Magnesia (mineral)

Periclase
White to light greenish Periclase with black Srebrodolskite from Ronneburg, Thuringia, Germany (Picture size 5 mm)
General
Category	Oxide minerals
Formula (repeating unit)	MgO
Strunz classification	4.AB.25
Crystal system	Isometric
Crystal class	Hexoctahedral (m3m) H-M symbol: (4/m 3 2/m)
Space group	Fm3m
Unit cell	4.21 Å; Z = 4
Identification
Color	Colorless, grayish white, yellow, brownish yellow, green, black
Crystal habit	Granular, generally occurs as anhedral to subhedral octahedral crystals in matrix
Cleavage	{001} perfect; {111} imperfect, may exhibit parting on {011}
Fracture	Conchoidal
Tenacity	Brittle
Mohs scale hardness	6
Luster	Vitreous
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	3.56–3.68 (meas.) 3.58 (calc.)
Optical properties	Isotropic
Refractive index	n = 1.735–1.745
Other characteristics	Fluorescent, long UV=light yellow.
References

{001} perfect; {111} imperfect, may exhibit parting on

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 (MgO). In nature it usually forms a solid solution with wüstite (FeO) and is then referred to as ferropericlase or magnesiowüstite.

It was first described in 1840 and named from the Greek περικλάω (to break around) in allusion to its cleavage. The type locality is Monte Somma, Somma-Vesuvius Complex, Naples Province, Campania, Italy.

The old term for the mineral is magnesia. Stones from the Magnesia region in ancient Anatolia contained both magnesium oxide and hydrated magnesium carbonate as well as iron oxides (such as magnetite). Thus these stones, called Stones from Magnesia in antiquity, with their unusual magnetic properties were the reason the terms magnet and magnetism were coined.

Periclase is usually found in marble produced by metamorphism of dolomitic limestones. It readily alters to brucite under near surface conditions.