The North Haig olivine pigeonite achondrite (ureilite) is a polymict breccia consisting of major olivine, low-Ca pyroxene and an intergranular carbonaceous matrix. Olivine and low-Ca pyroxene vary widely in composition, covering the ranges observed in all known ureilites. Minor granular enstatite clasts with diopside exsolution blebs, akin to enstatite achondrites, were also observed. Native metal in ureilites is normally kamacite of variable Ni content, in some cases with up to 2% Si in solid solution. However, kamacite with trace amounts of Si is extremely rare in North Haig, only a few 1–2 μm grains within silicates were observed. Instead, the common metallic phase is approximately Fe3Si, a new mineral which we have named suessite in honor of Professor Hans E. Suess. Suessite occurs as minor anhedral vein fillings in interstitial cracks, in silicates, and in the intergranular carbonaceous material and ranges in size from 1 μm blebs to elongated grains about 30 × 150 μm in size. Suessite is cream white in reflected light, isotropic, ferromagnetic, and shows no cleavage. Reflectance in percent (determined by G. A. Desborough) at the 4 standard wavelengths of 470, 546, 589 and 650 nm for 2 grains is 48.5(5), 51.6(3), 53.5(7), 50(2), and 49.7(5), 53.4(4), 54.5(6), 52(1), respectively. Analyses (by EMX) indicate presence of dominating low-Ni and less common high-Ni varieties of suessite (in wt.%, mean in parentheses): Fe 84.7, 83.1 (84.2); Ni 1.6, 4.5 (2.5); Co 0.21, 0.27 (0.23); Cr 0.10, 0.04 (0.08); Si 15.3, 13.7 (14.7); P 0.06, 0.17 (0.10). Thus, suessite is (Fe, Ni, Co, Cr)2.84–3.14 (Si, P)1.0, mean (Fe, Ni, Co, Cr)2.96(Si, P)1.0, very close to Fe3Si. X-ray powder diffraction shows that suessite possesses a similar structure to alpha-Fe (kamacite) and the solid solution alloy (Fe3Si)ss in displaying three lines (relative intensities in parentheses): 2.005(10), 1.42(1), 1.160(3) (in Å). The calculated cell size is 2.841 ±0.002Å (V = 22.93Å3). Thus, suessite is the Si-rich end member of the α solid solution region of the Fe–Si phase diagram with composition close to Fe3Si. Silicates, particularly olivine, in ureilites have core compositions of Fo76–92 and thin (<100 μm) rims of essentially Fo100 formed by reduction and reaction with the carbonaceous matrix material. We suggest that Si and Fe liberated in this reduction process formed suessite, possibly also by reaction with preexisting kamacite.

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