Hollingworthite

Chemical formula: Rh³⁺(AsS)³⁻

Hollingworthite is a rare rhodium, platinum, and palladium sulfarsenide, forming small, metallic grains in ultramafic igneous rocks.

## Characteristics Hollingworthite is a rare mineral from the sulfarsenide group, belonging to the cobaltite group. Its composition primarily includes rhodium, platinum, and palladium, as well as arsenic and sulfur. It forms very small, most often submicroscopic, euhedral or anhedral grains, rarely exceeding 0.1 mm. It usually occurs as inclusions in other sulfides, such as chalcopyrite, pentlandite, or pyrrhotite. It is opaque and has a metallic luster. ## Physical Properties Due to the extremely small size of the grains, most physical properties are difficult to measure precisely. The Mohs hardness is estimated to be around 6. The mineral is characterized by a high density of approximately 7.92 g/cm³, which is typical for platinum-group minerals. ## Colors and Varieties Hollingworthite has a steel-gray to white color. In reflected light under a microscope, its color is neutral gray, sometimes with a slight, barely noticeable tint. No color varieties or commercial varieties are distinguished. ## History and Name The mineral was discovered in platinum-bearing concentrates from pyroxenite in the Driekop mine in South Africa. It was described and named in 1965 by E. F. Stumpfl and A. M. Clark. The name honors Professor Sydney E. Hollingworth (1899-1966), a geologist from University College London, in recognition of his contribution to geology. ## Applications Hollingworthite, as a component of certain platinum-group element deposits, has scientific and potentially economic significance as one of the sources of rhodium and other platinum-group metals. It has no direct collector's application due to the microscopic size of its crystals.

Properties

Mohs hardness
6-6.5
Color
Medium gray, slight bluish tint in oil
Luster
Metallic
Streak
Black
Density
0
Cleavage
None
Fracture
Uneven
Transparency
Opaque
Crystal system
Isometric

Diagnostic features

## Identification Identification of hollingworthite is possible almost exclusively using advanced laboratory techniques. It requires analysis in reflected light (ore microscopy) and chemical methods, such as X-ray microanalysis (EDS/WDS), to confirm its chemical composition rich in rhodium, platinum, arsenic, and sulfur. ## Distinguishing from Similar Minerals Under the microscope, hollingworthite can be confused with other platinum-group minerals (PGM) such as irarsite, osarsite, ruarsite, or platarsite. Differentiation is based on precise measurement of reflectivity and, crucially, on chemical analysis, which will show the dominance of rhodium. ## Crystal Forms This mineral crystallizes in the isometric system. Observed crystals are usually very small, with an isometric habit, often in the form of cubes, octahedra, or combinations thereof. However, it most often occurs as irregular, anhedral grains.

Geological environment

## Genesis Hollingworthite is a mineral of magmatic origin. It forms as a result of crystallization from residual, sulfur- and arsenic-rich melts within ultramafic and mafic igneous intrusions. It is a typical component of platinum-group element (PGE) deposits associated with layered igneous complexes. ## Mineral Associations It co-occurs with other platinum-group minerals, such as braggite, sperrylite, laurite, as well as with base metal sulfides, mainly chalcopyrite, pentlandite, pyrrhotite, and pyrite. Associated minerals also include chromite and rock-forming silicates (pyroxenes, olivines, plagioclases). ## Localities The most important occurrences of hollingworthite are associated with large igneous complexes. These include, among others, the Bushveld Complex in South Africa (Driekop, Onverwacht mines), the Sudbury Complex in Canada, the Duluth Intrusion in the USA (Minnesota), the Oktyabrskoye deposit (Norilsk) in Russia, as well as deposits in Finland (Kemi, Penikat) and Brazil.

Rarity

Very rare

For collectors

## Quality Criteria Hollingworthite is not a mineral collected for traditional collecting purposes. Its value is purely scientific and economic. Specimens are typically rock fragments or polished samples (sections) in which the mineral is visible only under high magnification. The value of such a specimen depends on the confirmed presence of the mineral, the richness of the association of other platinum-group minerals, and the quality of the analytical documentation. ## Popular Localities Research samples mainly come from classic localities for PGE minerals, such as the Bushveld Complex in South Africa, Sudbury in Canada, or the Norilsk region in Russia. However, they are not traded on the collector's market.

Care and storage

## Cleaning Specimens containing hollingworthite, typically rock fragments or polished sections, do not require specialized cleaning. If necessary, they can be wiped with a dry, soft cloth or blown with compressed air to remove dust. ## What to Avoid Avoid contact with strong acids and chemicals that could react with sulfides and arsenides. There are no specific requirements regarding temperature or light, but as with all minerals, extreme conditions should be avoided. ## Storage Rock samples containing hollingworthite are best stored under stable conditions, in closed boxes or display cases, to protect them from dust and contaminants. Polished sections should be protected from scratching.

External references

Sources

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