Magnesium oxide or magnesia, is a white hygroscopic

Hygroscopy

Hygroscopy is the ability of a substance to attract and hold water molecules from the surrounding environment. This is achieved through either absorption or adsorption with the absorbing or adsorbing material becoming physically 'changed,' somewhat, by an increase in volume, stickiness, or other...

 solid mineral

Mineral

A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...

 that occurs naturally as periclase

Periclase

Periclase occurs naturally in contact metamorphic rocks and is a major component of most basic refractory bricks. It is a cubic form of magnesium oxide ....

 and is a source of magnesium

Magnesium

Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...

 (see also oxide

Oxide

An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....

). It has an empirical formula

Empirical formula

In chemistry, the empirical formula of a chemical compound is the simplest positive integer ratio of atoms of each element present in a compound. An empirical formula makes no reference to isomerism, structure, or absolute number of atoms. The empirical formula is used as standard for most ionic...

 of and consists of a lattice of Mg²⁺ ions and O^2– ions held together by ionic bond

Ionic bond

An ionic bond is a type of chemical bond formed through an electrostatic attraction between two oppositely charged ions. Ionic bonds are formed between a cation, which is usually a metal, and an anion, which is usually a nonmetal. Pure ionic bonding cannot exist: all ionic compounds have some...

s. Magnesium hydroxide

Magnesium hydroxide

Magnesium hydroxide is an inorganic compound with the chemical formula Mg2. As a suspension in water, it is often called milk of magnesia because of its milk-like appearance. The solid mineral form of magnesium hydroxide is known as brucite....

 forms in the presence of water (MgO + H₂O → Mg(OH)₂), but it can be reversed by heating it to separate moisture.

Magnesium oxide was historically known as magnesia alba (literally, the white mineral from Magnesia

Magnesia Prefecture

Magnesia Prefecture was one of the prefectures of Greece. Its capital was Volos. It was established in 1899 from the Larissa Prefecture. The prefecture was disbanded on 1 January 2011 by the Kallikratis programme, and split into the peripheral units of Magnesia and the Sporades.The toponym is...

), to differentiate it from magnesia negra, a black mineral containing what is now known as manganese

Manganese

Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...

.
A refractory material is one that is physically and chemically stable at high temperatures.