Abstract
The natural Na-montmorillonite (Tsukinuno mine, Yamagate Prefecture, Japan) was treated under hydrothermal conditions. Temperature ranged between 250 and 500°C at 100MPa, and run durations ranged between 1 and 150 days. The identification of phases was carried out by the X-ray powder diffraction method. The Greene-Kelly test was applied to distinguish among montmorillonite, beidellite and saponite. The phases identified in the products were plotted in a time-temperature diagram. The diagram showed that montmorillonite below 350°C reacted to the assemblage of mixed-layered phase of smectite and the 1:1 regular interstratification of montmorillonite/beidellite (S/r.i.M-B) +montmorillonite+quartz+analcime, and with increasing temperature transformed to the assemblage of Na-rectorite+quartz+feldspar, through the intermediate phase assemblage of beidellite+ saponite+ quartz+ feldspar. The diagram indicated also that montmorillonite and S/r.i.M-B might be stable clay minerals below 300°C, and that the decomposition of montmorillonite to an assemblage of beidellite, saponite and quartz can proceed in nature below the temperature where rectorite is formed.
