Ezochiite, Cu⁺(Rh³⁺Pt⁴⁺)S₄, a new mineral in the thiospinel group from Hokkaido, Japan

抄録

Ezochiite, a newly-discovered platinum-group mineral in the thiospinel group having the ideal formula Cu⁺(Rh³⁺Pt⁴⁺)S₄, was discovered in samples from the Tomamae coast near Tomamae town, Hokkaido, Japan. Additional specimens were later found in the Shosanbetsu river, Ainusawa river and Obira coast, Hokkaido. Ezochiite crystallized in melt pockets trapped in isoferroplatinum grains, occurring in the form of anhedral grains less than 5 µm in length. Ezochiite is associated with sulfide minerals such as braggite, cooperite, torryweiserite, and chalcopyrite. It 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 by analogy with related thiospinel group minerals and a density of 6.66 g·cm⁻³ was calculated from the empirical formula and powder X-ray diffraction data. The empirical formula, on the basis of 7 apfu was (Cu⁺_0.85Fe³⁺_0.15)_Σ1.00(Rh³⁺_1.09Pt⁴⁺_0.78Ir³⁺_0.08Pt²⁺_0.05)_Σ2.00S_4.00 for a specimen obtained from the Tomamae coast. The powder X-ray diffraction study indicated that the mineral exhibits the spinel structure, space group Fd3m, with lattice parameters a = 9.8559(14) Å and V = 957.4(4) ų (Z = 8). Evidence for a spinel structure was also provided by Raman spectra. Data from samples of ezochiite and cuprorhodsite from samples sourced in Hokkaido showed a compositional relationship based on coupled Fe³⁺_0.5Rh³⁺-Cu⁺_0.5Pt⁴⁺ substitution. Ezochiite is not rare. It is also found various other geological environments, including in ophiolites, Ural-Alaskan intrusions and mafic-ultramafic intrusions.