Shiranuiite

shiranuiite

isoferroplatinum

cuprorhodsite

michitoshiite-(Cu)

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Formula: Cu+(Rh3+Rh4+)S4
Sulphide, carrollite subgroup, thiospinel group, spinel supergroup, rhodium-bearing mineral
Crystal system: Isometric
Specific gravity: 5.78 calculated
Hardness: 5
Environments

Plutonic igneous environments
Placer deposits

Shiranuiite is a new mineral, approved in 2023 and pending publication as of May 2025.

Localities

The type locality is Haraigawa, Misato, Shimomashiki District, Kumamoto Prefecture, Japan. Shiranuiite is a new (in 2023) member of the carrollite subgroup of the thiospinel group representing an extremely rare example of a mixed-valence rhodium sulphide. It was found in Haraigawa within a grain of isoferroplatinum containing associated tulameenite, tetraferroplatinum and cuprorhodsite.
The validity of the mineral, however, has been called into question; it has been argued that shiranuiite and ezochiite are not new minerals, but platinum- and rhodium- rich varieties of cuprorhodsite (AM 110.5.830).
Isoferroplatinum-based grains are often accompanied by black to grey coloured nubs from 20 to 500 μm in size. Shiranuiite and cuprorhodsite occur as dominant minerals of such nubs, although both are indistinguishable based on the occurrence mode and can only be distinguished by chemical analysis.
The clinopyroxenite mass at the type locality has a depression near the centre, and a small river, approximately 2 to 3 m wide and 2 km long, passes through this area and joins the Shakain River. Platinum group minerals can be collected as placers in this small river. Most of the Platinum group mineral placers collected were composed of isoferroplatinum-based grains that are irregularly shaped with an average size of less than 1 mm, although grains up to 4 mm were also obtained.
Minerals associated with shiranuiite include isoferroplatinum, bowieite, cuprorhodsite and michitoshiite-(Cu) (Mindat).

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