Mgriite

Chemical formula: Cu<sub>3</sub>AsSe<sub>3</sub>

Mgriite is a very rare copper selenoarsenide with a metallic luster, occurring as microscopic aggregates in Kamchatka.

## Characteristics Mgriite is a copper selenoarsenide with a metallic luster and a steel-gray to black color. It occurs as very fine-grained, compact aggregates, as well as thin coatings, rims around other minerals, or small veinlets. Due to the microscopic size of its grains, its macroscopic features are difficult to observe, and identification requires advanced analytical techniques. ## Physical Properties This mineral is opaque and has a hardness ranging from 3.5-4 on the Mohs scale. Its density, calculated based on its chemical composition and unit cell parameters, is 5.51 g/cm³. The luster is distinctly metallic, and the streak is black. ## Colors and Varieties Mgriite is monochromatic – its color ranges from steel-gray to black. No color varieties or commercial varieties are distinguished. ## History and Name The mineral was first described in 1983 by a team of Soviet scientists (V. S. Gruzdev, V. I. Volgin, E. M. Spiridonov, and others). Its name honors the Moscow Geological Prospecting Institute (abbreviated MGRI from Russian Московский геолого-разведочный институт), where research on the newly discovered mineral was conducted. ## Applications Mgriite has no industrial applications. It is solely an object of scientific interest and is valued by specialized collectors of rare minerals (so-called microminerals).

Properties

Mohs hardness
3.5-4
Luster
Metallic
Streak
Black
Density
5.51
Transparency
Opaque
Crystal system
Tetragonal

Diagnostic features

## Identification Identification of mgriite with the naked eye is impossible. Due to its occurrence in the form of small, inconspicuous aggregates, the only reliable method of identification is chemical analysis using an electron microprobe (EDS/WDS) to determine the composition (presence of copper, arsenic, and selenium in appropriate proportions). ## Differentiation from Similar Minerals Mgriite can be confused with many other associated metallic minerals of gray or black color, such as eskebornite, gruzdevite, or other sulfides and selenides. Differentiation is based solely on chemical analysis. ## Crystal Forms Mgriite does not form well-developed crystals. It is observed only in the form of very fine-grained, compact aggregates, as well as thin veinlets and reaction rims around grains of other minerals, with a size not exceeding 0.1 mm.

Geological environment

## Genesis Mgriite forms under low-temperature hydrothermal conditions. It is a product of crystallization from solutions rich in selenium, arsenic, and copper in active geothermal systems. ## Mineral Associations This mineral co-occurs with other rare selenides, sulfides, and arsenides. Its most common associated minerals include eskebornite, arsenoselenohauchecornite, cadmoselite, gruzdevite, laffittite, routhierite, as well as realgar, orpiment, and native arsenic. ## Localities The only confirmed locality of mgriite in the world is its type locality – the Uzon caldera on the Kamchatka Peninsula in Russia. This is a classic site for many rare selenium and arsenic minerals.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of mgriite specimens, being a micromineral, is assessed differently than for large crystals. The most important criterion is the richness of the aggregates – the more visible mgriite on the sample, the more valuable it is. Specimens with well-identified associated minerals, especially other rare selenides, are also highly valued. ## Popular Localities The only source of mgriite specimens is the Uzon caldera in Kamchatka. Material from this locality is extremely difficult to obtain and is a great rarity on the collector's market.

Care and storage

## Cleaning Mgriite specimens are typically microscopic samples that do not require cleaning. If necessary, dusty specimens can be cleaned with a gentle stream of compressed air. Avoid contact with water and any chemical agents. ## What to Avoid The mineral is relatively soft and brittle, so it should be protected from impacts and scratching. Contact with acids and other chemicals, as well as ultrasonic cleaning, should be strictly avoided. ## Storage Mgriite specimens are best stored in specialized, sealed micromount boxes, which protect them from dust, moisture, and mechanical damage. Each specimen should be precisely labeled.

Sources

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