Abstract
Themoanalytical studies of dehydration of fibrous clay minerals (sepiolite and palygorskite) and fibrous zeolites are briefly reviewed mainly based on the works in our laboratory.
Fibrous clay minerals;
It is well established that sepiolite and palygorskite contain three types of water (zeolitic and coordination water and hydroxyl) in the structure. As already confirmed by many workers, these waters eliminate in discrete four steps in sepiolite, including the two-step dehydration of coordination water. Further, there is a good agreement between the theoretical water losses calculated from the structure formula (the Brauner and Preisinger model) and the observed ones from TG curves. However, palygorskite dehydrates in three steps due to a superposition of the temperature regions where the waters are lost. This is presumably because palygorskite has dioctahedral nature. In addition, the mechanism of the two-step dehydration of the coordination water in sepiolite are studied by TG under controlled water vapour pressures and kinetic analysis of TG data.
Fibrous zeolites;
Dehydration behaviour and thermal stability of (1) natrolite and its K-exchanged forms and (2) tetranatrolite-gonnardite- Ca-gonnardite series are examined by TG-DTA and high temperature X-ray powder diffraction.
In the former case, the dehydration temperature is decreased from 330°C for natrolite to 150°C for the K-exchanged form, and the destruction temperature is increased from about 800°C for the former to about 1000°C for the latter, by K-exchange for Na.
In the latter case, by the cation-exchange of natural gonnardite with Na and Ca, a series of Na_??_Ca exchanged forms was obtained. Ca-gonnardite, corresponding to the Ca-end member in the series, dehydrates in two steps like scolecite. The destruction temperature is about 240°C and lower by 70°C than those of tetranatrolite and gonnardite.
Finally, diverse dehydration behaviours of natural thomsonite are examined in relation to crystallo-chemical properties by TG-DTA, X-ray powder diffraction, EDS and 29Si MAS NMR. As a result, it has been confirmed that thomsonite with nearly ideal chemical composition (Si/Al ratio, about 1.0, Na/Ca ratio, about 0.5) dehydrates in three steps below 450°C, and the weight loss due to dehydration at each step is approximately equal.
