Ruarsite

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

Ruarsite is a rare ruthenium arsenosulfide with a metallic luster, occurring as microscopic, cubic crystals.

## Characteristics Ruarsite is a very rare mineral from the sulfide group, specifically a ruthenium arsenosulfide. It occurs as very small, most often submicron, cubic crystals or as granular inclusions in other minerals, mainly in laurite. Its crystals rarely exceed 100 micrometers. Due to its size, it is a mineral that can be identified almost exclusively using advanced laboratory techniques, such as electron microscopy. ## Physical Properties This mineral is characterized by a metallic luster and is opaque. Its Mohs hardness is 6-7, making it a relatively hard mineral. The density of ruarsite is high, approximately 7.5 g/cm³, which is typical for minerals containing platinum group elements. ## Colors and Varieties Ruarsite is gray to white in color. In reflected light under a microscope, its color is described as white. There are no distinct color varieties or commercial forms of this mineral. ## History and Name The name "ruarsite" directly refers to its chemical composition – ruthenium (Ru) and arsenic (As). The mineral was first described in 1979 by P.J. Dunn, L.J. Cabri, J.H.G. Laflamme, and J.A. Criddle based on material from two localities: the Onverwacht platinum deposit in the Republic of South Africa and the Horokanai River area in Japan. It was approved by the International Mineralogical Association (IMA) in the same year. ## Uses Ruarsite has no industrial application. Its significance is purely scientific and collectible, and due to its microscopic size, it is an object of interest for specialized micromineral collectors.

Properties

Mohs hardness: 6-7
Luster: Metallic
Streak: Black
Density: 7.5
Cleavage: None
Fracture: Uneven
Transparency: Opaque
Crystal system: Cubic

Diagnostic features

## Identification Identification of ruarsite under amateur or field conditions is impossible due to its microscopic size. Recognition requires specialized equipment, such as a scanning electron microscope (SEM) with an EDS analyzer to confirm its chemical composition (presence of ruthenium, arsenic, and sulfur). In reflected light under a ore microscope, it is white and isotropic. ## Distinguishing from Similar Minerals Ruarsite is difficult to distinguish from other microscopic platinum group sulfides and arsenides, such as laurite, irarsite, or osarsite, with which it often co-occurs. Definitive differentiation is possible only through chemical analysis. Its cubic crystal form can be a clue, but it is not a unique feature. ## Crystal Forms Ruarsite crystals are very small, usually below 100 micrometers. They adopt cubic or octahedral forms, which is typical for the isometric system. It is most often observed as fine, granular inclusions in other minerals.

Geological environment

## Genesis Ruarsite is a mineral of magmatic origin. It forms in ultramafic magmatic complexes, crystallizing from magma rich in sulfides and platinum group elements (PGE). It also occurs in placer deposits, where it is transported as a result of weathering and erosion of primary magmatic rocks. ## Mineral Associations This mineral most commonly co-occurs with other platinum group minerals (PGM), such as laurite, irarsite, osarsite, native platinum, as well as chromite, chalcopyrite, and pentlandite. ## Localities The most important confirmed occurrences of ruarsite worldwide include: - Onverwacht Mine in the Bushveld Complex, South Africa (type locality). - Horokanai River, Hokkaido, Japan (type locality). - Nickel deposits in the Klaipėda region, Lithuania. - Stillwater Igneous Complex in Montana, USA. - PGM deposits in Freetown, Sierra Leone.

Rarity

Very rare

For collectors

## Quality Criteria For ruarsite, collectible specimens are actually fragments of host rock or other minerals (mainly laurite or chromite) containing identified, microscopic inclusions of ruarsite. Highly valued are specimens in which ruarsite crystals, despite their microscopic size, are well-formed (e.g., in cubic shape) and visible under magnification. However, the key here is not aesthetics, but the confirmation of the presence of this rare mineral itself, often documented by chemical analysis. ## Popular Localities Specimens from type localities, i.e., from the Onverwacht mine (South Africa) and Japan, have the greatest historical and scientific value for specialized collectors.

Care and storage

## Cleaning Specimens in which ruarsite has been identified usually do not require cleaning, as this mineral occurs as microscopic inclusions in the host rock or in other, more stable minerals. Any mechanical or chemical intervention could irrevocably destroy the tiny crystals. If dust needs to be removed from the host specimen, compressed air can be used from a safe distance. ## What to Avoid Avoid contact with any chemicals, especially acids, which can react with sulfides. Also avoid ultrasonic cleaners and mechanical cleaning (e.g., with brushes), which would destroy the microscopic crystals. ## Storage Specimens containing ruarsite should be stored in stable conditions, away from moisture and chemical contaminants, preferably in sealed "micromount" boxes. This protects delicate crystals from physical damage and dust.

Sources

IMA Mineral List (CNMNC)