Allabogdanite, (Fe,Ni) (sub 2) P, a new mineral from the Onello Meteorite; the occurrence and crystal structure

Allabogdanite, (Fe,Ni) (sub 2) P, is a new mineral from the Onello iron meteorite (Ni-rich ataxite). It occurs as thin lamellar crystals disseminated in plessite. Associated minerals are nickelphosphide, schreibersite, awaruite, and graphite. Crystals of the mineral, up to 0.4X0.1X0.01 mm, are flattened on (001) with dominant {001} faces, and other faces that are probably {110} and {100}. Mirror twinning resembling that of gypsum is common, with possible twin composition plane {110}. Crystals are light straw-yellow with bright metallic luster. Polished (001) sections look silvery-white against an epoxy background. In reflected light in air, the mineral has a creamy color, with distinct anisotropy from light to dark creamy tint. No bireflectance was observed. R (sub 1) /R (sub 2) (lambda , nm) in air: 48.4/37.2(440), 46.7/36.8(460), 47.0/37.6(480), 47.5/38.1(500), 47.6/38.8(520), 48.2/39.2(540), 49.0/39.9(560), 49.6/40.7(580), 50.1/41.6(600), 50.5/41.9(620), 51.9/43.0(640), 52.3/44.3(660), 53.3/45.0(680), 54.4/46.2(700). No cleavage or parting was observed. Moh's hardness is 5-6; the mineral is very brittle, and its calculated density 7.10 g/cm (super 3) . Its chemical composition (determined by microprobe methods, average of nine analyses) is: Fe 57.7, Ni 20.7, Co 1.4, P 20.4, Total 100.2 wt%, corresponding to (Fe (sub 1.51) Ni (sub 0.50) Co (sub 0.03) ) (sub 2.04) P (sub 0.96) (three atoms per formula unit). Crystal structure: R1 = 0.040 for 138 unique observed (F (sub 0) > or =4 (sub F) ) reflections. Orthorhombic, Pnma, unit-cell parameters refined from powder data: a = 5.748(2), b = 3.548(1), c = 6.661(2) Aa, V = 135.8(1), Aa (super 3) , Z = 4; unit-cell parameters refined from single-crystal data: a = 5.792(7), b = 3.564(4), c = 6.691 (8) Aa, and V = 138.1(3) Aa (super 3) . Strongest reflections in the X-ray powder diffraction pattern are [d in Aa, (1) (hkl)]: 2.238 (100)(112), 2.120(80)(211), 2.073(70)(103), 1.884(50)(013), 1.843(40)(301), 1.788(40)(113), 1.774(40)(020). The mineral is named for Alla Bogdanova, Geological Institute, Kola Science Centre of Russian Academy of Sciences.