In inorganic chemistry, mineral hydration is a reaction which adds water to the crystal structure of a mineral, usually creating a new mineral, commonly called a hydrate. In geological terms, the process of mineral hydration is known as retrograde alteration and is a process occurring in retrograde metamorphism. It commonly accompanies metasomatism and is often a feature of wall rock alteration around ore bodies. Hydration of minerals occurs generally in concert with hydrothermal circulation which may be driven by tectonic or igneous activity.

Processes

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

Examples of hydrated minerals include: (Fe(2+))5.4(Mg,Fe(2+))5S6(OH)10, also written 6 Fe0.9S * 5 (Mg,Fe(2+))(OH)2, in IMA notation (Fe(2+),Cu)4(Mg,Al)3S4(OH,O)6, or 4 (Fe,Cu)S * 3 (Mg,Al)(OH)2