Tongbaite

Chemical formula: Cr₃C₂

Tongbait is an extremely rare chromium carbide, found mainly in ultramafic rocks and meteorites.

## Characteristics Tongbait is a mineral with a metallic appearance, usually occurring as very fine, rounded grains or aggregates. Its size rarely exceeds 0.2 mm. Due to its nature, individual crystals are difficult to observe with the naked eye, and this mineral is most often identified by analytical methods within a larger rock mass. ## Physical Properties This mineral is characterized by very high hardness, which is typical for carbides. It has a metallic luster and is opaque. Its density is significantly higher than that of most rock-forming minerals. ## History and Name The name tongbait comes from its discovery locality in Tibet, near Tongba village in Qusum County. It was described and approved as a new mineral by the International Mineralogical Association (IMA) in 1983.

Properties

Mohs hardness: 8.5
Color: Brownish yellow
Luster: Metallic
Streak: Dark gray
Density: 0
Cleavage: None
Fracture: Uneven
Transparency: Opaque
Crystal system: Orthorhombic

Diagnostic features

## Identification Field identification of tongbait is impossible. Identification requires advanced laboratory techniques, such as scanning electron microscopy (SEM) with chemical composition analysis (EDS/WDS) or X-ray diffraction (XRD). In polished rock sections (ore polished sections), it can be identified under a reflected light microscope based on its high reflectivity and hardness. ## Distinguishing from Similar Minerals It can be confused with other rare, hard minerals with a metallic luster, such as other carbides (moissanite), nitrides (osbornite), or some sulfides. Definitive distinction is only possible based on chemical analysis confirming the dominance of chromium and carbon. ## Crystal Forms Tongbait forms very small, irregular or rounded grains, often intergrown with other minerals. Well-formed, idiomorphic crystals are not observed.

Geological environment

## Genesis Tongbait forms under specific reducing conditions in ultramafic rocks, such as chromitites and dunites within ophiolite complexes. Its presence is associated with processes occurring in the Earth's upper mantle. It has also been identified as an accessory phase in some meteorites, indicating its formation under extraterrestrial conditions. ## Mineral Associations It most commonly coexists with chromite, forsterite (olivine), diopside, as well as other rare minerals such as cohenite, graphite, native iron, awaruite, wairauite, and heazlewoodite. ## Localities The most important and type locality is the chromite deposits in the Luobusha ophiolite complex in Tibet (China). It is also known from the Ray-Iz ophiolite in the Polar Urals, Russia, and from several meteorite finds.

Rarity

Extremely rare

For collectors

## Quality Criteria Tongbait specimens are not evaluated in the traditional way, as this mineral does not form aesthetic crystals. Its collector value lies in its extreme rarity and scientific significance. The most valuable are rock samples (usually chromitite) with analytically confirmed, rich concentrations of tongbait grains, originating from the type locality in Tibet.

Care and storage

## Cleaning Due to the extremely small grain size and occurrence within solid rock, isolated cleaning of individual specimens is practically impossible and unnecessary. Specimens of host rocks containing tongbait can be cleaned mechanically, using a soft brush and distilled water. ## What to Avoid Avoid contact with strong acids, which could react with the mineral or the surrounding rock. Despite its high hardness, ultrasonic cleaners should be avoided as they could damage the delicate structure of the host rock. ## Storage Specimens containing tongbait should be stored in stable conditions, away from dust and moisture, preferably in sealed boxes or display cases. It is not sensitive to light.