The state of hydration of Ca-exchanged montmorillonite at a depth of 2-2.7 kilometers

Monte Carlo simulations in NPzzT and mu VT ensembles of the hydration of Ca-saturated Wyoming-type montmorillonite show that one-layer Ca-exchanged montmorillonite hydrate with a d(001) of 12.11 A is stable at 353 K, 300 bar, and -7.21 kcal/mol potential. Two- and three-layer hydrates do not appear stable. At 353 K and 625 bar, the one-layer hydrate is nearly stable. In the interlayer space, molecules of H2O are clustered on the interlayer midplane, alternatively oriented with their protons toward the siloxane surfaces on both sides and on the midplane. The Ca2+ cations are solvated, in outer-sphere coordination, and located 2.77 A from the H2O molecules. In sedimentary basins under normal geotheons, one-layer Ca-exchanged montmorillonite is the single hydrate stable at 2 km depth. In overcompacted sediments at 2.7 km depth, the hydrate could be unstable.