periclase

Periclase is the high-temperature polymorph of magnesium oxide, MgO. It is the mineral form of the compound that is commonly known as magnesia. Periclase crystallizes in the cubic system with the rock salt structure, meaning each magnesium ion is octahedrally coordinated by six oxygen ions, and vice versa. It is found in metamorphosed dolomitic limestones where it forms through the thermal decomposition of dolomite, CaMg(CO3)2, at temperatures exceeding approximately 600 degrees Celsius. The reaction is: CaMg(CO3)2 -> CaO + MgO + 2CO2. The CaO then reacts with any remaining dolomite to form more periclase and calcium carbonate. Periclase is typically colorless to gray and has a vitreous luster. It is relatively hard, with a Mohs hardness of 5.5 to 6.0, and has a density of around 3.58 g/cm³. It is insoluble in water but reacts with acids. While periclase itself is a mineral, the material produced industrially by calcining magnesite (magnesium carbonate) or brucite (magnesium hydroxide) is also referred to as periclase or dead-burned magnesia. This industrial product is extensively used as a refractory material in the lining of furnaces, kilns, and incinerators due to its high melting point and chemical inertness. The presence of periclase in rocks can indicate a history of high-temperature metamorphism.