GAS VENT MINERALOGY OF COAL FIRES BURNING AROUND THE WORLD

Gas vents and ground fissures at coal fires burning around the world are frequently encrusted with rare and beautiful sulfate and sulfide minerals. The minerals form in association with coal-fire gas during exhalation and cooling at the surface. Examples include coquimbite, voltaite, and godovikovite from the Wuda coalfield in Inner Mongolia; hydrobasaluminite, voltaite, and alunogen from the Centralia mine fire in Pennsylvania; pentlandite and hazelwoodite from the Southern Ute Indian Reservation in Colorado; letovicite and mascagnite from the Witbank coalfield in South Africa.

The gas-vent phases form by a variety of thermochemical processes. Gas and mineral analyses strongly suggest that sublimation is the simplest and probably least common of such processes. More complex thermochemical processes include gas altered substrate (GAS), gas-liquid-altered substrate (GLAS), and gas reaction +/- liquid-solidification (GRLS). During GAS, coal-fire gas reacts with the substrate adjacent to the vent and alters it. This is analogous to igneous-pneumatolytic alteration. During GLAS, coal-fire gas condenses to a liquid on the substrate. The liquid then chemically alters the substrate, resulting in the crystallization of one or more minerals. GRLS occurs by two different processes: (1) select components in the gas react, resulting in the crystallization of one or more minerals or (2) select gas components react and form a liquid on the substrate adjacent to a vent. The liquid then reacts with the substrate and alters it, resulting in the crystallization of one or more new minerals.

Reactions involving coal-fire-gas components have never been identified. The identification of such reactions would make it possible to calculate P-T stability diagrams for GRLS by using thermodynamic loop analysis. Such stability diagrams are useful environmental indicators for determining the tendency of gas components to either condense adjacent to a vent or be absorbed by the atmosphere.