Thermodynamic and structural aspects of the dehydration of smectites in sedimentary rocks

Abstract

A B S T R A C T:The diagenetic conversion of smectite to illite in shales has been proposed as a mechanism for generating overpressure (fluid pressure above hydrostatic) in sedimentary basins. However, the mechanism and rate-controlling factors of the reaction and the magnitude of the resulting volume change are not known. In this paper the thermodynamics of the reversible hydration/dehydration of smectite are analysed in the pressure/temperature regime of sedimentary rocks. If the interlamellar water is more dense than bulk water, and the pressures on the solid and fluid phases are equal, the dehydration temperature should increase with increasing confining pressure via the Clapeyron-Clausius equation. However, either fluid pressures below the pressure on the solid matrix or high pore-fluid salinities can reduce the dehydration temperature. Experiments on Ca2+-montmorillonite compacted in the presence of distilled water in a steel autoclave fitted with a sensitive pressure transducer failed to detect any overall volume change due to dehydration up to 185~ and 1400 bar. This finding, which is consistent with recent high-pressure DTA and XRD studies on Na+-montmorillonite, suggests that reversible loss of the last two water layers does not occur in sedimentary basins except as part of the more complex diagenetic conversion of smectite to illite. Some recent evidence from high-resolution TEM and microprobe analyses regarding the mechanism of the reaction is also discussed.

In situ XRD study of Ca2+ saturated montmorillonite (STX-1) exposed to anhydrous and wet supercritical carbon dioxide

“…Nonetheless, the difference in behavior between N 2 and scCO 2 clearly shows that scCO 2 was the active agent responsible for partially dehydrating the clay and not the increase in pressure. In fact, previous work has shown that increasing pressure tends to stabilize interlayer water and can offset the effect of increasing temperature (Vangroos and Guggenheim, 1984;Hall et al, 1986;de Siqueira and Skopper, 1998;de Siqueira et al, 1999). The solubility of water in scCO 2 is significant and under these experimental conditions (50 • C and 90-180 bar) could contain up to 0.14 and 0.27 wt% before becoming fully saturated (Spycher et al, 2003).…”

Adsorption of Water by Homoionic Exchange Forms of Wyoming Montmorillonite (SWy-1)

“…Thus, as discussed by Hall et al (1986), in most sedimentary rocks the loss of the last two interlamellar water layers from smectites is most probably normally associated with the process ofillitization, usually at a temperature substantially below the thermodynamic dehydration temperature of smectite. The thermodynamic considerations provide an upper stability limit for hydrated montmorillonite, which might conceivably be approached in hot but relatively young sediments, e.g., in hydrothermal environments, in which there has been insufficient time for illitization to have occurred, or where the illitization has been kinetically hindered, as, e.g., in K-poor environments.…”

“…An attempt to explore the PT phase diagram directly for the hydration of homoionic calcium montmorillonite by monitoring the fluid pressure change associated with the dehydration is reported elsewhere (Hall et al, 1986). The method detected no change in the state of hydration at elevated pressures (<20,000 psi) to the maximum temperature investigated (185~ These results are in accord with high-pressure XRD studies by Colten (1986), which showed no collapse of the basal spacing in a bilayer hydrate of sodium montmorillonite <200~ and pressures ---6700 psi.…”

The pressure dependence of the dehydration of gypsum to bassanite

“…The apparatus used was that described by Hall et al (1986) and comprised a stainless steel autoclave of 100 cm 3 interior volume, a needle valve, bursting disc and pressure transducer all having a pressure rating of 2 kbar. The autoclave was wholly submerged in an oil-bath filled with silicone oil.…”