Olkhonskite

Chemical formula: Cr₃+₂Ti₄+₃O₉

Olkhonskite is an extremely rare oxide mineral, a member of the pseudobrookite group, found exclusively in one location worldwide – on Olkhon Island in Russia.

## Characteristics Olkhonskite is an extremely rare oxide mineral belonging to the pseudobrookite group. It occurs as very fine, tabular or prismatic crystals, reaching sizes up to 0.3 x 0.1 x 0.05 mm. It usually forms granular aggregates or occurs as inclusions in other minerals, mainly in calcite. Its crystals are black and opaque, with a metallic luster. ## Physical Properties This mineral is characterized by high hardness, estimated at 6-7 on the Mohs scale. It has a metallic luster and is opaque. Its density, calculated based on the chemical formula and unit cell parameters, is 4.85 g/cm³. It does not exhibit cleavage, and its fracture is uneven. ## Colors and Varieties Olkhonskite is an idiochromatic mineral, meaning its color is constant and directly results from its chemical composition. It is always black. No varieties have been distinguished. ## History and Name The mineral's name comes from its discovery location – Olkhon Island on Lake Baikal in Russia. It was approved as a new mineral species by the International Mineralogical Association (IMA) in 2010 (IMA2010-061). It was described by a Russian team of mineralogists: L.Z. Reznitsky, E.V. Sklyarov, Z.F. Ushchapovskaya, and E.V. Sokol. ## Uses Due to its extreme rarity and microscopic crystal sizes, olkhonskite has no industrial applications. It is solely an object of scientific interest and a valuable acquisition for specialized collectors of rare minerals (so-called "micromounts").

Properties

Mohs hardness: 6-7
Luster: Metallic
Streak: Black
Density: 4.85
Cleavage: None
Fracture: Uneven
Transparency: Opaque
Crystal system: Orthorhombic

Diagnostic features

## Identification Identification of olkhonskite is possible only through advanced laboratory methods, such as X-ray microanalysis (EDS/WDS) in a scanning electron microscope (SEM) or X-ray diffraction (XRD). In collecting conditions, identification relies on trusting a label from a reliable source confirming the analysis. Visually, they are simply fine, black, metallic crystals. ## Distinguishing from Similar Minerals Olkhonskite can be visually indistinguishable from other black, metallic minerals occurring in similar environments, such as chromite, ilmenite, magnetite, or other minerals from the pseudobrookite group (e.g., armalcolite, pseudobrookite). Definitive differentiation requires specialized analytical studies. ## Crystal Forms Olkhonskite crystals are very small, typically tabular or prismatic. They form irregular, granular aggregates or occur as single, isolated inclusion crystals in calcite.

Geological environment

## Genesis Olkhonskite forms under conditions of high-grade contact metamorphism, in the granulite facies. It occurs in calcite-diopside veins cutting rocks of the Olkhon metamorphic complex. Its formation is associated with metasomatic interaction with ultramafic protoliths rich in chromium and titanium at very high temperatures. ## Mineral Associations This mineral co-occurs with many other rare and unusual minerals. The most common associations include: calcite, diopside, tremolite, chromite, uvarovite, hexagonal silicon carbide polymorph (moissanite-6H), graphite, pyrite, chalcopyrite, pentlandite, shulamite, tazheranite, and other rare titanium and zirconium minerals. ## Localities The only confirmed locality of olkhonskite in the world is its type locality – the Priolkhonie tectonic zone on the western coast of Olkhon Island on Lake Baikal, in the Irkutsk Oblast, Russia.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of an olkhonskite specimen is determined by several factors. The most important is the certainty of mineral identification, confirmed by analysis. From a collector's perspective, specimens where crystals, despite their microscopic size, are as well-formed and visible as possible are valued. The richness of associated minerals on the rock matrix also enhances attractiveness, especially if they are other rare species. Crystal size, even if measured in fractions of a millimeter, is important – the larger, the more valuable the specimen. ## Popular Localities The only source of olkhonskite specimens is its discovery site on Olkhon Island in Russia. All specimens available on the collector's market originate from this single locality.

Care and storage

## Cleaning Olkhonskite specimens, due to their microscopic size and embedding in rock matrix, usually do not require cleaning. If necessary, compressed air can be used to remove dust. Contact with water and chemicals, which could damage co-occurring minerals like calcite, should be avoided. ## What to Avoid Ultrasonic cleaners and all acids and solvents should be avoided, as they could destroy not only olkhonskite itself but primarily the surrounding rock. Specimens should not be heated or subjected to sudden temperature changes. ## Storage Specimens containing olkhonskite, being typical micromounts, should be stored in specialized, sealed boxes (so-called "perky boxes") that protect them from dust, moisture, and mechanical damage. Exposure to direct sunlight should be avoided, although the mineral is stable.

Sources

IMA Mineral List (CNMNC)