Chromomphacite

Chemical formula: (Ca,Na)(Mg,Cr³⁺,Al)Si₂O₆

Chromian omphacite is a rare, light green or emerald green mineral from the clinopyroxene group, being a chromium-rich variety of omphacite.

## Characteristics Chromian omphacite is a mineral belonging to the clinopyroxene group, classified as a chromium-rich variety of omphacite. In its chemical composition, chromium replaces aluminum. It forms granular or fibrous aggregates; well-formed crystals are rarely encountered. Its most characteristic feature is an intense, light green to emerald green color, resulting from the presence of chromium ions. ## Physical Properties This mineral is characterized by a hardness in the range of 5-6 on the Mohs scale. It has a vitreous luster and is usually translucent. Its density is slightly higher than that of quartz, at approximately 3.35 g/cm³. ## Colors and Varieties Chromian omphacite occurs in shades of green, from light green to intense emerald. Its color is its main distinguishing feature. No named varieties are distinguished for it, as it is itself considered a specific variety of omphacite. ## History and Name The mineral's name directly refers to its chemical composition – the presence of chromium – and its relationship to the structurally similar omphacite. It was formally described and recognized by the International Mineralogical Association (IMA) in 1987. The type locality is considered to be the Sayan ultramafic massif near the village of Sayan in Siberia, Russia. ## Applications Due to its rarity and small occurrences, chromian omphacite has no industrial applications. It is solely an object of interest for collectors of rare minerals and scientists studying high-pressure metamorphic rocks.

Properties

Mohs hardness: 5-6
Luster: Vitreous
Streak: White
Density: 0
Cleavage: Good on {110}
Fracture: Uneven
Transparency: Translucent
Crystal system: Monoclinic

Diagnostic features

## Identification The key diagnostic feature of chromian omphacite is its intensely green color, vitreous luster, and occurrence in a specific geological environment – eclogite and ultramafic rocks. A hardness in the range of 5-6 helps distinguish it from softer minerals. ## Distinguishing from Similar Minerals Chromian omphacite can be confused with other green pyroxenes, such as jadeite or chromian diopside, as well as uvarovite (chromium garnet). It is distinguished from jadeite by subtle differences in hue and optical properties, and from chromian diopside often by a brighter color. Uvarovite typically forms small, well-formed crystals with a regular habit, while chromian omphacite occurs mainly as granular aggregates. ## Crystal Forms Well-formed, single crystals of chromian omphacite are extremely rare. It usually forms granular, massive, or fibrous aggregates within the host rock.

Geological environment

## Genesis Chromian omphacite is a metamorphic mineral, forming under conditions of very high pressure and relatively high temperature. Its formation is associated with the metamorphism of ultramafic (rich in magnesium and iron) and mafic rocks in subduction zones. ## Mineral Associations It most commonly co-occurs with other high-pressure minerals, such as pyrope (garnet), chromian diopside, uvarovite, rutile, coesite, and also with diamonds in some kimberlites and eclogites. ## Localities The most important and classic localities for chromian omphacite are in Russia, in the Sayan ultramafic massif in Siberia (type locality) and in kimberlites in Yakutia. Occurrences have also been reported in the Dora Maira massif in Italy, in Guatemala, and in China.

Rarity

Very rare

For collectors

## Quality Criteria The most prized specimens by collectors are those in which chromian omphacite forms rich, intensely colored aggregates, contrasting with the lighter host rock. Due to its rarity, any well-documented specimen, even in the form of microscopic grains in a polished thin section, has scientific and collector value. Specimens from the type locality (Sayan, Russia) are particularly sought after.

Care and storage

## Cleaning Chromian omphacite specimens should only be cleaned with lukewarm distilled water and a very soft brush. Ultrasonic cleaners should be avoided, as they can damage aggregates. ## What to Avoid The mineral is sensitive to strong acids and bases. Avoid sudden temperature changes and prolonged exposure to direct sunlight, although its clear effect on fading is not known. ## Storage It is recommended to store specimens in separate, padded boxes or display cases to prevent scratches from harder minerals. Protect it from dust and moisture.

External references

https://www.mindat.org/min-27602.html