Stillwaterite

Chemical formula: Pd₈As₃

Stillwaterite is a rare palladium arsenide, forming microscopic, metallic grains in platinum-group element deposits.

## Characteristics Stillwaterite is a palladium arsenide, occurring as small, irregular grains, rarely exceeding 0.1 mm. It is typically anhedral (does not exhibit its own crystal faces) and occurs as inclusions in other minerals. It has a metallic luster and a tin-white color, often with a pinkish hue. ## Physical Properties The mineral is characterized by a hardness of 4.5 on the Mohs scale. It has a high, metallic luster. It is opaque. Its density has not been precisely measured, but the calculated theoretical value is approximately 10.38 g/cm³. ## Colors and Varieties Stillwaterite is tin-white; on fresh surfaces, it may exhibit a pinkish or brownish hue. No varieties are distinguished. ## History and Name The mineral's name comes from its discovery locality – the Stillwater igneous complex in Montana, USA. It was described and approved as a new mineral in 1974. ## Uses Stillwaterite, due to its rarity and microscopic size, has no direct industrial application. However, it is a component of palladium and platinum ores, making it an object of scientific and collector interest.

Properties

Mohs hardness: 4.5
Luster: Metallic
Streak: Black
Density: 0
Cleavage: None
Fracture: Uneven
Transparency: Opaque
Crystal system: Trigonal

Diagnostic features

## Identification Identification of stillwaterite requires advanced laboratory techniques, such as reflectivity analysis in reflected light, microhardness testing, and chemical analysis using an electron microprobe (EDS/WDS). Its tin-white color with a pinkish hue and high metallic luster are indicative features but insufficient for certain identification. ## Distinguishing from Similar Minerals It can be confused with other arsenides and platinum-group metal alloys, such as sperrylite, arsenopalladinite, or palladoarsenide. Differentiation is almost exclusively possible based on chemical composition analysis and crystallographic data. ## Crystal Forms Stillwaterite forms irregular, anhedral grains, often as inclusions in other minerals, mainly sulfides. Well-formed crystals are not observed.

Geological environment

## Genesis Stillwaterite is a mineral of magmatic origin. It forms in ultramafic and mafic layered intrusions, in zones rich in platinum-group elements (PGE). It crystallizes in the late stages of magma differentiation. ## Mineral Associations This mineral co-occurs with other platinum-group minerals, such as braggite, kotulskite, sperrylite, as well as with base metal sulfides like chalcopyrite, pentlandite, and pyrrhotite. ## Localities The most important and classic locality is the Stillwater Complex in Montana, USA. It has also been reported in other large igneous complexes worldwide, including the Bushveld Complex in South Africa, the Noril'sk region in Russia, and Sudbury in Canada.

Rarity

Very rare

For collectors

## Quality Criteria As a "micromount" type mineral, the quality of a specimen is primarily determined by the abundance and size of stillwaterite grains within the host rock. The most prized samples are those where grains are visible under low magnification and stand out clearly from the background. Certainty of identification, confirmed by analysis, is also crucial. ## Popular Localities The most classic and sought-after specimens by collectors come from the type locality – the Stillwater Complex in Montana, USA.

Care and storage

## Cleaning Stillwaterite specimens are generally not cleaned due to their microscopic occurrence and the risk of damage. If absolutely necessary, compressed air can be used to remove dust. ## What to Avoid Avoid contact with chemicals, especially acids, which can react with arsenides. Also, avoid ultrasonics and mechanical cleaning (e.g., with brushes), which could damage the fine mineral grains. ## Storage Specimens should be stored under stable conditions, in closed "micromount" boxes, to protect them from dust and mechanical damage. Humidity should be avoided.