Electronic microdiffraction study of structural modifications resulting from the dehydration of gypsum. Prediction of the microstructure of resultingpseudomorphs

Endothermic decomposition reactions (solid 1 --> solid 2 + gas) generally a ffect only part of the inter-atomic bonds of the solid 1 structure. In cons equence, the morphology and external dimensions of particles remain unchang ed (the particles of solid 2 are referred to as pseudomorphs of those of so lid 1). The gas release normally leads to a decrease of the molar volume of the precursor solid, resulting in cracking and formation of intra-particle porosity, This work deals with such a reaction: the dehydration of gypsum into sub-hydrated and anhydrous phases. Ultrathin (010) gypsum cleaved plat es have been dehydrated under controlled conditions and studied by electron microdiffraction, All reactions have been shown to preserve four different sets of three-dimensional orientation relations between all structures. Th ese topotactic orientation relations allow us to clarify the associated ato mic rearrangements. The reactions are described in a simplified net common to all structures, which allows to predict the microstructure of the result ing pseudomorphs. (C) 2001 Elsevier Science B.V. All rights reserved.