Abstract: Clay minerals undergo significant changes when exposed to weathering agents. Some of these changes were revealed by studying a sequence of samples collected from weathering profiles developed on shales. Illite and chlorite are modified by the development of expandable layers within the illite and chlorite structures. The suggested mechanism of alteration of chlorite is the oxidation and removal of octahedral iron. Chemical analyses indicate that interlayer potassium is removed from illites. Removal of potassium may be accomplished by an exchange reaction between potassium ions and hydronium ions in the environment.
The origin of underclays is controversial. In an attempt to shed some light on this subject, three shale-underclay profiles were studied. The changes in clay minerals from shales to underclays are very similar to those in weathered shale profiles. This similarity lends support to the theory that some underclays are products of weathering.