Minakawaite and platinum-group minerals in the placer from the clinopyroxenite area in serpentinite melange of Kurosegawa belt, Kumamoto Prefecture, Japan

Minakawaite, a new mineral with a RhSb composition, in association with a platinum-group mineral (PGM) placer is found from a small stream crossing the clinopyroxenite mass in serpentinite melange of the Kurosegawa belt on the northeast side of Hikawa Dam, Haraigawa, Misato machi, Kumamoto Prefecture, Japan. Almost all PGM placer grains are based on isofeftoplatinum, of which the rims are often covered by tulameenite and tetrafeftoplatinum. This isofeftoplatinum-based grain contains small inclusions and accessories consisting mainly of osmium, erlichmanite, laurite, bowieite, cuprorhodsite, and ferhodsite-like mineral. Minakawaite occurs as the outmost surface layer with a rose gray metallic luster on the nub consisting of cuprorhodsite, ferhodsite-like mineral and/or Rh(Ge,Cu,Fe) mineral in association with an isoferroplatinum-based grain. The density of minakawaite is 10.04 g/cm(3), calculated using the empirical formula and powder X-ray diffraction (XRD) data. Minakawaite has a pale gray color under the microscope in reflected light, and pleochroism is weak as a variation from pinkish pale gray to bluish pale gray. Anisotropy is moderate as reddish gray to bluish gray. Average results often energy dispersive X-ray spectroscopy (EDS) analyses give Rh 46.83, Sb 48.97, As 4.08 and total 99.88 wt%. The empirical formula is Rh-0.(998)(Sb0.882As0.120)(Sigma 1.)(002), based on 2 atoms per formula unit. Minakawaite is orthorhombic (Pnma) with a = 5.934(7) angstrom, b = 3.848(3) angstrom, c = 6.305(4) angstrom, and V = 144.0(2) angstrom(3) (Z = 4). The seven strongest lines of minakawaite in the powder XRD pattern [d in (I/I-0) (hkl)] are 2.860(63) (111), 2.774(35) (102), 2.250(47) (112), 2.199(100) (211), 2.162(38) (202), 1.923(49) (020), and 1.843(51) (013). Minakawaite is identical to the synthetic RhSb phase with MnP-type structure. PGM including minakawaite may occur with chromite in the magma chamber of the clinopyroxenite. Minakawaite was named in honor of Japanese mineralogist, Prof. Tetsuo Minakawa (b. 1950) of Ehime University for his outstanding contribution to descriptive mineralogy from Kyushu and Shikoku, Japan.