Widely used geologic term denoting a process that involves some kind of transformation. In a petrological sense it refers to the partial or complete alteration of an original mineral to an aggregate of secondary minerals, by the diffusion of ions between the solid phases and an introduced, fluid (usually water-rich) phase. Such diffusion takes place easily when the temperature of the rock system is below the stability limit of an individual mineral and a fluid is present to act as a catalyst to initiate the diffusion reactions. The secondary minerals may be all of one type, or a combination of mineral types. For example, high-temperature, magnesium-rich olivine can be replaced by an aggregate of secondary serpentine and chlorite, while plagioclase can be replaced by a fine aggregate of white mica (sericite). See also fossilization.