Simulated Weathering of Lignite Overburden Shales from Northeast Texas

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Abstract
Unweathered overburden shale was continuously leached with water and sulfuric acid solutions (10−3 and 10−2N) for periods of up to 3 months. Clay samples from the shale were also subjected to artificial weathering by incubation in sulfuric acid solutions. Acidity resulted in increased leaching of the basic cations from the exchange complex, particularly Ca and Mg, and a decrease in percent base saturation from 100 to < 20. The more concentrated acid solutions appreciably increased release of structural cations, especially Al, Fe, and Mg. Release of Fe and Mg was mostly associated with weathering of ferromagnesian chlorite. Progressive disappearance of chlorite from clay suspensions aged in solutions of increasing acidity was observed. Differential dissolution of chlorite and smectite resulted in concentration of kaolinite and mica. The relative stability of the layer silicate minerals to acidic conditions decreased in the following order: kaolinite > mica > smectite > chlorite. Weathering of chlorite in the overburden materials is a potential source of Fe oxides in reclaimed strip‐mined lands.

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