Michitoshiite-(Cu), a new mineral from Haraigawa, Misato town, Shimomashiki District, Kumamoto Prefecture, Japan

This paper reports the discovery and description of a new mineral, michitoshiite-(Cu), from a placer platinum-group mineral (PGM) deposit in Haraigawa, Misato town, Shimomashiki District, Kumamoto Prefecture, Japan.Michitoshiite-(Cu) occurs as micrometer-sized grains within or on small surface nubs developed on isoferroplatinum grains derived from a clinopyroxenite body in a serpentinite mlange.The mineral is characterized by Rh as the dominant element, with Cu and Ge as essential constituents, a chemical feature not observed in previously known PGMs.The mineral was identified as an independent phase by SEM-EDS analyses and powder X-ray diffraction pattern, revealing a cubic CsCl-type structure in natural samples.This mineral was therefore initially interpreted to be formed under high-pressure conditions.However, subsequent discussions during the review process of the IMA Commission on New Minerals, Nomenclature and Classification (CNMNC) prompted systematic synthesis experiments in the Rh-Cu-Fe-Ge system.These experiments demonstrated that the CsCl-type structure is stabilized primarily by chemical composition rather than by pressure conditions.Consequently, michitoshiite-(Cu) was defined as a mineral