Shenzhuangite, NiFeS2, the Ni-analogue of chalcopyrite from the Suizhou L6 chondrite

Abstract

Shenzhuangite (IMA 2017-018), NiFeS₂, is a new mineral species found in the shocked Suizhou L6 chondrite. The mineral is closely associated with forsteritic olivine, pyroxene, plagioclase glass (maskelynite), taenite and troilite. It is opaque, and in reflected light shenzhuangite is weakly anisotropic with light brown to greenish rotation tints. Internal reflections are absent. The reflectance COM values (air, R₁, and R₂, in %) are: 24.8, 26.0 (471.1 nm); 34.9, 36.2 (548.3 nm); 37.7, 39.1 (586.6 nm); 40.4, 41.1 (652.3 nm). The average result of 4 electron-microprobe analyses is Ni 22.37(55), Fe 30.87(67), Cu 10.88(22), Co 0.07(4), S 35.42(58), total 99.61 wt%. The empirical formula (based on 4 atoms pfu and assuming the crystal-chemical exchange: Cu⁺+Fe³⁺↔Ni²⁺+Fe²⁺) is (Ni²⁺_0:69Cu⁺_0:31)(Fe²⁺_0:69Fe³⁺_0:69 0:31)S_2.00. The simplified formula is NiFeS₂, which requires Ni 32.85, Fe 31.26, S 35.89, total 100.00 wt%. Shenzhuangite is tetragonal, space group I4^-2d, with unit-cell parameters: a = 5.3121(4), c = 10.4772(7)Å, V = 295.65(4)ų, c:a = 1.9723, Z = 4. The five strongest observed X-ray powder-diffraction lines [d in Å(I/I₀) (h k l)] are: 3.05 (100) (1 1 2), 1.875 (20) (2 2 0), 1.591 (25) (3 1 2, 1 1 6), 1.215 (10) (3 3 2, 3 1 6), 1.080 (10) (4 2 4). The crystal structure showed that shenzhuangite is the Ni-analogue of chalcopyrite. It likely formed as alteration of pre-existing taenite when _pS₂/_pO₂ ratios allowed sulfurization of the FeNi metal. Shenzhuangite is named in honour of Prof. Shangyue Shen and Prof. Xiaoli Zhuang who first discovered the Ni-rich variety of chalcopyrite in the Suizhou meteorite.