DEHYDRATION AND HYDRATION OF MONTMORILLONITE AT ELEVATED-TEMPERATURESAND PRESSURES MONITORED USING SYNCHROTRON-RADIATION

Na-saturated montmorillonite hydrate with three layers of H2O in the i nterlayer (d001 = 17.65-18.56 angstrom) was found to be stable up to 3 40-degrees-C and 2 +/- 1 kbar of H2O pressure using a diamond-anvil ce ll by real-time diffraction using X-rays from the Cornell High Energy Synchrotron Source (CHESS). It dehydrates to montmorillonite with two H2O layers at higher temperature (d001 = 15.38 angstrom). The hydrate with two H2O layers, in turn, dehydrates to montmorillonite with one H 2O layer at 483-degrees-C and 4.4 +/- 1.3 kbar (d001 = 12.56 angstrom) . The montmorillonite hydrate with one H2O layer dehydroxylates or mel ts at 525-degrees-C and 5 +/- 1.4 kbar, conditions that are much lower than those found at lower pressures. Montmorillonite with basal spaci ng intermediate between two homogeneous states, which was interpreted as interstratification of two hydrates, is also stable at the elevated temperatures and pressures. Results confirm that the dehydration temp erature of montmorillonite significantly increases with increasing H2O pressure, consistent with the idea that the density of interlayer H2O in montmorillonite is higher than that of pore H2O. Kinetic data show that the dehydration of interlayer H2O is fast (on the order of minut es) even at high H2O pressure, whereas the rehydration reaction is slo wer. The rehydration rate significantly increases when the sample cool s to temperatures far below equilibrium. The preliminary experiments u sing micrograms of swelling clay, montmorillonite, affirm that synchro tron radiation can be a very useful tool to quantify the fluid-rock re actions occurring within a diamond-anvil cell.