Shiranuiite

Chemical formula: Cu₁+(Rh₃+Rh₄+)S₂-₄

Shiranuiite is an extremely rare copper-rhodium sulfide, discovered in Japan, forming microscopic, metallic grains in platinum-group metal concentrates.

## Characteristics Shiranuiite is a sulfide mineral, chemically classified as a copper-rhodium sulfide. It occurs as extremely small, anhedral (irregularly shaped) grains, typically not exceeding 60 micrometers in size. Observed in polished sections under a microscope, in reflected light, it has a grayish-white color with a slight creamy tint. ## Physical Properties Due to the microscopic size of the grains, most physical properties, such as hardness, density, luster, or transparency, have not been precisely determined. The luster is described as metallic, and the mineral is opaque. The Vickers hardness (VHN₅₀) is 388, suggesting a Mohs hardness in the range of 4-5. ## History and Name The mineral was officially recognized by the International Mineralogical Association (IMA) in 2018 (IMA2018-038). Its name comes from the Shiranui Sea in Japan, near which the type locality is situated. The discovery was made during the study of platinum-group metal concentrates originating from an unidentified deposit in Japan.

Properties

Mohs hardness: 4-5
Luster: Metallic
Transparency: Opaque
Crystal system: Cubic

Diagnostic features

## Identification Identification of Shiranuiite is possible only using advanced laboratory techniques. It requires analysis in reflected light (ore microscopy) to observe its color and co-occurrence with other sulfides. Final confirmation is obtained through chemical composition analysis using an electron microprobe (EDS/WDS) and crystal structure analysis by electron diffraction. ## Distinguishing from Similar Minerals Under the microscope, Shiranuiite can be confused with other platinum-group metal sulfides, such as cuproiridsite, cuprorhodsite, or malanite. Differentiation is based on precise measurement of chemical composition, especially the ratio of copper to rhodium and iridium. ## Crystal Forms This mineral forms only very fine, anhedral grains, often intergrown with other platinum-group minerals. It has not been observed in the form of well-developed crystals.

Geological environment

## Genesis Shiranuiite forms in ultramafic rock complexes, in chromite-rich zones. Its genesis is associated with magmatic processes, during which platinum-group elements (PGE) concentrate and sulfides crystallize under specific physicochemical conditions. ## Mineral Associations This mineral occurs in close association with other platinum-group minerals (PGM). It most often co-occurs with laurite, iridium, osmium, cuproiridsite, cuprorhodsite, malanite, irarsite, as well as with chromite and base metal sulfides. ## Localities The only confirmed occurrence (type locality) is an unspecified deposit in Japan, from which the studied metal concentrates originated. The mineral has not yet been identified anywhere else in the world.

Rarity

Extremely rare

For collectors

## Quality Criteria Shiranuiite does not appear on the collector's market in the form of macroscopic specimens. Its value is purely scientific. The only existing samples are research material (so-called type material) stored in scientific institutions, such as the National Museum of Nature and Science in Tsukuba, Japan. For research purposes, the "quality" of a specimen is defined by grain size (the larger, the better for analysis) and chemical purity.

Care and storage

## Cleaning Due to its extremely small size and occurrence as inclusions in other minerals, individual Shiranuiite specimens are not isolated and cleaned in a traditional manner. Preparation is limited to laboratory techniques, such as polishing of thin/polished sections. ## What to Avoid Specimens containing Shiranuiite should be protected from aggressive chemicals, especially strong acids, which can react with sulfides. Ultrasonic cleaners and abrasion of the sample surface should be avoided. ## Storage Samples (most often in the form of polished sections or resin blocks) should be stored in a dry place, at a stable temperature, away from dust and chemical contaminants. It is best to keep them in specialized boxes for storing microminerals or microscopic preparations.

Sources

IMA Mineral List (CNMNC)