The Reversible Dehydroxylization of Clay Minerals

Rehydroxylization of the anhydrous modification of dioctahedral montmorillonites can be carried out under DTA conditions with controlled atmosphere techniques. The structural character of the rehydrate prepared in this way is determined from X-ray and DTA data. The mechanism of the reactions by which the rehydrate can be formed is also considered. Thermal analysis curves of the rehydrate form of several pure montmorillonite samples failed to show any large variation such as is shown by the curves of the original clays themselves. The possibility is considered that such large variations in the dehydroxylization curves of montmorillonites are related to structural instead of compositional variations within the sample. It is suggested that montmorillonites showing two reactions attributable to dehydroxylization may be mixtures of the rehydrate form and montmorillonite. The addi t ion of cont ro l led-a tmosphere techniques to classical differential thermal analysis ( D T A ) has served to s t imulate new interes t in its use in clay minera l research. The amount of addi t ional in fo rmat ion made accessible wi th control led a tmosphere can of ten make the process a profi table research toot fo r problems in which the classical methods would fail to reveal any usefu l data. Not only can the usual D T A react ions be modified by heat ing in a control led a tmosphere , for example , ana lyz ing specimens r ich in organic mater ia l in an a tmosphere of iner t gas such as argon, but also new react ions can be p romoted by cooling high t empera tu re phases in an active a tmosphere . Cont ro l led-a tmosphere D T A was appl ied in s tudying the react ions of clay minera ls which involve crys ta l l ine or hydroxy l water . S t a n d a r d D T A condi t ions of heat ing in an a tmosphere of o rd ina ry a i r at a ra te of about 1 2 ~ per minute were used to p roduce an anhydrous fo rm i f the clay minerals . The anhydrous fo rm was normal ly completely developed by 750 ~ C, as indicated by the r e tu rn of the D T A curve to base line fol lowing the endothermic react ion associated with the removal of crysta l l ine or h y d r o x y l water . The anhydrous mater ia l was then cooled at a control led ra te of about 10 ~ C per minute to a t empera tu re of 400 ~ C. Dur ing the cooling an a tmosphere of wate r vapor was main ta ined a round the sample. T h e appara tus which provided these exper imenta l condit ions was s imilar to the one descr ibed by Stone (1952) in his thermal analysis of kaolinites, * Published with permission of the Chief, Illinois State Geological Survey, Urbana, Illinois. t Present address, The University of Texas, Austin, Texas.