Organic minerals in a self-heating coal-waste dump in Upper Silesia, Poland: Structure, formation pathways and environmental issues

The study presents research on the unusual appearance of purple-colored organic minerals, ravatite (phenanthrene) and freitalite (anthracene), occurring in the migrating front wall of a heating spot in the Bytom coal waste dump (Upper Silesia Coal Basin, Poland). These minerals are known to be sublimation products, but their formation mechanism remains unclear. Additional minor components are fluorene, dibenzothiophene, naphthothiophenes, dibenzofuran, and their alkyl-derivatives, and n-C₁₇ – n-C₂₀ alkanes. Temperatures were surprisingly low (30–60 °C on the surface) at the sampling sites, though such large amounts of Polycyclic Aromatic Hydrocarbons (PAHs) typically form in a burning environment where temperatures reach 800–1000 °C. The relatively low temperatures suggest that the primary mechanism of formation was not direct evaporation (desublimation) of phenanthrene and anthracene from coal-waste gases but that their occurrence may reflect a catalytical polymerization of ethylene on iron (III) chloride synthesized in a reaction between HCl and a common Fe mineral such as goethite. Subsequently, both minerals crystallized on the cold dump surface. High concentrations of phenanthrene and anthracene in self-heating products, testified by ravatite and freitalite, mean that self-heating of coal waste may significantly increase backgrounds of environmental pollution by PAHs.