Rhodarsenide

Chemical formula: Rh<sub>₂</sub>As

Rhodarsenide is a rare rhodium arsenide, forming tiny, metallic inclusions in platinum-group minerals.

## Characteristics Rhodarsenide is a rhodium arsenide, occurring extremely rarely. It usually forms very small, rounded or irregular grains and inclusions in other minerals, mainly in rutheniridosmine (a variety of iridosmine rich in ruthenium). Its identification is possible almost exclusively using advanced analytical methods, such as reflected light ore microscopy and electron microprobe analysis. It does not form macroscopic crystals or specimens in a form known to collectors. ## Physical Properties As an opaque mineral with a metallic luster, its Mohs hardness is estimated at approximately 5. The density is very high, calculated at 11.33 g/cm³, which is typical for minerals containing platinum-group metals. ## Colors and Varieties It has a tin-white color with a pinkish tint, visible on fresh surfaces in reflected light. No varieties are known. ## History and Name The name rhodarsenide directly refers to its chemical composition – the content of rhodium (Latin: *rhodium*) and arsenic. It was discovered and described in 1956 by S. A. Hiemstra based on material from concentrates from gold mines in the Witwatersrand district in the Republic of South Africa. ## Uses Rhodarsenide has no industrial or commercial application. Its significance is purely scientific, as a component of ore parageneses associated with platinum-group metals.

Properties

Mohs hardness: 5
Luster: Metallic
Streak: Black
Density: 11.33
Cleavage: None
Fracture: Uneven
Transparency: Opaque
Crystal system: Cubic

Diagnostic features

## Identification Identification of rhodarsenide in the field or in a collection is impossible with the naked eye. It requires specialized laboratory equipment. In reflected light microscope studies, it is characterized by a tin-white color with a pinkish tint and a very high reflectance. It is isotropic. ## Distinguishing from Similar Minerals It can be confused with other small, metallic platinum-group minerals, such as sperrylite (PtAs₂) or other arsenides and sulfides. Final differentiation requires chemical analysis by electron microprobe to confirm the dominance of rhodium and arsenic. ## Crystal Forms It usually occurs as very small, rounded or irregular grains, often as inclusions in other minerals. Well-formed crystals have not been observed.

Geological environment

## Genesis Rhodarsenide is a hydrothermal mineral associated with platinum-group metal (PGM) deposits. It forms in paragenesis with other PGM minerals in ultramafic igneous rocks or in placer deposits formed by their weathering. ## Mineral Associations It co-occurs with other platinum-group metal minerals, such as iridosmine (especially the rutheniridosmine variety), native platinum, irarsite, laurite, as well as native gold, chromite, and various sulfides. ## Localities The most important confirmed localities for this mineral are the Witwatersrand region in South Africa (type locality), the Bushveld Complex (South Africa), nickel and copper deposits in the Sudbury region in Canada, and some placer deposits in Borneo (Indonesia) and Colombia.

Rarity

Very rare

For collectors

## Quality Criteria Rhodarsenide does not form collector specimens in the traditional sense. Its value is purely scientific. In the context of micromounting, a "specimen" can be considered a fragment of host rock with an analytically confirmed, microscopically visible grain of rhodarsenide. In such a case, the value is determined by the quality of the analytical documentation and its association with other rare minerals. ## Popular Localities As a microscopic mineral, there are no "popular localities" in the collector's sense. Material for scientific research comes mainly from the Bushveld Complex in South Africa and the Sudbury region in Canada.

Care and storage

## Cleaning Due to its microscopic nature of occurrence, individual grains of rhodarsenide are not subject to cleaning. Specimens of host rocks in which it may occur should be cleaned mechanically, using a soft brush and distilled water, and then thoroughly dried. ## What to Avoid Avoid contact with strong acids, which can react with arsenides. Store away from moisture to prevent potential surface oxidation. ## Storage Specimens containing rhodarsenide should be stored under stable conditions, in a dry place, preferably in closed display boxes, to protect them from dust and contaminants.

Sources

IMA Mineral List (CNMNC)