Water can exist within solids in many forms : adsorbed on the surface or within micropores, occluded in voids, trapped within a molecular cage as in clathrates, or chemically bonded to species within the crystal lattice. Only in the latter case does it markedly alter the microscopical properties of a crystal. The water molecule possesses an unusually low molecular refraction coefficient. When bound to a crystal, it strongly depresses the principal refractive indices from those of the parent (anhydrous) compound. Examination of optical-crystallographic data from several hundred compounds shows a linear relationship between the per cent water and the mean refractive index for a given series of crystal hydrates. Moisture evolved on heating may be identified by its characteristictemperature and rate of evolution and by other physical and microchemical tests. Hot stage and micro DTA methods are discussed, along with special techniques for the examination of hydrated crystals in the scanning and transmission electron microscopes. © 1972.
Stevens, R. E. (1972). Microscopical identification of water in crystals. Analytica Chimica Acta, 60(2), 325–334. https://doi.org/10.1016/S0003-2670(01)95009-3