The origin of gas-vent minerals: Isochemical and mass-transfer processes
Published:January 01, 2007
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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Geology of Coal Fires: Case Studies from Around the World
The “sedimentary cover” refers to the stratified rocks of youngest Proterozoic and Phanerozoic age that rest upon the largely crystalline basement rocks of the continental interior. The early chapters of the volume present data and interpretations of the geophysics of the craton and summarize, with sequential maps, the tectonic evolution of the craton. The main body of the text and accompanying plates and figures present the stratigraphy, structural history, and economic geology of specific sedimentary basins (e.g., Appalachian basin) and regions (e.g., Rocky Mountains). The volume concludes with a summary chapter in which the currently popular theories of cratonal tectonics are discussed and the unresolved questions are identified.