Dehydration of a Ca- and a Mg-exchanged montmorillonite (SWy-1) at elevated pressures Available to Purchase

The dehydration of a Ca- and a Mg-exchanged montmorillonite obtained by ion exchange of a Na-montmorillonite was determined at pressures to 1 kbar using high-pressure dilferential thermal analysis. Interlayer dehydration reactions of Ca-exchanged montmorillonite occur at about 70°C and 110°C above the liquid-vapor curve of water, whereas Mg-exchanged montmorillonite dehydrates at about 50°C and 110°C above this curve. At pressures above the critical point of water, the dehydration temperatures increase only slightly. These two reactions are interpreted to represent dehydration of two distinctly dilferent H₂O molecule configurations around the interlayer cation. The data were used to determine the enthalpy of the dehydration ΔH_dh and the enthalpy of the interlayer water ΔH_iw at 1 atm. These are, for Ca-exchanged montmorillonite, first dehydration reaction, ΔH_dh = 49.5 ± 0.5 kJ/mol and ΔH_iw = 12.1 ± 0.5 kJ/mol and second dehydration reaction ΔH_dh = 58.2 ± 2 kJ/mol and ΔH_iw = 22.6 ± 2 kJ/mol; and, for Mg-exchanged montmorillonite, first dehydration reaction, ΔH_dh = 49.3 ± 0.5 kJ/mol and ΔH_iw = 11.1 ± 0.5 kJ/mol and second dehydration reaction, ΔH_dh = 66.2 ± 2 kJ/mol and ΔH_iw = 31.1 ± 2 kJ/mol. Comparison with Na-montmorillonite and a K-exchanged montmorillonite shows that the enthalpy of the first dehydration reaction of the alkali montmorillonites is significantly lower than for Ca- and Mg-exchanged montmorillonites. The high value of ΔH_iw of 31.1 kJ/mol suggests a unique environment for H₂O molecules around Mg, perhaps indicating that these molecules are located partly within the sixfold silicate ring of the 2:1 layer in this montmorillonite.

In sedimentary sequences, smectites often coexist with brines. Also, PH₂O is often less than P_load. In these systems, the activity of H₂O is significantly reduced and may be buffered by the dehydration of montmorillonite. Substantial amounts of smectite in altered ocean floor and in ocean sediments may have important bearing on the introduction of water to the upper mantle.