Montmorillonite/illite Stability Diagrams

Chemical activity diagraoas , prepared to illustrate the properties expected if mixed-layer montmorillonite/illite is regarded as a solid solution, are compared to those derived from a treatment of these materials as a mixture of two phases. If the system is a solid solution, the coexisting aqueous solution should range from higher dissolved silica contents in the presence of kaolinite and a montmorillonite end member to lower dissolved silica in the presence of kaolinite and an illitic end member. Silica concentration in the aqueous solution might vary by a factor of as much as six. If the system is two phase, the silica content of a solution in equilibrium with kaolinite and both phases would be fixed at a given T and P, as would a solution equilibrated with both phases and K-feldspar. In the absence of a third phase, silica in equilibrium with both phases should be nearly constant, but increase with increasing ratio of K+/H + in solution. Available data on coexisting aqueous solutions apparently are more nearly consistent with two phases than with a solid solution. Key Words--Chemical activity diagram, Illite, Mixed layer, Montmorillonite, Phase, Solid solution, Stability. I N T R O D U C T I O N The genesis and phase relations of mixed-layer montmorillonite/ill i te clays have been the subjects of much research and comparable controversy over the last 20 years. The following aspects of genesis, taken chiefly from the works of Hower and his colleagues (Hower and Mowatt, 1966; Perry and Hower, 1970; Hower et al., 1976), seem to be generally accepted. I l l i te /montmori l loni te interlayered materials are composed of two discrete species; one is montmori llonite with low interlayer charge. Its inteflayers are hydrated by at least two layers of water molecules and have interlayer spacings at unit water activity of 15 or more. Most of the negative charge of montmorillonites is attributable to substitution of divalent cations in octahedral positions; in some montmorillonites there is no substitution of other ions for silicon in tetrahedral positions and hence no resultant tetrahedrally based negative charge. Cations exchange freely in the hydrated inteflayers of montmorillonites; exchange energies for divalent cations are usually less than 1 kJ per formula weight (Garrels and Tardy, 1982). Energies for exchange between smaller, hydrated ions, such as Na +, and larger, slightly hydrated ions, such as Cs +, may be as large as 3 kJ, but the overall selectivity among cations by hydrated inteflayers is so slight that montmorillonite interlayers characteristically are occupied simultaneously by several cations in significant percentages (> 10%). The second species, illite, is made up of layers interstratified with the montmorillonite layers and formed by diagenetic alteration of original montmoril lonite layers. Illite layers have a 10-/~ spacing. The interlayers are dry and are occupied almost exclusively by K § The K + is exchangeable, but so slowly and only by such Copyright 9 1984, The Clay Minerals Society strong solutions that it is usually described as "fixed potassium." Illitic layers have a total interlayer charge of 0 . 8 0 to 0 . 8 2 per formula weight (Hower and Mowatt, 1966). Most of the charge stems from replacement oftetrahedral silicon by trivalent cations, chiefly AI 3§ In nature, a typical layer of montmoril lonite is converted to a typical layer of illite by a reaction such as the one below: K+o.33(mll.aoFe3+o.z3Mg2+o.27)(mlo.o6Si3.940 1 o(OH)2