Wl. Huang et al., DEHYDRATION AND HYDRATION OF MONTMORILLONITE AT ELEVATED-TEMPERATURESAND PRESSURES MONITORED USING SYNCHROTRON-RADIATION, The American mineralogist, 79(7-8), 1994, pp. 683-691
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.