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During the Holocene, the natural combustion of coal seams in the western United States has produced large amounts of intensely altered sedimentary rock overlying the burning coal. This thermal alteration has heated the rocks to temperatures of 1300°C or higher, as inferred from the mineral assemblages. Such temperatures are achieved by no other common metamorphic process at atmospheric pressure. As a result the altered rocks contain unusual, hightemperature mineral phases. In cases of extreme high temperature, an ironrich liquid slag forms, separates from the parent rock, and then accumulates by flowing into puddles. The slag specimens analyzed have an overall iron oxide content of 35-40 percent, with Fe+^ about ten times the abundance of Fe . Chemically, the slag might be likened to an iron-rich basalt or andesite. Conditions of intense heating coupled with rapid cooling are reflected in the mineralogy of the slag, with examples such as exsolution/oxidation lamellar intergrowths between hematite and magnetite, with Ti concentrated in the hematite, Mg in the magnetite, and both containing significant Al. At least four iron oxide phases can be identified in the slag. Flowing at slightly above its melting point, the slag forms drip-like structures where it flows into a cooler zone of the fire and quickly cools, preserving the shape of the drops. Fe-Mg oxide crystals located toward the outside of the slag droplets contain up to 15 weight percent ZnO. The likely source of the Zn is release from coal in or sediments adjacent to the burning seam; it is likely that other volatile trace metals from the coal may be incorporated into the slag as well. The bulk iron content of the sedimentary rocks overlying the coal appears insufficient to produce the iron-rich slag; large quantities of these rocks with their relatively low and well-disseminated iron content would be needed. Instead, local enrichments of iron, such as in pyrite or iron-rich concretions in the rock overlying the coal, likely provided the iron to produce the slag.

Unusual, high-temperature, iron-rich, mineral phases produced by natural burning of coal seams; analytical data