Usturite

Chemical formula: Ca₃Zr₄+Sb₅+Fe₃+₃O₁₂

Usturite is an extremely rare mineral from the garnet supergroup, distinguished by a unique combination of zirconium and antimony in its chemical composition.

## Characteristics Usturite is a mineral belonging to the garnet supergroup. It forms small, well-formed crystals, most often in the shape of rhombic dodecahedra, reaching up to 0.3 mm in size. It also occurs as irregular grains. Its crystals are usually embedded in other minerals, mainly larnite. ## Physical properties This mineral is characterized by an adamantine luster. It is translucent in thin fragments and opaque in larger pieces. Its density, calculated based on the chemical formula and unit cell parameters, is 4.81 g/cm³. ## Colors and varieties Usturite has a color ranging from dark brown to almost black. No color or commercial varieties have been distinguished. ## History and name It was officially recognized as a new mineral by the International Mineralogical Association (IMA) in 2009 (IMA2009-034). The name comes from Mount Ustur near the discovery site. It was found and described by an international research team led by Irina Galuskina. ## Applications Due to its extreme rarity and microscopic crystal size, usturite has no commercial application. It is solely an object of scientific and collecting interest for specialized micromount mineral collectors.

Properties

Luster: Adamantine
Streak: Light brown
Density: 4.81
Cleavage: None
Fracture: Conchoidal
Transparency: Translucent to opaque
Crystal system: Isometric

Diagnostic features

## Identification Identification of usturite is impossible without advanced analytical techniques. Chemical analysis using electron microprobe (EDS/WDS) is necessary to confirm the presence of calcium, zirconium, antimony, and iron in appropriate proportions. A helpful feature is the regular (garnet) habit of the crystals and a very high luster. ## Differentiation from similar minerals Usturite can be confused with other dark garnets with which it co-occurs, such as kimzeyite (containing zirconium but not antimony) or kerimasite. Final differentiation requires chemical analysis. It differs from other common garnets (e.g., andradite) by the presence of zirconium and antimony. ## Crystal forms Usturite crystallizes in the isometric system, forming characteristic garnet shapes, mainly small rhombic dodecahedra {110}. Crystals are usually well-formed but microscopic in size.

Geological environment

## Genesis Usturite forms under very specific conditions of contact metamorphism. Its only known locality is carbonate rock xenoliths (limestones) entrained by volcanic lava and metamorphosed at high temperature and low pressure (fumarolic exhalations in the sanidinite facies). ## Mineral associations This mineral occurs in paragenesis with other rare minerals typical of this environment. It is most often accompanied by larnite, wadalite, kimzeyite, kerimasite, srebrodolskite, lakargiite, as well as members of the ettringite group. ## Localities The only confirmed occurrence of usturite in the world is its type locality – xenolith No. 8 in the Chegem volcanic caldera (Upper Chegem Volcanic Structure) in Kabardino-Balkaria, North Caucasus, Russia.

Rarity

Extremely rare

For collectors

## Quality criteria The collector's value of usturite specimens primarily depends on the abundance of microcrystals on the rock matrix and their development. Specimens with numerous, well-formed dodecahedra with a strong luster are most valued. The association with other rare minerals from this locality is also of great importance. ## Popular localities The only source of usturite specimens is its type locality in the Chegem caldera in Russia. Material from this location is extremely difficult to obtain and rarely enters the collector's market, mainly through specialized dealers or exchanges between scientific institutions.

Care and storage

## Cleaning Usturite specimens should be cleaned very carefully, preferably using compressed air to remove dust. Due to the microscopic size of the crystals, mechanical cleaning (brushes) and washing in water are not recommended, as they could damage delicate mineral associations. ## What to avoid Avoid contact with chemicals, acids, and bases. Ultrasonic cleaners are not recommended, as they can cause microcrystals to detach from the substrate. Specimens should be protected from sudden temperature changes. ## Storage The safest storage method is a specialized "micromount" box, which protects the specimen from dust and mechanical damage. Exposure to strong light is not advisable, although there is no detailed data on its photosensitivity.

Sources

IMA Mineral List (CNMNC)