The beam of an electron microscope has been used to dehydrate single crystals of magnesium hydroxide to magnesium oxide. Electron diffraction photographs and electron micrographs were taken at various stages to follow the crystallographic and morphological changes which accompany decomposition. The decomposition may be considered to occur in two stages. First, there is a small shrinkage in the basal plane, and the resulting strain causes a maze of cracks in the crystal. This change is followed by a collapse of the planes down the original [0001] of magnesium hydroxide. The collapse is controlled by the migration of water molecules from between the planes to a surface where they can escape. The product is a highly oriented aggregate of micro-crystallites of magnesium oxide. More intense irradiation in the electron beam occasionally causes bulk movement of the solid.
