In regard to natural causes of coal fires, another cause not mentioned in the article involves the oxidation of pyrite, an iron sulfide that commonly occurs in coal beds. When oxygenated groundwater percolates through fractures in the coal, the sulfide in pyrite will be oxidized to sulfate. This reaction is exothermic and may produce enough heat to spontaneously ignite the coal.

Bruce Bartleson
Gunnison, Colo