Applied to a phase which is apparently stable but is capable of reaction if disturbed, a state usually due to the slowness of a system to attain equilibrium. The term is applied, for example, to supersaturated solutions. A phase is said to be metastable if it exists in the same temperature range in which another phase with lower vapour pressure is stable. Many minerals that occur at room temperature and pressure are metastable (e.g. diamond) and most metamorphic rocks formed under high temperatures and pressures are metastable at the surface under normal temperatures and pressures. A metastable system (mineral assemblage) is in a temporary state of equilibrium and requires only a minimal disturbance to initiate a change to a state of true equilibrium. A gabbro mineral assemblage which crystallized at high temperatures and moderate pressures, and is thus in equilibrium under these conditions, would be in a metastable state when exposed by erosion to the low-temperature, low-pressure environment at the Earth’s surface. Rainwater falling on the exposed mineral assemblage would act as the disturbance or catalyst initiating the reaction of the gabbro mineral assemblage to a low-temperature, low-pressure, chlorite–clay mineral assemblage, thus re-establishing the equilibrium.