A mineral formed by the subsolidus alteration of a pre-existing primary mineral in an igneous rock. Minerals which have crystallized from a magma are stable only at high temperature and can readily alter to low-temperature, secondary minerals when a fluid, e.g. water, is introduced into the rock system. The fluid acts as a catalyst to initiate the alteration reaction. Most secondary minerals are hydrated silicates. A typical example is the alteration of primary olivine to secondary chlorite and serpentine.