NEW LOW TEMPERATURE PHASE DIAGRAMS FOR NOW LOW TEMPERATURE PHASE DIAGRAMS FOR PHYLLOSILICATES DEFINING STABLE AND METASTABLE PHASI

摘要

An alternative methodology has been elaborated here for theoretical derivation of phase diagrams of phyllosilicates in a low temperature, aqueous contact system. These are somewhat different from the equilibrium activity diagrams that are generally used for such systems. The particular feature of these diagrams is that they incorporate the basic features of Schreinemaker's rules to distinguish between stable, metastable and unstable clay mineral phases under ambient conditions. Diagrams have been drawn to show stable and metastable phases for a system with the constituent components, Si(OH)_4, K~+, Mg~(3+), H~+ Up, AlO(OH), FeO(OH) and a clay mineral at STP. Results indicate that only kaolinite, muscovite, pyrophyllite, chlorite, talc and phlogopite (i.e., end members) are stable phases. All other (non-stoichiometric) minerals are either metastable or unstable. The diagrams help in understanding the stability / metastability status of non-stoichiometric clay minerals such as illites and smectites, from a thermodynamic viewpoint. Experimental data on aqueous solution-mineral equilibria were collected from the literature and plotted in the diagrams. Such mineral-equilibrated solution compositions show good compatibility with the derived phase diagrams. They also show that data for non-stoichiometric clays occur mostly within the region of the closest end member.