A CHEMICAL AND THERMODYNAMIC MODEL OF ALUMINOUS DIOCTAHEDRAL-2 1 LAYER CLAY-MINERALS IN DIAGENETIC PROCESSES - REGULAR SOLUTION REPRESENTATION OF INTERLAYER DEHYDRATION IN SMECTITE/

Consideration of experimental data reported in the literature indicate s that dioctahedral aluminous smectite can be regarded as a regular so lid solution of hydrous and anhydrous thermodynamic components that di ffer in stoichiometry only by 4.5 moles of H2O in the interlayer posit ion (per O10(OH)2).1 All H2O that may be present in the smectite inter layer in excess of 4.5 moles is considered to have the properties of b ulk water. Equilibrium constants (K) and regular solution Margules par ameters (W(s)) for the dehydration of Na-and Ca-smectite at 25-degrees -C and 1 bar were generated from vapor-pressure isotherm data reported by Keren and Shainberg (1975). The calculations were carried out assu ming the stoichiometric number of moles of interlayer H2O in the smect ite components to be independent of the composition of the 2:1 silicat e layer, as well as the number and identity of the interlayer cations. The thermodynamic calculations resulted in values of log K and W(s) o f -3.61 and -2883 cal/mol, respectively, for Ca-smectite and -0.767 an d -3254 cal/mol, respectively, for Na-smectite. These values, together with the electrostatic properties of the interlayer cations were used to estimate values of log K and W(s) for the dehydration of K-, NH4-, Rb-, Cs-, Mg-, Sr-, and Ba-smectite. Equilibrium hydration states at 25-degrees-C and 1 bar predicted from the estimated values of log K an d W(s) are in close agreement with those reported in the literature. T his observation strongly supports the general validity of describing s mectite dehydration in terms of regular solution theory, which require s random mixing of the hydrous and anhydrous components of the mineral s. Because the hydration states of smectite predicted in the present s tudy are in close agreement with those inferred from XRD patterns of n atural clay samples, the hydrous and anhydrous components of smectite may well correspond to hydrous and anhydrous layers in the mineral. Un der these circumstances, smectites in partial states of dehydration sh ould generate diffractograms similar to those of randomly interstratif ied mixed-layer clays. In the case of K-saturated smectites, the XRD p atterns would be identical to those commonly ascribed to randomly inte rstratified illite/smectite, despite the fact that no layers of illiti c composition are present in the mineral.