne et al, 1993;Ertl et al, 2003;Bosi and Lucchesi, 2004;Bosi and Skogby, 2013;Bačík, 2015;Pieczka et al, 2018;Scribner et al, 2021;Bosi et al, 2022;Sȩk et al, 2022).…”

Magnesio-lucchesiite from the Kowary vicinity, Karkonosze Mountains, SW Poland: the third occurrence worldwide

“…These B-bearing fluids could have been also sources for other quartz–tourmaline veins and veinlets in the Karkonosze granite metamorphic envelope of the Kowary region, for example, at the Wołowa Góra Mt. near to the Budniki camp, containing oxy-dravite, fluor-dravite and dravite (Lis et al , 1965; Pieczka et al , 2018).…”

“…Crystals of magnesio-lucchesiite have subsequently been found within quartz veins cross-cutting an amphibolite body in proximity of the Kowary town, SE Karkonosze-Izera Massif, south-western Poland, ~1 km to west of the oxy-dravite occurrence on Wołowa Góra Mt. (Pieczka et al , 2018), thus representing the third documented occurrence of magnesio-lucchesiite worldwide. This work addresses the chemical and structural characterisation of this peculiar oxy-tourmaline species.…”