Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original Paper

Zdeněk Losos, Julie B. Selway

Tourmaline of dravite-uvite series in graphitic rocks of the Velké Vrbno Group (Silesicum, Czech Republic)

Journal of the Czech Geological Society, volume 43 (1998), issue 1-2, 45 - 52

Graphite gneisses of the Velké Vrbno Group, metamorphosed in lower amphibolite facies (garnet to staurolite zone), from the Petříkov and Malé Vrbno graphite deposits contain accessory tourmaline (dravite I). Dravite I columns contain fine graphite inclusions (euhedral graphite flakes up to 0.003-0.01 mm) in the core of the crystals. The second type of tourmaline (dravite II) occurs in massive pyrrhotite ore from dolomitic marbles of the Petříkov graphite deposit. Both types of tourmaline range in composition from dravite₉₅ uvite₅ to dravite₆₁ uvite₃₈ with minor, but constant, schorl component (0.5-3 mol.%). Rims of dravite I crystals are enriched in uvite component relative to the core. The contents of V (0.00-0.40 wt.% V₂O₃), and Cr (0.05-0.14 wt.% Cr₂O₃) are very low in tourmaline.
The tourmaline and graphite in the host rocks were formed during progressive regional metamorphism (T about 500-540 °C, P about 0.5 to 0.7 GPa). The tourmaline's chemical compositions fall into a field between low- and medium-grade metamorphosed metapelites and marbles. The primary boron content in these metasediments can be explained by an increased salinity within a regressive shelf-sea.