THE STATE OF HYDRATION OF Ca-EXCHANGED MONTMORILLONITE AT A DEPTH OF 2 2.7 KILOMETERS

Abstract

Monte Carlo simulations in NP zz T and μVT 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 H 2 O are clustered on the interlayer midplane, alternatively oriented with their protons toward the siloxane surfaces on both sides and on the midplane. The Ca 2+ cations are solvated, in outer-sphere coordination, and located 2.77 A from the H 2 O molecules. In sedimentary basins under normal geotherms, 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.