Nanoscale mineralogical evolution of pyrite and secondary phases during and after spontaneous coal combustion in coal reject deposits.

Abstract

Spontaneous coal combustion (SCC) occurs globally and releases particulate matter, acid gases (e.g., SO₂ and NO_x), and metals into the environment. This study characterizes nanometer-scale processes that occur during and after SCC in Urussanga, Santa Catarina, Brazil. A multi-analytical approach using various surface- and electron-microscopy techniques allowed the identification of secondary mineral processes and the formation of secondary minerals. Weathering of thermally altered pyrite and coal rejects forms secondary weathering crusts composed predominantly of Fe-oxides (hematite and spinel-group minerals), amorphous silica, clay minerals (illite-smectite, I-S, and kaolinite), sulfides (pyrite), and sulfates (gypsum and jarosite-alunite group minerals). The crusts are enriched in metals common in pyrite, form efflorescent minerals, or show high structural compatibility with Fe-oxides. Pyrite transforms into pyrrhotite, mikasaite, Fe₂(SO₄)₃, and hematite under different O₂ and hydrocarbon activities. Porous coal matrices allowed inferred meteoric water to infiltrate pyrite-coal-clay mineral associations. This promoted the non-stoichiometric dissolution of I-S into amorphous silica and the weathering of pyrite to nanometer-thick secondary Fe-sulfate layers in confined pore spaces along pyrite-coal interfaces. The high abundance of clay minerals and amorphous silica resulted in the entrapment/armoring of pyrite-, TiO₂-, and jarosite-alunite nanoparticles and the heterogeneous nucleation of jarosite on the (001) basal surface of kaolinite. These findings indicate complex interactions between the original minerals in coal rejects (pyrite and I-S), thermal alteration products (pyrrhotite, mikasaite, and TiO₂), and post-combustion weathering products (amorphous silica, Fe-sulfates and -(hydr)oxides), as well as the geochemical redistribution of environmentally significant elements during and after SCC.