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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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