Two crystals are said to be isostructural, if they have the same structure, but not necessarily the same cell dimensions nor the same chemical composition, and with a 'comparable' variability in the atomic coordinates to that of the cell dimensions and chemical composition. For instance, calcite CaCO 3, sodium nitrate NaNO 3 and iron borate FeBO 3 are isostructural. One also speaks of isostructural series, or of isostructural polymorphs or isostructural phase transitions. The term isotypic is synonymous with isostructural.