Mica-Derived Vermiculites as Unstable Intermediates*

Abstract

Stability determinations were made by solubility methods on two trioctahedral mica-derived vermiculites. The phlogopite-'derived vermiculite was found to be unstable under acid solution conditions, where stabilities of montmorillonite, kaolinite and gibbsite had previously been determined. An attempt was next made to locate a possible montmorillonite-vermiculite-amorphous silica triple point. This triple point involved conditions of alkaline pH, high pH4SiO 4 and high Mg 2+. These are conditions where phlogopite and biotite-derived vermiculites are most likely to control equilibria/f they are stable minerals. The montmorillonite-vermiculite-amorphous silica samples went to the montmorillonite-magnesite- amorphous silica triple point, leaving no stability area whatsoever for the vermiculites. These large par- ticle-size, trioctahedral, mica-derived vermiculites appear to be unstable under all conditions of room T and P. Arguments are presented indicating that micas are unstable in almost all weathering environments. A hypothesis is proposed that mica-derived vermiculites result from the unique way in which unstable micas degrade in these environments. It is proposed that vermiculite derives from a series of reactions whose relative rates often result in an abundance of vermiculite. These relative reaction rates are slow for mica dissolution, rapid for K removal and other reactions pursuant to vermiculite formation, and slow for ver- miculite dissolution. In chemical terms, mica-derived vermiculites may be considered fast-forming un- stable intermediates.