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a mineral that consists of natural magnesium oxide (MgO) with occasional admixtures of FeO, MnO, and ZnO. Periclase, whose crystal structure resembles that of rock salt, crystallizes in the isometric system to form cubic and octahedral crystals as well as irregular grains; cleavage is perfect along the cube. Periclase is grayish white, yellow, dark green, or black with a hardness of 5.5–6 on Mohs’ scale and a density that ranges from 3,600 to 3,900 kg/m3. It is characteristic of meta-morphic limestones and dolomites. As a result of weathering, periclase alters to brucite and hydromagnesite. Artificial periclase, which is usually prepared by sintering natural magnesite at 1600”-1650oC or by electrically smelting natural magnesite, is important in manufacturing magnesite refractories.