Formation and Occurrence of Clay Minerals

Clay minerals may be si<j'nifieant indieatoi's of earth processes. They form throufrh a range which involves at one extreme the action of compressed water vapor at a temperature of several hundred degrees centigrade, and at the other extreme, the action of atmospheric agencies at an ordinary temperature. Not recognized as original products of magmatic crystallization, clay minerals are prima facie evidence of a process subsequent to the original crystallization. Yet their relation to the magmatic process may be close: the last of the fluids and vapors of the magma may react with wall rock to form claymineral masses. This is probably the extreme of hightemperature, hypogene clay-mineral formation. Between this and the low temperature limit the transition is gradual, the minerals formed falling into gradational groups. In time clay aggregates of hydrothermal origin become distinguishable with difficulty from clay aggregates formed under atmospheric conditions—that is, those of supergene origin.