Five unique mineral assemblages that include the sulfates millosevichite, alunogen, anhydrite, tschermigite, coquimbite, voltaite, and godovikovite, as well as the halide salammoniac and an unidentiÞ ed phase, according to X-ray diffraction and EDS data, were found as encrustations on quartzofeldspathic sand and sandstone adjacent to coal-Þ re gas vents associated with underground coal Þ res in the Wuda coalÞ eld of Inner Mongolia.The mineral assemblage of alunogen, coquimbite, voltaite, and the unidentiÞ ed phase collected from the same gas vent, is documented for the Þ rst time. Coquimbite also occurs as rosettes secondarily nucleated on a cryptocrystalline mass of alunogen, coquimbite, voltaite, and the unidentiÞ ed phase during storage in a sealed container at room temperature.Field observations, analyses of vent gases, SEM images, and mineral compositions suggest that the sulfates millosevichite, alunogen, coquimbite, voltaite, godovikovite, and the unidentiÞ ed phase, crystallized in response to a complex sequence of processes that include condensation, hydrothermal alteration, crystallization from solution, ß uctuating vent temperatures, boiling, and dehydration reactions, whereas the halide salammoniac crystallized during the sublimation of coal-Þ re gas. Tschermigite and anhydrite formed by the reaction of coal-Þ re gas with quartzofeldspathic rock or by hydrothermal alteration of this rock and crystallization from an acid-rich aqueous solution.Variations in the mineral assemblages found at Þ ve gas vents are possibly due to differences in coal-bed chemistry, exchange reactions involving coal-Þ re gas, and the composition of sediment, rock, and aqueous solutions prior to the exhalation of gas at the surface, as well as the temperature and cooling rate at a vent.Few studies have addressed the interaction of coal-Þ re gas with sediment, rock, and aqueous solutions and the subsequent mineralization processes. Coal Þ res present opportunities for discovering rare and new mineral occurrences. These minerals have potentially important environmental signiÞ cance and may be vectors for the transmission of toxins. Coal Þ res also provide insight for the recognition in the geologic record of preserved mineral assemblages that are diagnostic of ancient Þ res.Brought to you by | Rice University Authenticated Download Date | 10/7/15 4:07 AM
