Hydration of Ca-montmorillonite at basin conditions: A Monte Carlo molecular simulation

Abstract

Monte Carlo simulations in NP_zzT and mVT ensembles of the hydration of Wyoming-type Ca- montmorillonite have shown the interlayer configurations. Ca-montmorillonite may hydrate to one-, two- and three-layer hydrates of d₀₀₁ spacing 11.83, 13.73, and 15.60 Å at 353 K and 625 bar. At lower temperatures and pressures the spacing increases. Grand canonical simulations show that the one-layer Ca-montmorillonite hydrate of d₀₀₁ spacing 12.11 Å is stable at 353 K, 300 bar, -7.21 kcal/mol potential, at a 2.0 km depth of normally compacted sediments. Two- and three-layer hydrates do not form. At  353 K, 625 bar, -5.58 kcal/mol potential, the one-layer hydrate is nearly stable. In the clay interlayer, the water molecules are clustered on the midplane, with their protons pointing towards the siloxane surfaces on both sides and on the midplane. The Ca²⁺ cations are solvated in outer-sphere coordination, separated 2.77 Å from the water molecules. In sedimentary basins under normal geotherms, one-layer Ca-montmorillonite is the single hydrate stable at 2 km depth; under over-compacted sediments at 2.7 km depth it becomes unstable.