Chengdeite—A New Mineral of Ordered Natural Iron- iridium Alloy

Abstract Chengdeite occurs in chromite orebodies in dimite as well as in placers in their neighbourhood. The mineral occurs as granular aggregates in association with inaglyite and in some cases occurs as graphic intergrowths with native iridium. It is opaque with a metallic lustre, colour steel-black, streak black, H_M= 5.2, VHN₅₀=452 kg/mm², cleavage not observed, fracture not observed, strongly magnetic. Its reflection colour is bright white with a yellowish tint. It has no internal reflection, bireflectance or pleochrism, and shows isotropism.

Thirteen chemical analyses were carried out by means of the electron microprobe. The mean percentages of the data obtained in the 13 analyses are: s 0.001, Fe 7.9, Ni 0.03, Co 0.03, Cu 0.83, As 0.02, Rh 0.19, Pd 0.00, Os 0.06, Ir 88.5, Pt 2.2 and Pb 0.00. The simplified formula is Ir₃Fe, which requires Ir 91.17 and Fe 8.83, the total being 100.00 (%).

Five strongest lines of X-ray powder diffraction (hkl, d, I) are: 111, 2.18 (80);200, 1.89 (60); 220, 1.34 (70); 311, 1.142 (100); 222, 1.094 (80).

Through indexing of the X-ray powder diffraction data, the mineral has been determined to be cubic with Pm 3m, a=0.3792 (5)nm, V=0.05453nm and Z=1.