ILLITE EQUILIBRIA IN SOLUTIONS .3. A REINTERPRETATION OF THE DATA OF SASS ET-AL (1987)

In a recent solubility study of Goose Lake and Beavers Bend illite, SASS et al. (1987) inferred the existence of three components of natural illites [K0.24/O10(OH)2], [K0.67/O10(OH)2], and [K0.90/O10(OH)2] which were interpreted to be smectite, illite, and K-mica, respectively. They also speculated that illite-smectite equilibrium is metastable under diagenetic conditions except between 90 and 110-degrees-C where it is stabilized by an ordering transition. A re-interpretation of the data of SASS et al. (1987) indicates that the solubility-controlling phases have the following K atoms per half cell: 0.29, 0.52, 0.69, 0.84, and 1.0. Furthermore, solution equilibration investigations of kaolinite-microcline mixtures have shown that these two minerals do not coexist stably. Thus, the question of an ordering transition whose main effect is to stabilize illite-smectite equilibria relative to kaolinite-microcline assemblage does not arise.