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Minerals derived from coal-fire gas and found encrusting vents and ground fissures form by isochemical or mass-transfer processes. Isochemical processes include sublimation and gas-liquid–solidification (GLS). Evidence for sublimation includes elements found in both the gas and minerals as well as the occurrence of euhedral crystals. Textural evidence for GLS includes drip and flow condensates.

Mass-transfer processes are more complex and include gas-altered substrate (GAS), gas-liquid–altered substrate (GLAS), gas-liquid–precipitation (GLP), and gas reaction ± liquid-solidification (GRLS) processes. During GAS, the depositional substrate is altered by reaction with one or more components of coal-fire gas, whereas during GLAS, the substrate is altered by reaction with a liquid solution. Evidence for GLAS includes vesicles and minerals with ions acquired from the underlying substrate. GRLS is characterized by reaction among select-gas components, ultimately responsible for mineralization. Differentiating GAS, GLAS, and GRLS from one another is difficult if the mineralized by-products of combustion are dissolved by water or windswept from an altered substrate. Amorphous bulb-like masses at the end of elongated crystals are evidence for GLP.

Minerals reported to be coal-fire sublimates on the basis of textural evidence may actually be the reaction products of select-gas components exhaled from a vent or fissure. Reactions involving coal-fire gas components have never been identified. Additional mineral forming reactions based on isochemical and mass-transfer processes are possible. Some coal-fires minerals may be vectors for the transmission of toxins to humans by inhaled dust particles or by food grown in soils that contain these minerals.

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