Acta Musei Moraviae

Supergene sulfate association from the Konstantin Quarry near Velké Vrbno (Moravia, Czech Republic)

Konstantin Quarry, extracting graphite from 1977 until 2009, is situated on the outcrop of the graphite seams of the Velké Vrbno Unit, with uncertain Devonian age. The rock series includes metapelites, metacarbonates (calcite and dolomite marbles), quartzites and metavolcanites. It underwent Variscan metamorphosis with the peak at ca. 11 kbar and 660 °C, i.e., in the amphibolite facies. Two graphite seams were extracted in the Konstantin Quarry. Both contain significant amount of sulfur – up to 11.1%. Up to now, only jarosite was identified as a secondary phase. New sampling, field emission scanning selectron microscopy and energydispersive X-ray microanalysis revealed varied association of secondary sulfates. Namely, abundant botryoidal aggregates and crusts of white fibroferrite and pale-yellow slavíkite, partly coroded gypsum aggre gates, and coatings of powdery hydroniumjarosite accompanying schwertmannite. Rosettes are infrequent and crusts of magnesiocopiapite, rare microscopic alunogen, hexahydrite and epsomite. This association shows rather early stage of oxidation alteration with the predominant Fe³⁺-sulfates, but with lack of reaction products with the surrounding rocks (metacarbonates). In this very low-pH environment, most of the recognized products are not final stages of Fe³⁺ hydrolysis and still have significant acidification potential. We expect that during the time, mineral association will shift towards pickeringite-halotrichite series minerals and epsomite.