SYNTHESIS AND PHASE RELATIONS IN MONTMORILLONITE-STEVENSITE SERIES UNDER HYDROTHERMAL CONDITIONS

Abstract

Smectites and related phases were synthesized systematically from quenched glasses with stoichiometric dehydrated Na-smectite compositions in a montmorillonite-stevensite pseudo-binary join under hydrothermal conditions. Experiments were conducted at temperatures from 250 to 500℃ and at 100 MPa, and durations ranged from 1 to 151 days. The existence of a chemical composition gap in the pseudo-binary join between dioctahedral and trioctahedral smectites at 350℃ or lower was confirmed from the time-temperature-transformation relationships obtained. The smectite with intermediate composition was recognized to be metastable; it changed to rectorite through coexisting dioctahedral and trioctahedral smectites. Regularly interstratified chlorite-smectite was then obtained above 500℃. In the montmorillonite composition system, a reaction sequence comparable to that of the natural system was obtained from montmorillonite to rectorite through beidellite plus saponite. Above 500℃, a mica and mica-like phase expanded by ethylene glycol appeared. In the region of trioctahedral composition, unusual high-crystallinity smectite appeared after the low-crystallinity smectite (which is comparable to natural smectite), and then mica was obtained. In the stevensite composition system, stevensite as an initial phase was transformed to a novel regularly interstratified talc-talc-stevensite (r.i.TTS) and then to mica. The structure of r.i.TTS was confirmed by a computer simulation of the XRD line profile.