R1: Characterization and description of minerals (Joint Session with The Gemmological Society of Japan)

[R1-P-06] Michitoshiite-(Cu), a new Ge-containing platinum group mineral from Haraigawa, Misato machi, Kumamoto Prefecture, Japan

*Takahiro TANAKA¹, Daisuke Nishio Hamane², Tadashi Shinmachi (1. Nittetsu Mining Co., Ltd., 2. ISSP, Univ. of Tokyo)

Keywords:Michitoshiite-(Cu), Germanium, Platinum group mineral, Synthetic experiment, Kumamoto

Michitoshite-(Cu) is a new mineral of Ge-containing platinum group minerals (PGM) discovered from the Haraigawa, Kumamoto Prefecture, Japan. It is named in honor of Michitoshi Miyahisa (1928-1983), a former professor of the Ehime University. The mineral and name have been approved by the IMA Commission on New Minerals, Nomenclature and Classification (IMA2019-029a). The PGM placer deposit where the michitoshiite-(Cu) was found is located in a small stream that crossing a clinopyroxenite mass (Nishio-Hamane et al., 2019). Michitoshite-(Cu) is opaque, has a metallic luster, and is silver-gray in color. It is light gray with brownish tints and shows no pleochroism and anisotropy in reflected light. The Mohs hardness is 5, and the density calculated from the empirical formula and powder X-ray diffraction (XRD) data is 10.78 g/cm³. The empirical formula is calculated on the basis of 2 apfu is (Rh_0.95Pt_0.03Ir_0.01)_Σ0.99[( Cu_0.36Fe_0.24)_Σ0.60Ge_0.41]_Σ1.01. The powder XRD pattern shows five strong peaks [d in A (I/I0) hkl], 2.103 (100) 110, 1.717 (3) 111, 1.487 (15) 200, 1.332 (6) 210, 1.216 (70) 211, and can be indexed as the cubic Pm-3m (#221) with the lattice parameters a = 2.9771(11) A and V = 26.39(3) A3 (Z = 1). From previous studies, the RhCu-RhFe-RhGe system has a face-centered cubic (fcc) structure for RhCu, a CsCl-type structure for RhFe, and a MnP-type structure for RhGe, with the end-member of each compound having a different structure, but the solid solution structure has not been investigated. Since Rh[(Cu_0.35Fe_0.25)_Σ0.60Ge_0.40], which was synthesized to mimic the empirical formula of the natural sample, had a CsCl-type structure, the crystal structure of michitoshite-(Cu) was considered to be CsCl-type. Considering that the synthetic materials of Rh(Cu,Fe) composition synthetics were not bcc-based crystal structures but rather fcc structures in the synthesis experiments, the presence of Ge is considered essential for the formation of the bcc-based crystal structure. Therefore, based on the relationship between composition and crystal structure obtained from the behavior of other synthetics, the ideal formula for michitoshite-(Cu) is Rh(Cu_1-xGe_x) 0 < x <= 0.5.