Abstract

The rare thiospinel siegenite (CoNi2S4) has been found in drill-core samples of quartz-norite as primary oriented intergrowths in millerite from the Morokweng impact site, South Africa. The mineral occurs as elongate, sub-parallel grains (10 × 50 μm on average, up to 100 × 500 μm) that are light purple under reflected light. Quantitative reflectance data compares well to current data sets and shows little variation among other Ni-Co thiospinel endmembers. The average major element chemistry of siegenite (n = 19) includes (wt.%): Fe 1.69 (0.65–2.99), Co 12.46 (7.92–15.72), Ni 43.84 (39.29–47.68), and S 41.90 (41.25–42.26), corresponding to the average formula (Co0.64Ni0.28Fe0.09)Σ1.01Ni2.00S3.98 (based on 7 atoms per formula unit). Laser-Raman spectroscopy along with single-crystal X-ray diffraction results supports the forumla space group assignment, with no evidence to suggest a phase transformation. The X-ray diffraction data also support the inverse cation distribution for siegenite. Electron backscattered diffraction (EBSD) indicates that the siegenite (sgn) and millerite (mlr) are present in one of two possible crystallographic orientations (shared plane, orientation of plane): (1) (100)[012]mlr||(111)[1forumla0]sgn or (2) (001)[1forumla0]mlr||(110)[1forumla1]sgn. Further examination with EBSD suggests that the siegenite is twinned on {111} forming simple lamellar twins. Based on the data presented, it is suggested that the sequence of crystallization was millerite → siegenite → chalcopyrite, initiated by nucleation of Ni1–xS (millerite, at high T) and followed by the formation of siegenite between 356 and 282 °C which crystallized alongside metal-deficient Ni1–xS. This process caused the co-crystallization of millerite and siegenite, forming the oriented crystal-controlled intergrowth observed in the Morokweng quartz-norite impact melt sheet.

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