Eltyubyuite

Chemical formula: Ca₆Fe₃+₅Si₂O₁₆Cl₃

A very rare mineral from the wadalite group, forming microscopic, dark brown to black grains in volcanic rocks.

## Characteristics Eltyubyuite is a very rare, complex silicate with sulfate and chloride admixture, belonging to the wadalite group. It occurs as very small, anhedral (irregular) or subhedral grains, rarely exceeding 0.2 mm in size. It forms granular aggregates or occurs as single, dispersed grains. Its appearance is inconspicuous, and identification requires specialized equipment. ## Physical Properties The hardness of the mineral is estimated at approximately 5 on the Mohs scale. It is characterized by a vitreous to slightly greasy luster. It is an opaque mineral, only translucent in very thin fragments. Its density, calculated based on the formula and cell parameters, is 3.55 g/cm³. ## Colors and Varieties Eltyubyuite has a uniform, dark color – from dark reddish-brown to almost black. Due to its extreme rarity and microscopic nature, no color or commercial varieties have been distinguished. ## History and Name The mineral was approved by the International Mineralogical Association (IMA) in 2001. Its name comes from the nearby village of Eltyubyu (Эльтюбю) in Kabardino-Balkaria (Russia), where its only known occurrence is located – the Verkhnechegemskaya volcanic caldera. ## Applications Eltyubyuite has no industrial applications. Its significance is purely scientific and collectible, being an object of interest for specialists in rare minerals and systematics.

Properties

Mohs hardness: 5
Luster: Vitreous to Greasy
Streak: Brown
Density: 3.55
Cleavage: None
Fracture: Uneven
Transparency: Opaque, Translucent in thin fragments
Crystal system: Tetragonal

Diagnostic features

## Identification Visual identification of eltyubyuite in the field or even in a collection is practically impossible due to its microscopic size and uncharacteristic appearance. Certain identification requires advanced analytical techniques, such as chemical composition analysis (EDS/WDS) and X-ray diffraction (XRD). A key clue is its specific geological environment and associated minerals. ## Distinguishing from Similar Minerals It can be confused with other dark, granular minerals occurring in the same association, such as srebrodolskite or some iron oxides. Differentiation is based on chemical analysis, which shows the simultaneous presence of silicon, iron, calcium, chlorine, and sulfur in the eltyubyuite structure. ## Crystal Forms This mineral mainly forms irregular, anhedral grains or subhedral grains. They occur as dense, granular aggregates or as single inclusions in the host rock.

Geological environment

## Genesis Eltyubyuite forms under specific conditions of high-temperature (up to 800-950°C) and low-pressure contact metamorphism. It forms within xenoliths of carbonate-silicate rocks, entrained and altered by hot ignimbrites. ## Mineral Associations It co-occurs with a rich assemblage of other rare high-temperature minerals, such as larnite, spurrite, wadalite, rondorfite, srebrodolskite, bultfonteinite, as well as calcite and fluorite. ## Localities The only confirmed locality of eltyubyuite in the world is its type locality – the Verkhnechegemskaya volcanic caldera in Kabardino-Balkaria, North Caucasus, Russia.

Rarity

Very rare

For collectors

## Quality Criteria In the case of such a rare micromineral, the main criterion for collectible value is the richness of the specimen – i.e., the quantity and size of eltyubyuite grains on the rock matrix. Specimens with well-identified, rare associated minerals are also highly valued. The purity and crystal habit of the grains, although rare, also increase the value. ## Popular Localities All specimens known to collectors come from one place in the world: the Verkhnechegemskaya caldera in Russia. This locality is accessible only to specialized research expeditions, and the material on the market comes from very limited distribution.

Care and storage

## Cleaning Eltyubyuite specimens are usually micromounts that do not require cleaning. If necessary, dust can be gently removed with compressed air. Avoid contact with water and any chemical agents. ## What to Avoid As a mineral containing chlorine and sulfates, it may be sensitive to acids and other aggressive chemicals. It should be protected from moisture and extreme temperatures, although as a high-temperature mineral, it is stable at room conditions. ## Storage The safest way to store it is in a closed micromount box, which protects the delicate specimen from dust, mechanical damage, and sudden changes in humidity. Store in a dry place.

Sources

IMA Mineral List (CNMNC)