Authigenic Kaolinite and Mica as Evidence for Phase Equilibria at Low Temperatures

Abstract

Abstract Electron micrograph, X-ray diffraction, and chemical studies of secondary clay minerals formed within sandstones provide data supporting a portion of the phase relationships proposed by Garrels (Garrels and Christ, 1965) for kaolinite and K-mica growing in equilibrium with K-feldspar and quartz at about 25°C. A true phase distinction between mica and kaolinite is found at the phase boundary and not a continuous solid-solution series, such as proposed by Keeling (1961). Some of the micas, however, show evidence for the presence of an interlayer impurity, such as small amounts of hydroxymagnesium cation proxying for K+. Electron micrographs and diffraction studies show epitaxial growth of oriented mica crystals on kaolinite providing evidence for the existence of these two distinct phases in mutual equilibrium with the solution from which they crystallized.