Formation and stability of endmember illite: II. Solid equilibration experiments at 100 to 250°C and Pv,soln
One of several natural muscovites or illites, kaolinite, and quartz or amorphous silica were equilibrated in 2 M KCl/HC1 solutions in hydrothermal experiments designed to investigate the stability, composition, and structure of endmember illite at 100 to 250°C. Equilibrium solution compositions determined in previously conducted low solution-to-solid ratio experiments were used in these experiments. High solution-to-solid ratios (120:1) in the present study served to bring about changes in solids that could be characterized by powder XRD and TEM/ATEM. Average alkali-site occupancies of muscovite starting materials decreased from 0.97 ± 0.04 to 0.88 ± 0.05K/O 10(OH) 2, reflecting an increase in the ( Si/Al) iv ratio and a decrease in K for all muscovites, as well as a decrease in Mg for a phengifc muscovite. The illites, with initial alkali-site occupancies close to 0.88K/O 10(OH) 2, remained unchanged during equilibration. The formation of mica-like phases with a composition of 0.88 K/O 10(OH) 2 in this study confirms the results of previous solution equilibration experiments in which a solubility-controlling phase of the same composition was inferred. The retrograde reaction, muscovite-to-illite, represents a reversal of the commonly observed prograde transition from smectite-to-illite. The change in muscovite compositions to 0.88K/O 10(OH) 2, while illite (0.87 ± 0.03K) compositions remained unchanged, suggests that end-member illite [0.88K/O 10(OH) 2] is stable with respect to muscovite between 100 and 250°C under the conditions of this study.