Periclase

Chemical formula: MgO

Periclase is magnesium oxide (MgO), occurring as isometric crystals, formed mainly under conditions of contact metamorphism of carbonate rocks.

## Characteristics Periclase is magnesium oxide with a simple chemical composition. It forms well-developed, cubic or octahedral crystals, often with rounded edges. It also occurs as rounded grains embedded in rock. It is a typical mineral of high-temperature, silica-poor environments. Pure periclase is colorless and transparent, but impurities of iron and other elements can give it various hues. ## Physical Properties This mineral is characterized by a hardness of about 6 on the Mohs scale and a vitreous luster. It is relatively heavy, with a density of approximately 3.56-3.68 g/cm³. Crystals exhibit perfect cleavage in three mutually perpendicular directions, which is typical of its isometric crystal structure. It is brittle, and its fracture is uneven to conchoidal. ## Colors and Varieties Periclase is most commonly found colorless, white, grayish, yellowish, or greenish. Less frequently, it is black. The presence of iron ions (Fe²⁺) substituting for magnesium causes a color change to green to black and is referred to as ferropericlase. No other named varieties are distinguished. ## History and Name The name periclase comes from the Greek words *peri* (περί) - around and *klasis* (κλάσις) - to break, referring to its perfect, cubic cleavage. The mineral was first described in 1840 by the Italian mineralogist Arcangelo Scacchi based on specimens found in volcanic material from the Monte Somma-Vesuvius complex in Italy. ## Uses Synthetic magnesium oxide, with the same structure as periclase, is a key raw material for the production of refractory materials, such as magnesia bricks used in metallurgical furnaces. Natural periclase is primarily of scientific and collector's interest.

Properties

Mohs hardness: 5.5
Color: Colorless, grayish white, yellow, brownish yellow, green, black
Luster: Vitreous
Streak: White
Density: 3.55
Cleavage: {001} perfect; {111} imperfect.
Fracture: Irregular/Uneven
Transparency: Transparent,Translucent
Crystal system: Isometric

Diagnostic features

## Identification Periclase is most easily recognized by its isometric, cubic crystals and perfect cleavage in three perpendicular directions. Its vitreous luster and occurrence in metamorphic rocks, such as marbles, are also characteristic. It reacts with acids, which can be helpful in identification, but this is a destructive test. ## Distinguishing from Similar Minerals Periclase is sometimes confused with other colorless or lightly colored minerals with an isometric crystal form. It is distinguished from halite (rock salt) by its significantly greater hardness (halite can be scratched with a knife) and lack of a salty taste. It is distinguished from spinel by its lower hardness (spinel has a hardness of 8) and perfect cleavage, which spinel lacks. It differs from fluorapophyllite in its crystallographic system and the absence of characteristic pearly luster on the basal faces. ## Crystal Forms Periclase crystals most often occur as cubes and octahedra and their combinations. They frequently appear as rounded grains embedded in rock or in granular aggregates.

Geological environment

## Genesis Periclase is a mineral of high-temperature contact metamorphism. It forms as a result of the thermal decomposition of dolomite or magnesite in contact zones of magmatic intrusions with carbonate rocks (e.g., in marbles). It is an indicator of the sanidinite facies. It also occurs as a product of volcanic exhalations, where it crystallizes directly from hot gases, and in some meteorites. ## Mineral Associations Minerals co-occurring with periclase in metamorphic rocks are most often forsterite, magnesite, brucite (as a weathering product), calcite, dolomite, spinel, and minerals from the humite group. In volcanic products, it may be accompanied by, for example, sylvite. ## Localities The classic locality from which the mineral was described is the Monte Somma-Vesuvius volcanic complex in Italy. Other known occurrences include Crestmore in California (USA), where it occurs in marbles, the Broadford area on the Isle of Skye in Scotland, Kopeysk in the Chelyabinsk Oblast (Russia), and Långban in Sweden. In Poland, its presence has been noted in basalts from Góra Świętej Anny.

Rarity

Not very common

For collectors

## Quality Criteria Specimens with well-formed, sharp-edged, and transparent crystals of vivid color (e.g., greenish or yellow), set on a contrasting rock matrix, are most valued by collectors. Large, single crystals of regular, cubic or octahedral shape are particularly sought after. The purity of the specimen is also important – the absence of white brucite coatings indicates good preservation. ## Popular Localities Classic, prized specimens come from the historical locality of Monte Somma-Vesuvius in Italy. High-quality crystals have also been found in Crestmore, California, USA. Specimens from Kopeysk, Russia, are also known for their well-formed crystals.

Care and storage

## Cleaning Periclase specimens can be cleaned with lukewarm water and mild soap, using a soft brush. Strong detergents and acids should be avoided. After washing, the specimen should be thoroughly rinsed with distilled water and allowed to dry completely. ## What to Avoid Periclase is sensitive to moisture and carbon dioxide from the air, in the presence of which it slowly alters to brucite (magnesium hydroxide), which manifests as a white, powdery coating and loss of luster. Contact with acids, which rapidly dissolve it, must be absolutely avoided. It is not sensitive to light, but extreme temperature changes should be avoided. ## Storage Due to its reactivity with atmospheric moisture, the best way to store periclase is in tightly sealed containers (e.g., "perky boxes") or in display cases with a desiccant. This protects the specimen from slow degradation and helps preserve its natural appearance.