5 Photos
Periclase
Chemical Formula	MgO
Species	Oxides & Hydroxides
Crystal System	Isometric
Mohs Scale	5
Specific Gravity	3.56-3.68
Color	Colorless, grayish white, yellow, brownish yellow, green, black
Streak	White
Luster	Sub-Vitreous
Refractive Index	n = 1.732
Diaphaneity	Transparent, Translucent
Cleavage	Perfect perfect; imperfect.
Fracture	Irregular/Uneven
Crystal Habit：Granular, generally occurs as anhedral to subhedral octahedral crystals in matrix
Geological Setting：contact metamorphosed limestone or dolomite

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).

It was first described in 1840 and named from the Greek περικλ?ω (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.

In addition to its type locality, it is reported from Predazzo, Tyrol, Austria; Carlingford, County Louth, Ireland; Broadford, Skye and the island of Muck, Scotland; León, Spain; the Bellerberg volcano, Eifel district, Germany; Nordmark and L?ngban, Varmland, Sweden; and Kopeysk, southern Ural Mountains, Russia. In the USA it occurs at the Crestmore quarry, Riverside County, California; Tombstone, Cochise County, Arizona; Gabbs district, Nye County, Nevada. In Canada, it occurs at Oka, Quebec and in Australia, west of Cowell, Eyre Peninsula, South Australia.