The rate of silica removal from two montmorillonites (Chambers and Polkville) has been measured as a function

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of time, temperature, solution composition, and exchange ion on the clay. Solution compositions ranged from 400 to 4,000 ppm potassium in all samples. Sodium concentration ranged from 0 to 9,400 ppm, calcium from 0 to 380 and magnesium from 0 to 10 ppm. Silica removal rate increased as the temperature increased from 200 to 350°C, decreased with time, and could be approximated initially by a parabolic rate law. Within the time range (from 1 to 10 days) approximated by the parabolic-rate law, comparison of rate constants allows quantitative evaluation of the effects of solution chemistry and exchange ion. Calcium-saturation of the clay reduced the value of the rate constant, relative to sodium-saturation, by about 50%. In all analyses, increasing solution concentration of an ion de reased the rate of silica removal. On an equimolar basis, magnesium was most effective at inhibiting dissolution, followed by calcium, sodium, and potassium. Reductions of the rate constant by 50 to 75% were observed for a Na-clay with 9,400 ppm sodium and for Ca-clay with 380 ppm calcium, relative to the sodium and calcium-free solutions. Activation energies for silica removal range from 3 to 12 kcal/mole. The highest values are associated with the largest concentrations of ions in solution, thus suggesting dissolution-inhibition by an ion adsorption mechanism. These results demonstrate that silica dissolution rate depends dramatically on solution composition. This relation should be incorporated into models constructed to describe sandstone cementation or porosity enhancement by dissol tion and transport of dissolved silica from clays in sandstones or interbedded shales and sandstone sequences.

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