FREE ENERGIES OF FORMATION OF ILLITE SOLID SOLUTIONS AND THEIR COMPOSITIONAL DEPENDENCE

Abstract--Electron probe analyses of diagenetic llites in Eocene sandstones from Kettleman North Dome, California, along with analyses of coexisting interstitial waters were used to calculate apparent molal free energies of formation of the illites at the in situ conditions of 100 ~ and 150 bars. Various triangular and rectangular compositional p ots, once contoured for free energy, crudely indicate that the free energy de-creases as the potassium content increases, decreases as the Al-for-Si substitution i creases, and appears to be a minimum along a narrow composition valley having-3 % Fe203 of octahedral cations. Qualitatively, the shape of the free energy surface suggests only small departures from ideality. The illites and water samples used in the calculations were chosen as close to each other as possible and are in equilibrium. The waters contain mainly CI-, SO4 ~-, Na § and Ca 2+ and have a total salinity o f- 10,000 ppm. The illites, which coexist in the sandstones with diagenetic quartz, albite, and K-feldspar, are phen-gitic and contain 5-9 % K~O, 1-3 % total FeO, 1 % MgO, and minor Na20, MnO, and TiO2. The FezOJFeO ratio has been set arbitrarily to 7 by comparison with published analyses. Log K (100 ~ was first calculated for the equilibrium illite ~ ions from the ionic activities obtained through speciation calculations for each water sample. The apparent free energies of formation of illites at 100 ~ were then calculated. The effect of instrumental uncertainty on the values of AG s was assesse