Evaluation of the effect of pressure on the temperature of interlayer water loss (dehydration) by smectites under diagenetic conditions indicates that smectites are stable as hydrated phases in the deep subsurface. Hydraulic and differential pressure conditions affect dehydration differently. Smectites under hydraulic pressure conditions, such as in the pores of a sandstone, will retain at least two water layers (basal spacing, d₀₀₁ = 15A). Smectites under differential pressure for vertical effective stress conditions, such as in compacting shales, are stable as two-water-layer complexes to temperatures of 67°-81°C (153°-178°F), at which one water layer will be expelled. Loss of the remaining water layer requires temperatures of 172°-19 °C (342°-377°F).

The temperatures of dehydration increase with pore fluid pressure and interlayer water density. The stability of hydrated smectite under hydraulic-pressure conditions indicates that simple thermal dehydration of smectite is not important in the development of abnormally high subsurface fluid pressures. Loss of water by smectite in a shale of low permeability may cause overpressuring; however, the resulting increase in fluid pressure will inhibit further dehydration.

The temperatures of dehydration under differential-pressure conditions are inversely related to pressure and interlayer water density. The temperature range for interlayer water loss by smectite under differential-pressure conditions is approximately coincident with the beginning of the smectite-to-illite transformation and hydrocarbon generation. The model presented assumes the effects of pore fluid composition and 2:1 layer reactivity to be negligible. Agreement between theoretical and experimental results validate this assumption. However, changes in the chemical stability of the 2:1 layer with decreasing interlayer water content, increasing pressure and temperature, and changing pore-fluid chemistry may be important in initiating clay and organic matter transformations. Changes in clay stability with interlayer water loss may also be responsible for the different rates of smectite-to-illite transformation observed in interbedded sandstones and shales.