Osarsite

Chemical formula: Os<sup>₃+</sup>(AsS)<sup>₃-</sup>

Osarsite is a rare osmium arsenosulfide, occurring as microscopic, metallic grains in ultramafic rocks.

## Characteristics Osarsite is a very rare mineral from the sulfide group, chemically classified as osmium arsenosulfide. It occurs as extremely small, rounded or irregular grains, rarely exceeding 0.1 mm in diameter. It is usually intergrown with other platinum-group minerals or with chromite. Due to its size, its visual features are difficult to observe without specialized equipment, such as an electron microscope. ## Physical Properties As a metallic mineral, osarsite is characterized by a strong, metallic luster. It is opaque. Its Mohs hardness is estimated at about 7, making it a hard mineral. The density is very high, calculated at 8.56 g/cm³, which is typical for minerals containing platinum-group metals. ## History and Name The name "osarsite" directly refers to its chemical composition – it contains osmium (Os) and arsenic (As). The mineral was first described in 1972 by E. H. Stumpfl. The discovery was made on material from alluvial deposits in Gold Bluff, Humboldt County, California, USA, where it occurs in platinum concentrates. ## Uses Osarsite has no industrial application due to its extreme rarity and microscopic size. Its significance is purely scientific and collectible, as a component of rare mineral parageneses associated with platinum-group metals.

Properties

Mohs hardness: 7
Luster: Metallic
Streak: Black
Density: 8.56
Cleavage: None
Fracture: Uneven
Transparency: Opaque
Crystal system: Trigonal

Diagnostic features

## Identification Identification of osarsite is impossible without advanced analytical techniques. In polished ore sections under a polarizing microscope in reflected light, it exhibits a creamy white color and is strongly anisotropic. Final confirmation requires chemical composition analysis using an electron microprobe (EDS/WDS). ## Distinguishing from Similar Minerals It can be confused with other platinum-group metal arsenosulfides, such as ruarsite (RuAsS) or irarsite ((Ir,Ru,Rh,Pt)AsS). Differentiation is possible only based on precise chemical analysis, which will show the dominance of osmium. ## Crystal Forms Osarsite forms very fine, rounded or irregular grains. Well-formed crystals have not been observed.

Geological environment

## Genesis Osarsite is a mineral of magmatic origin. It crystallizes in ultramafic rock complexes, in rocks such as dunites and chromitites, often in association with ophiolites. It also occurs secondarily, as weathering-resistant grains in alluvial deposits (river sands and gravels), from where it is recovered along with other heavy minerals, including native gold and platinum. ## Mineral Associations It most commonly co-occurs with other platinum-group minerals (PGM), such as laurite, irarsite, ruarsite, osmiridium, as well as with chromite, magnetite, and native gold. ## Localities The most important osarsite localities worldwide include deposits in the Bushveld Complex in South Africa, the Stillwater Complex in Montana (USA), numerous occurrences in the Urals (Russia), and in river sediments in California (USA), British Columbia (Canada), and Borneo (Indonesia).

Rarity

Very rare

For collectors

## Quality Criteria The quality of an osarsite specimen is determined not by the appearance of the mineral itself (which is microscopic), but by the richness of the mineral association and the confirmation of its presence through analysis. Most valued are polished sections where osarsite is clearly visible surrounded by other rare platinum-group minerals. The size of the grains, even if it is a fraction of a millimeter, also increases scientific and collectible value. ## Popular Localities For collectors specializing in platinum-group minerals, specimens from classic localities such as the Bushveld Complex (South Africa) or the Urals (Russia), which have provided the best-studied parageneses, are most sought after.

Care and storage

## Cleaning Specimens with microscopic osarsite are generally not mechanically cleaned due to the risk of damage or loss of fine grains. If cleaning is absolutely necessary, only compressed air should be used to remove dust. ## What to Avoid Avoid contact with chemicals, acids, and cleaning agents that could react with sulfides or associated minerals. Specimens should not be subjected to ultrasonic cleaning. ## Storage Specimens containing osarsite, typically in the form of polished sections or small rock fragments, are best stored in specialized "micromount" boxes, which protect them from dust and mechanical damage. Avoid humid environments to prevent potential oxidation of sulfides.

Sources

IMA Mineral List (CNMNC)