Chromium

Chemical formula: Cr

Chromium is a native element and a rare mineral with a metallic luster, found mainly in ultramafic rocks and some meteorites.

## Characteristics Native chromium is a mineral from the native elements group, consisting of pure chromium. It occurs extremely rarely as small, isolated grains or as a component of alloys with other metals, e.g., iron. Its crystals, usually microscopic in size, have a regular, cubic structure. It is most often observed as inclusions in other minerals, and its identification requires advanced analytical techniques. ## Physical Properties Chromium is characterized by high hardness, reaching 8.5 on the Mohs scale, which makes it one of the harder minerals. It has a steel-gray color and a strong, metallic luster. It is an opaque and malleable mineral, meaning it can be hammered and shaped. Its density is significant, approximately 7.19 g/cm³. ## Colors and Varieties This mineral has a uniform, steel-gray color, which may be slightly lighter or darker depending on impurities. No color varieties or commercial varieties are distinguished, as it occurs in quantities too small to be commercially significant as a mineral. ## History and Name The name chromium comes from the Greek word "chroma" (χρῶμα) meaning "color," referring to the intensely colored chemical compounds it forms. As an element, it was discovered by Louis Nicolas Vauquelin in 1797. Its occurrence in native form (as a mineral) was confirmed much later, with the first detailed descriptions dating back to the 1980s. ## Applications Native chromium has no direct industrial application due to its extreme rarity. It is solely an object of scientific and collecting interest. Industry obtains chromium on a mass scale from richer and more accessible ores, mainly from the mineral chromite.

Properties

Mohs hardness: 8.5
Luster: Metallic
Streak: Steel-gray
Density: 7.19
Cleavage: None
Fracture: Hackly
Transparency: Opaque
Crystal system: Cubic

Diagnostic features

## Identification Identification of native chromium in field conditions is practically impossible. It requires specialized laboratory equipment, such as an electron microscope with an EDS analyzer or an electron microprobe, to confirm its chemical composition. It is distinguished by very high hardness and metallic luster, but these features are difficult to observe given the microscopic size of the grains. ## Distinguishing from Similar Minerals Native chromium can be confused with other metallic minerals of gray color, such as native platinum, native iron (kamacite, taenite), or graphite. Differentiation is possible almost exclusively based on chemical analysis. In a geological context, its occurrence in specific ultramafic rocks can be a diagnostic indicator. ## Crystal Forms It crystallizes in the isometric system, theoretically forming cubes or octahedra. In nature, however, it is observed almost exclusively as very fine, irregular grains or dendritic aggregates, usually not exceeding several tens of micrometers in size.

Geological environment

## Genesis Native chromium forms under specific reducing conditions, with low oxygen fugacity. Its main environment of formation is ultramafic rocks, such as dunites and peridotites, where it crystallizes from magma. It is also found in serpentinites formed by the alteration of these rocks. Furthermore, its presence has been confirmed in some meteorites, especially enstatite chondrites. ## Mineral Associations It most often co-occurs with minerals typical of ultramafic rocks and serpentinites, such as chromite, olivine, pyroxenes (diopside), serpentine, as well as with other native elements and alloys, e.g., native iron (kamacite), awaruite (nickel-iron alloy), and moissanite (silicon carbide). ## Locations Confirmed occurrences of native chromium are very few. The most important include deposits in the Luobusa massif in Tibet (China), where it occurs in podiform dunites. It has also been found in the ophiolite complex in the Polar Urals (Russia) and in the Kumtor gold deposit in Kyrgyzstan. Occurrences in meteorites have been reported, among others, in the Qingzhen chondrite (China).

Rarity

Extremely rare

For collectors

## Quality Criteria Due to its extreme rarity and microscopic nature, specimens of native chromium are not evaluated according to typical criteria such as color or clarity. The most valuable samples are those in which the chromium grains are relatively large (visible under low magnification) and well-documented scientifically. Precise localization and confirmation of authenticity through chemical analysis are crucial. A specimen consists of a fragment of host rock with chromium inclusions. ## Popular Localities The only source of specimens that occasionally appear on the collector's market is the Luobusa massif in Tibet. Samples from this locality are considered reference material for this mineral.

Care and storage

## Cleaning Specimens of native chromium, usually in the form of microscopic inclusions in the host rock, practically do not require cleaning. If there is dust on the rock surface, it can be removed with a soft brush or compressed air. Avoid contact with water and chemical agents. ## What to Avoid Chromium is a stable element, but contact with strong acids, which can damage it, should be avoided. The host rock in which it occurs may be sensitive to changes in temperature and humidity, so the specimen should be protected from extreme conditions. ## Storage Specimens containing native chromium are best stored in stable conditions, in closed boxes or display cases, to protect them from dust and mechanical damage. Due to the microscopic size of the grains, they often require proper description and localization marking under a microscope.