Dehydroxylation of Ca- and Mg-exchanged montmorillonite Available to Purchase

Montmorillonite is a phase in the metastable binary system (montmorillonite dehy-droxylate)-H₂O. By using high-pressure differential thermal analysis (hp-dta) of Ca- and Mg-exchanged montmorillonite (Clay Minerals Society Source Clay SW_y-1) to pressures of 1.2 kbar, dehydroxylation and melting reactions were found involving montmorillonite (M), an aqueous vapor (V), and a silicate liquid (L). In addition, one dehydroxylated phase (DH) is present in the Ca-rich system, and two dehydroxylated phases (DH1, DH2) are present in the Mg-rich system. These differences are caused by the ability of Mg to move close to the residual oxygen upon partial dehydroxylation, whereas Ca is limited in its ability to migrate. The DH 1 phase is an intermediate dehydrated structure with Mg paired with the residual oxygens, and DH2 is the fully dehydrated phase.

The dehydroxylation reaction of Ca-exchanged montmorillonite (M = MD + V) occurs at 715 °C and 1 bar and increases to 800 °C and 30 bars where this reaction terminates in an invariant point. Three additional reactions involving a liquid phase emanate from this point. The reaction M + DH = L is nearly temperature independent, reaction M = L + V terminates at slightly higher temperature and pressures in a singular point, and MD + V = L intersects 1 bar at about 920 °C. Two additional melting reactions emanate from the singular point. One reaction, M + V = L, decreases to 683 °C at 735 bars, and the second, M = L, is assumed to lie close to the reaction M + DH = L. In the Mg-rich system, as a result of the partly dehydrated phase (DH1), three invariant points and two singular points are present. The general topology is analogous to the Ca-rich system, although much more complex. The dehydroxylation reaction for the Mg-exchanged montmorillonite occurs at about 725 °C at 1 bar and increases to 730 °C at 1.3 bars, to terminate in an invariant point. Two dehydroxylation reactions emanate from this point, each terminating in invariant points (at 795 °C, 25 bars; 785 °C, 65 bars). At these points, a series of reactions involving silicate liquids emanates in analogy with the Ca-rich system. Below the invariant point at 1.3 bars, DH2 is more stable than DH1 because of the inability of the Mg cation to effectively interact with a newly formed residual oxygen upon dehydroxylation. An expanded interlayer region at low pressures accounts for this.

Estimates for the enthalpy of dehydroxylation range from 350 ± 50 kj/mol for Ca- exchanged SWy-1 to 500 ± 300 kj/mol for the complete dehydroxylation reaction at low pressures for Mg-exchanged SWy-1, 600 ± 100 kj/mol for the partial dehydroxylation M = DH1 + V, and 400 ± 100 kj/mol for the loss of the remaining hydroxyls in DH1 = DH2 + V.