- Mineral hydration
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.
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 in the hydration of feldspars to clay minerals, garnet to chlorite or kyanite to muscovite.
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 Portland cement develops strength.
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