Anhydrite is not a common mineral, as it easily alters to the much more common mineral Gypsum from the addition of water into its chemical structure. Anhydrite and Gypsum are chemically almost the same mineral, except Gypsum has the addition of water. In fact, the name of Anhydrite is derived from "An" and "Hydra" - meaning "without water" - in reference to its similarity to Gypsum but the fact that it lacks water. Some specimens only partly alter to Gypsum, leaving one part Anhydrite and the other part Gypsum. Many deposits that once contained much Anhydrite now contain an abundance of Gypsum which was formed by the alteration of the Anhydrite. Anhydrite also exists as a relic of the past in several traprock occurrences, where the Anydrite dissolves and leaves a hollow cast around its original form. Epimorphs of Quartz and Prehnite over Anhydrite frequently form at certain localities, with the original Anhydrite totally replaced or dissolved. Anhydrite sometimes occurs in arid regions, forming from the dehydration of Gypsum. Fine but usually small crystals may come from the rock area above salt domes, where the domes absorb all underground water and prevent it from entering the structure of the Anhydrite, which would otherwise cause it to alter to Gypsum.