The Polymer Model of Thermochemical Clay Mineral Stability

Garrison Sposito
Department of Soil and Environmental Sciences, University of California Riverside, California 92521

Abstract: The Nriagu polymer model of 2:1 layer type clay minerals develops from the premise that clay minerals are condensation copolymers of solid hydroxides. In the Mattigod-Sposito formulation of the model, standard state chemical potentials (standard Gibbs energies of formation from the elements) of 2:1 clay minerals are predicted quantitatively with a linear correlation equation relating the standard Gibbs energy of the polymerization reaction (ΔG0r to the half-cell layer charge of the clay mineral and to the valence and ionic radius of the exchangeable cation. It is now shown that this correlation equation can be derived from two basic assumptions: (1) that the standard Gibbs energy change for the transfer of a cation in a pure hydroxide solid to a hydroxide component in the tetrahedral or octahedral sheet of a 2:1 clay mineral is independent of the nature of the cation and (2) that the difference between ΔG0r for the polymerization reaction to form a 2:1 clay mineral and ΔG0r for the same reaction to form the zero layer-charge analog of the clay mineral is proportional to the number of interlayer exchangeable cations per unit cell of the clay mineral and to the radius of its exchangeable cation. Both of these assumptions can be tested experimentally, independent of the polymer model.

Key Words: Chemical potential • Illite • Layer charge • Polymer model • Smectite • Stability • Vermiculite

Clays and Clay Minerals; April 1986 v. 34; no. 2; p. 198-203; DOI: 10.1346/CCMN.1986.0340210
© 1986, The Clay Minerals Society
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