Periclase

Formula: MgO simple oxide
Specific gravity: 3.55 to 3.57
Hardness: 5½
Streak: White
Colour: Colourless, grayish white, yellow, brownish yellow, green, black
Solubility: Insoluble in water, soluble in hydrochloric, nitric and sulphuric acid
Environments:

Metamorphic environments

Periclase is found as a product of contact metamorphism of dolomite and magnesite.
It is a mineral of the amphibolite facies.

Alteration

During the progressive metamorphism of silica-rich dolostones the following approximate sequence of mineral formation is often found, beginning with the lowest temperature product: talc, tremolite, diopside, forsterite, wollastonite, periclase, monticellite

brucite to periclase and H2O
Mg(OH)2 ⇌ MgO + H2O
The equilibrium temperature for this reaction at 10 kbar pressure is about 840oC (granulite facies), with the equilibrium to the right at higher temperatures, and to the left at lower temperatures (for the same pressure).

Common impurity: Fe