Ramesh Dayal

The rate and extent of cation sorption and desorption were studied in the laboratory for kaolinite-, illite- and montmorillonite-sea water systems at 22^oC and pressure ranging from 1 to 1000 atmospheres. Kaolinite and illite exhibit decreasing sorption of magnesium, potassium, calcium and strontium with increasing pressure of reaction. In the case of montmorillonite, a similar sorption pattern was observed for magnesium and potassium, and the opposite pattern for calcium and strontium. Decreasing cation sorption by clay with increasing pressure of reaction is attributed to the effect of pressure on the ion exclusion properties of the structured water in the vicinity of the clay particles. It is possible that these electrolyte exclusions properties of clay minerals in addition to the effects of overlapping electrical double layers in compacting sediments result in the high salt concentration of formation water in sedimentary rocks.

The rate data indicates that the cation sorption and desorption reactions are initially rapid and reaction is virtually complete after a period of hours to a few days. Pressure does not affect the reaction rates significantly.

The silica results show that the solubilities of montmorillonite, illite and kaolinite increase with increasing pressure of reaction in the pressure region from 335 to 1000 atmospheres. The clay solubility was found to be a linear function of the pressure in this pressure range. The extent of clay solubility at pressure is a function of the clay mineral type. Using the experimental clay solubility data at different pressures, the partial molal volume change for the kaolinite and illite dissolution reactions was calculated to be -56 cm³/mole and -106 cm³/mole respectively. The molar volume for hydrated kaolinite was found to be 128 cm³/mole and that for hydrated illite 388 cm3/mole. Comparison of the molar volume values for hydrated clays with those for unhydrated clays suggest that hydrated kaolinite contains approximately 1.6 molecules of water and hydrated illite approximately 5.8 molecules.

Kinetic data obtained from the silica-depleted and silica-enriched - clay mineral experiments at 1 to 670 atmospheres indicate that diffusional transfer of H₄SiO₄ aq. through a silica-depleted or silica-enriched reaction rim determines the rate of incongruent clay dissolution reaction and silica sorption reaction. The rates of incongruent clay dissolution and silica sorption reactions are in the order of 10-8mg SiO2/sec1/2 cm²; similar to those reported for the incongruent dissolution of other aluminosilicates. The direction and extent of the silica-clay reactions are affected by the type of clay mineral and the pressure of the experiment. Silica sorption at atmospheric pressure exhibited by kaolinite changes to silica release at 670 atmospheres. The rate of silica-clay reactions are not affected by pressure. The coefficient of diffusion of silicic acid through the silica-depleted reaction rim resulting from the incongruent of kaolinite at 670 atmospheres was found to be of the order of 10-18 cm²/sec. The calculated thickness of the silica-depleted reaction rims is of the order of Angstrom units.

Equilibrium and kinetic considerations of silica data indicate that clay minerals sustain and buffer the interstitial silica concentration of sediments depleted in biogenic silica. Reconstitution of detrital clays and authigenic formation of other silicates may be responsible for the commonly observed low interstitial silica concentrations, relative to the equilibrium solubility of amorphous silica, in sediments containing abundant biogenic silica.

Changes in the solid phase resulting from reaction with sea water were also studied and some significant changes in the resultant reaction products were detected by X-ray diffractometry. For partially mixed layer illite clay, increasing pressure of reaction results in the reconstitution of 10 A illitic layers, in terms of the crystallinity and the potassium content, at the expense of the expandable component. A trend of polytype conversion (1 Md - 2M) towards higher stability forms with increasing pressure of reaction was observed. The octahedral layer of illite is being enriched in aluminium presumably as a result of increasing release of iron with increasing pressure. Decrease in the crystallinity of montmorillonite with increasing pressure of reaction was observed and this may reflect structural change to chlorite-like derivatives. Kaolinite exhibits increasing crystallinity with increasing pressure of reaction, but thermodynamic and kinetic considerations of the potassium and silica data suggest the development of amorphous and semi-crystalline potassium aluminosilicate layers on kaolinite.

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Supervisor: Gunter Muecke