<p>Mineral hydration has fundamental geological and technological implications. Despite extensive research, its reaction mechanism(s) and effects on the progress of key metamorphic reactions such as serpentinization, the damage of ceramics, or the setting/hardening of cements are not fully understood. Here, we studied the hydration of periclase (MgO) single crystals and powders forming brucite (Mg(OH)₂) as an analogue for key mineral hydration reactions. Our results show that hydration occurs through an intermediate amorphous phase, as in other oxide and silicate minerals, and results in an epitaxial relationship between periclase and brucite. Mg(OH)₂ precipitates on MgO despite the bulk solution remaining undersaturated with respect to both the amorphous precursor and brucite. This is related to the development of strong concentration gradients at the periclase-solution interface. Remarkably, the transformation of the amorphous precursor into brucite within periclase etch pits and precritical microcracks generates enough supersaturation and crystallization pressure to fracture MgO crystals, enabling further progress of the hydration reaction. This previously unrecognized mechanism for reaction-induced fracturing has relevant natural and technological implications.</p>