The Thermodynamic Status of Compositionally-Variable Clay Minerals: A Discussion

Stephen U. Aja and Philip E. Rosenberg
Department of Geological Sciences, McGill University 3450 University Street, Montréal, Canada H3A 2A7
Department of Geology, Washington State University Pullman, Washington 99164, U.S.A.

Abstract: According to Lippmann (1977, 1982) wide compositional variations and excess enthalpies of mixing calculated with electrostatic models imply that clay minerals of variable composition are dis-equilibrium solids. However, recent ATEM analyses of illite samples indicate compositional homogeneity of single illite grains and limited compositional variations in sedimentary basins. Moreover, Lippmann's electrostatic model may be inadequate inasmuch as it neglects polarization energy which is known to be a significant component of lattice energy even in dominantly ionic structures. Contrary to the assumptions of Lippmann, I/S minerals have also been shown to undergo Ostwald ripening.

May et al. (1986) reported that smectites do not reversibly control equilibria and further argued that conceptual and experimental deficiencies inherent in the solubility method prevent the attainment and demonstration of equilibrium in experiments with complex aluminosilicates of variable composition. However, equilibrium may be assumed if: (1) steady states are approached from both under- and over-saturation, (2) the slopes of univariant lines representing mineral-solution equilibria are rational over a wide range of solution compositions and temperature, and (3) results are reproducible in experiments of long duration. Recent solubility studies of smectites, chlorites, and illites meet these criteria indicating that clay minerals of variable composition are true phases capable of attaining equilibrium.

Key Words: Illite • Montmorillonite • Thermodynamic status • Excess lattice energy • Solubility • Compositional variation

Clays and Clay Minerals; June 1992 v. 40; no. 3; p. 292-299; DOI: 10.1346/CCMN.1992.0400307
© 1992, The Clay Minerals Society
Clay Minerals Society (www.clays.org)