18. Bird, P. (1984) Hydration-phase diagrams and friction of montmorillonite under laboratory and geologic conditions, with implications for shale compaction, slope stability, and strength of fault gouge, Tectonophysics, 107, 235-260.
Abstract. Absorption of water into the crystal structure of smectite clays causes a reversible volume increase exceeding 80% and a friction decrease by a factor of three. This behavior can be predicted if clay swelling is approximated as the formation of successive hydration phases and the generalized Clapeyron law is applied. In this study, new data on two standard montmorillonites saturated with sodium and calcium (Na-MM and Ca-MM) are combined with published results to define relative humidity boundaries of proposed phases at 20° C and 1 bar. From molar volumes, the effect of confining pressure is predicted; only phases with an integral number of full interlaminar water layers are stable at high pressures. In contact with liquid water, the number of interlaminar layers is governed by the "effective normal stress" on crystals. Next, vapour pressure measurements at 20° -60° C are used to calculate dehydration enthalpies, from which the effect of geologic temperatures is predicted. Under typical crustal conditions, Na-MM does not fully dehydrate until 7 km depth, and Ca-MM retains one water layer to 11 km. Since the zones of hydrocarbon maturation and montmorillonite dehydration overlap, this effect may play a part in petroleum geology, controlling migration and/or reservoir pressurization.
For the first time, the friction of individual hydration phases of montmorillonite has been measured without excess water and within their stability fields. Stable-sliding deformation with negligible rate-dependence and definite work-hardening is observed, but the pressure-dependence is less than in classical friction. Na-MM phases are always weaker than their Ca-MM analogues. In both clays, the addition of the first water layer causes the greatest strength decrease, but two- and three-layer phases are successively weaker. The friction coefficient of three-layer Na-MM at low pressure is near 0.27, consistent with most observed bedding plane rockslides in shales. If Ca-MM is abundant in fault gouge, average friction in the top 10 km of strike-slip and normal faults may be as low as 0.34. Such weak faults would permit major deformation of continental crust by regional shear stresses as low as 27 MPa. This model would also explain the frequent reactivation of ancient faults by later stresses of diverse orientations.