Alunites in the red beds of the “Marcel” coal mine, Upper Silesian Coal Basin, Poland

Within the so-called red beds of the “Marcel” coal mine, Upper Silesian Coal Basin (USCB),the authors have recorded the presence of alunite and natroalunite using XRD, IR and SEM/EDS. Both ofthem contain probably isomorphic substitutions of barium, strontium, phosphorus and, possibly, thehydronium ion. These two minerals preferably occur at the bottom of the red beds, in clay laminae overlyinga partly preserved coal seam; they are cryptocrystalline: their rhombohedral crystals, with a cube-like habit,are below 1 m. The sulphates coexist mainly with clay minerals (illite, halloysite, kaolinite-D, smectite) andhematite, while quartz, siderite and sphalerite are subordinate or occur in traces. The genesis of the aluniteand natroalunite has been linked to the final stage of the alteration of Carboniferous rocks and coal into thered beds in complex weathering and diagenetic-epigenetic processes. Both minerals must have been formedby metasomatosis of a clay precursor, affected by sulphuric acid resulting from supergenic oxidation ofsulphides (mainly pyrite) dispersed in the Carboniferous strata.