There are two main ways in which minerals hydrate. One is conversion of an
oxide to a double
hydroxide, as with the hydration of
calcium oxide—CaO—to
calcium hydroxide—Ca(OH)2. The other is with the incorporation of water molecules directly into the crystalline structure of a new mineral, as with the hydration of
feldspars to
clay minerals,
garnet to
chlorite, or
kyanite to
muscovite. Mineral hydration is also a process in the
regolith that results in conversion of silicate minerals into
clay minerals. Some mineral structures, for example,
montmorillonite, are capable of including a variable amount of water without significant change to the mineral structure. Hydration is the mechanism by which
hydraulic binders such as
Portland cement develop strength. A hydraulic binder is a material that can set and harden submerged in water by forming insoluble products in a hydration reaction. The term hydraulicity or hydraulic activity is indicative of the
chemical affinity of the hydration reaction. == Examples of hydrated minerals ==