Abstract
High-pressure differential thermal analysis (hp-dta) of the dehydroxylation of a Na-rich montmorillonite (Clay Minerals Society Source Clay SWy-1) and of the same montmorillonite after being K-exchanged was carried out at pressures to 2.5 kbar. The dehydroxylation can be treated as a reaction in the binary system montmorillonite-H2O. The dehydroxylation reaction of SWy-1, montmorillonite (M) = montmorillonite dehydroxylate (MD) + vapor (V), occurs at about 710 °C at 1-2 atm and increases to 793 °C at approximately 40 bars, where it terminates in an invariant point. At this point, three other reactions emanate. One of these, the reaction M + MD = liquid (L), also occurs at approximately 800 °C at 500 bars. At a slightly higher but unknown temperature and pressure, a second reaction (M = L + V) terminates in a singular point, from which two additional melting reactions originate: M + V = L, which decreases to 674 °C at 2460 bars, and M = L, which is assumed to lie close to the reaction M + MD = L. The fourth reaction emanating from the invariant point, MD + V = L, also occurs at approximately 920 °C at 1 atm. The reactions involving K-exchanged SWy-1 are analogous to those of SWy-1. The dehydroxylation occurs at about 705 °C at 1-2 atm, increasing to 800 °C at approximately 40 bars. The melting reaction M + V = L decreases in temperature to 728 °C at 770 bars. For both SWy-1 and K-exchanged SWy-1, the temperature range over which the reaction M + MD = L occurs becomes narrower at higher pressures. This is especially noticeable in runs with K-exchanged SWy-1. The enthalpy of dehydroxylation for SWy-1 is estimated at 300 ± 50 kJ/mol and for K-exchanged SWy-1 at 260 ± 50 kJ/mol. The above reactions represent metastable equilibria.
