Abstract: Dissolution data on five of the six illites reported by Reesman and Keller (1968) indicate that these illites are more stable than was previously thought. The revised Gibbs free energies of formation ( Δ G f 0 ) with respect to the ‘ideal’ illite formula and the muscovite formula are: Sample Δ G f 0 ‘illite formula’ Δ G f 0 ‘muscovite formula’ Fithian illite −1319·7 kcal/fw −1334·8 kcal/fw Grundy illite −1322·7 −1338·0 Rock Island illite −1307·3 −1333·2 Marblehead −1310·8 −1334·2 Gage (No formula) −1337·6 Use of muscovite formula as an indirect comparator provides a means of predicting the relative stabilities of these chemically complex materials.
The response of illite-equilibrated solution to a kaolin Δ G f 0 was found in all samples in which a 7 Å mineral phase was detected by X-ray diffraction. Stability diagrams based upon the Δ G f 0 with respect to ideal muscovite and kaolin formulas show a rather wide range in chemical conditions through which illites and kaolin minerals with differing Δ G f 0 would be stable. However, in carbonate rocks and sea water illite is stable relative to kaolin. During the weathering of carbonates the lower pH zones in the clay-rich residuum above the carbonates favor the transformation of illite to kaolin minerals.