EXPERIMENTAL DETERMINATION OF SMECTITE HYDRATION STATES UNDER SIMULATED DIAGENETIC CONDITIONS (HIGH-TEMPERATURE, X-RAY DIFFRACTION, DEHYDRATION)

The hydration states of a dioctahedral smectite in contact with saline solutions were determined at temperatures and hydraulic pressures that simulate burial diagenetic environments. A high-pressure high-temperature X-ray cell, mounted on an automated General Electric XRD3 X-ray diffractometer, was used to monitor the basal reflections.;Na-Cheto exists as a three water layer complex, d(001) = 18.4-18.9 (ANGSTROM), in 0.5 molal NaCl, at pressures to 750 psi and temperatures less than 100(DEGREES)C. Reversible loss of the third water layer occurs between 100 and 175(DEGREES)C. Thermodynamic estimates of the range of temperatures over which the third water layer might be expelled are in good agreement with the experimental results.;Cheto montmorillonite exhibits irreversible collapse of 30% of the 12.4 (ANGSTROM) layers upon K-saturation at room temperature and pressure. Experimental X-ray diffraction analyses showed that a mixed-layered clay is present. However, low intensities and lack of the first order reflection preclude further interpretation of the results.;The results of this investigation affirm the generally accepted inverse relationship between salinity and swelling. They also demonstrate the inhibiting effect of high layer charge. Within the range of pressures ((LESSTHEQ) 6700 psi), temperatures ((LESSTHEQ) 200(DEGREES)C), and salinities ((GREATERTHEQ) 1 m NaCl) common to areas where illitization of smectite is known to occur, Na-smectite is stable as a two water layer complex. This negates the previously proposed mechanism for stepwise loss of interlayer water at depth. The results of this investigation provide further evidence for the loss of the interlayer water during the illitization reaction, which requires layer charge build-up and fixation of K('+) as the interlayer cation.;Sodium-saturated Cheto montmorillonite exists as a two water layer complex, with basal spacing between 15.2 and 15.7 (ANGSTROM) in 1 and 5 molal NaCl brines at hydraulic pressures to 6700 psi and temperatures to 200(DEGREES)C. No change in hydration state was detected although thermodynamic analysis predicts that loss of interlayer water may occur at temperatures above 63 and 135(DEGREES)C for the 5 and 1 molal NaCl systems, respectively.