Okruginite

Chemical formula: Cu₂SnSe₂-₃

Okruginite is an extremely rare copper and tin selenide with a metallic luster, found as microscopic grains in gold-bearing volcanic deposits.

## Characteristics Okruginite is a seleno-stannide, a mineral from the sulfide group where sulfur is replaced by selenium. It occurs as anhedral (irregular) grains not exceeding 0.2 mm in size, most often intergrown with other selenides. It is an opaque mineral, steel-gray to black in color, with a strong metallic luster. Due to its microscopic nature, its visual characteristics can only be assessed under high magnification. ## Physical Properties The hardness of okruginite on the Mohs scale is approximately 4.5. The mineral is characterized by a metallic luster and is completely opaque. Its density, calculated based on its chemical composition and unit cell parameters, is 6.13 g/cm³. It does not exhibit cleavage, and its streak is black. ## Colors and Varieties The mineral is uniformly colored – always steel-gray to black. No colored or commercial varieties are known. ## History and Name Okruginite was officially recognized as a new mineral by the International Mineralogical Association (IMA) in 2015. Its name honors Viktor Valentinovich Okrugin (born 1946), a Russian geologist and specialist in volcanogenic deposits, who made significant contributions to geological research on the Kamchatka Peninsula, where this mineral was discovered. ## Applications As an extremely rare mineral occurring only in the form of microscopic grains, okruginite has no industrial applications. It is solely an object of scientific interest and a valuable acquisition for specialized collectors of systematic minerals and microminerals.

Properties

Mohs hardness: 4.5
Luster: Metallic
Streak: Black
Density: 6.13
Cleavage: None
Transparency: Opaque
Crystal system: Tetragonal

Diagnostic features

## Identification Identification of okruginite with the naked eye or using basic tests is impossible. Its appearance is very similar to many other metallic sulfides, sulfosalts, and selenides. Definitive identification requires advanced analytical techniques, such as X-ray microanalysis (EDS/WDS) to determine its chemical composition (presence of copper, tin, and selenium in appropriate proportions) and X-ray diffraction (XRD) to confirm its unique crystal structure. ## Differentiation from Similar Minerals Okruginite can be confused with other minerals from the stannite group, such as stannite itself (Cu₂FeSnS₄) or briartite (Cu₂(Zn,Fe)GeS₄), as well as with other selenides found in the same locality, such as kurilite (Ag₈Te₃Se) or clausthalite (PbSe). Differentiation is only possible based on precise chemical analysis. ## Crystal Forms This mineral does not form well-developed crystals. It occurs exclusively as irregular, anhedral grains, often intergrown with other minerals.

Geological environment

## Genesis Okruginite forms under low-temperature hydrothermal (epithermal) conditions. It is a product of crystallization from solutions rich in selenium, tin, and copper within gold, silver, tellurium, and selenium deposits associated with volcanic activity. ## Mineral Associations This mineral occurs in association with other rare selenides and tellurides. In its type locality, it co-occurs with native gold, kurilite, petrovskaite, tiemannite, clausthalite, naumannite, stannite, chalcopyrite, quartz, and pyrite. ## Localities The only confirmed occurrence of okruginite in the world is its type locality – the Ozernovskoe gold deposit, located in the central part of the Kamchatka Peninsula in the Russian Far East.

Rarity

Extremely rare

For collectors

## Quality Criteria For such a rare micromineral as okruginite, the main criterion for value is the certainty of the specimen's identification, confirmed by analysis. Collectors value specimens where okruginite grains are as large and numerous as possible. Rich associations with other rare minerals from this locality, such as kurilite or petrovskaite, are also highly desirable. ## Popular Localities As the mineral is known only from one place in the world, all collectible specimens come from the Ozernovskoe deposit in Kamchatka, Russia. Material from this locality is extremely difficult to obtain.

Care and storage

## Cleaning Specimens containing okruginite, typically microminerals, should not be wet cleaned. Dust can be removed using compressed air (from a safe distance) or a very soft brush. ## What to Avoid Avoid contact with all chemicals, acids, and detergents. The mineral is susceptible to mechanical damage, so avoid rubbing and scratching it. Ultrasonic cleaners should not be used. ## Storage Specimens should be stored in specialized "micromount" boxes, which protect them from dust, moisture, and physical damage. Each specimen should be precisely labeled, including its locality.

Sources

IMA Mineral List (CNMNC)