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

Monte Carlo simulations in NP=T and mu VT ensembles of the hydration of Wyoming-type Ca-montmorillonite have shown the interloyer configurations. Ca-montmorillonite may hydrate to one-, two- and three-layer hydrates of d(001) spacing 11.83, 13.73, and 15.60 angstrom at 353 K and 625 bat: At lower temperatures and pressures the spacing increases. Grand canonical simulations show that the one-layer Ca-montmorillonite hydrate of d(001) spacing 12.11 angstrom is stable at 353 K, 300 bat-, -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 bat: -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 angstrom from the water molecules. In sedimentary basins under normal geotherms, one-layer Ca-montmorillonite is the single hydrate stable at 2 kin depth; under over-compacted sediments at 2.7 kin depth it becomes unstable.