Observations on the formation of kaolinite in the St. Austell granite, Cornwall

Abstract

AbstractPartial chemical and X-ray analyses of specimens taken serially from three different rock types from the St. Austell granite show that potash feldspar is hydrothermally altered to a micaceous mineral. Sodic plagioclase alters to mica, montmorillonite and kaolinite. Furthermore, in the latter case the micaceous mineral and montmorillonite are intermediate products between feldspar and kaolinite. It is believed that the level of H+ion activity in the altering solutions controlled the removal of alkalis, Al2O3and SiO2so as to leave appropriate concentrations of these constituents to re-form as mica or montmorillonite. Further alteration of a similar kind produced kaolinite.