Shiranuiite, Cu⁺(Rh³⁺Rh⁴⁺)S₄, a new mineral in the thiospinel group from Kumamoto, Japan

Abstract

Shiranuiite, a newly-discovered platinum-group mineral in the carrollite-type thiospinel group with an ideal formula of Cu⁺(Rh³⁺Rh⁴⁺)S₄, was discovered at Haraigawa, Misato machi, Kumamoto Prefecture, Japan. This mineral was named after the ancient name for Kumamoto Prefecture, "Land of Fire", which appeared in an anecdote about Emperor Keiko's pilgrimage to Kyushu recorded in the Nihon Shoki: "the fire that everyone does not know about (= SHIRANUI, in the Japanese historical kana orthography)". Shiranuiite occurs as the most abundant mineral in nubs accompanied by isoferroplatinum-based grains and is occasionally associated with bowieite, cuprorhodsite, michitoshiite-(Cu), and oxidized platinum-group minerals. Shiranuiite is opaque and has a metallic luster with a bluish gray color in reflected light. The Mohs hardness of this mineral was estimated to be 5 according to the analogous thiospinel-group minerals, and a density of 5.78 g·cm^-3 was calculated from the empirical formula and powder X-ray diffraction data. The empirical formula, on the basis of 7 apfu is (Cu⁺_0.95Ni²⁺_0.01Fe³⁺_0.04)(Rh³⁺_1.19Rh⁴⁺_0.77Ir⁴⁺_0.06Pt⁴⁺_<0.01)₂S_3.99. Powder X-ray diffraction measurements indicated that the mineral has the spinel structure and belongs to the space group Fd3m with lattice parameters of a = 9.757(2) Å and V = 928.9(5) ų (Z = 8). Evidence for a spinel structure was also provided by Raman spectroscopy. Both shiranuiite and cuprorhodsite are altered products of bowieite and are highly immiscible in the Cu-Fe system. There is evidence suggesting that shiranuiite formed at a later stage than cuprorhodsite, i.e., these two minerals may form at different stages.