periclase, magnesium oxide mineral (MgO) that occurs as colourless to grayish, glassy, rounded grains in marble and in some dolomitic limestones, where it formed by the metamorphosis of dolomite at high temperatures. Rocks containing periclase have been identified at Monte Somma and Predazzo, Italy; Nordmark and Långban, Swed.; and Crestmore, Calif. Periclase may be a major component of the mantle of the Earth. For detailed physical properties, see oxide mineral (table).