Relation of Porosity and Permeability to the Origin of Diaspore Clay
The apparent porosity of flint, boehmite and diaspore clays bears no direct, consistent relation to the permeability of the same type of clay or to the alumina content of each type. The coefficient of permeability varies with the type and the structure of each flint clay. Diaspore clays have high permeability compared to flint clays, and the boehmite clays measured generally have a permeability intermediate between that of diaspore and that of flint clays. The apparent increase in permeability with increase in alumina content suggests that permeability has increased as silica was removed from flint clay to form boehmite and diaspore clays.
A vertical pipe of diaspore clay 1½ inches in diameter was collected by Fred Mertens of the Laclede Christy Company at the Shockley-Thompson diaspore pit at Belle, Missouri. The pipe contains 69.16 percent alumina compared with 55.30 percent alumina for the average of the walls a few inches from the pipe. Diaspore in the pipe has vertical banding indicating it was deposited along a vertical crack. The formation of cracks in brittle flint clay is an important process in increasing the permeability and providing openings along which ground water can migrate.
A restudy of boehmite and diaspore clays emphasizes not only a replacement of the colloform and oolitic structures of flint clays by diaspore and boehmite, a gradual alteration of flint clay to diaspore clay along fractures, and second-generation diaspore cutting early diaspore, but also the importance of secondary processes in the origin of diaspore and boehmite clays.
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