Hibonite-(Fe), (Fe,Mg)Al12O19, a new alteration mineral from the Allende meteorite

Hibonite-(Fe), (Fe,Mg)Al12O19, is the Fe2+-dominant analog of hibonite CaAl12O19, discovered in a highly altered Ca-,Al-rich refractory inclusion from the Allende meteorite. It occurs as scattered micrometer-sized single crystals within an aggregate of hercynite (Fe,Mg)Al2O4, adjacent to nepheline, ilmenite, ferroan spinel, perovskite, and hibonite. The mean chemical composition determined by electron microprobe analysis of hibonite-(Fe) is Al2O3 90.05 wt%, FeO 3.60, SiO2 2.09, MgO 1.61, Na2O 0.55, CaO 0.28, TiO2 0.04, V2O3 0.02, sum 98.25, corresponding to an empirical formula of (Fe2+ 0.34Mg0.27Na0.12Al0.11Ca0.03)Σ0.87 (Al11.77Si0.23)Σ12.00O19. Hibonite-(Fe) is hexagonal, P63/mmc; a = 5.613 Å, c = 22.285 Å, V = 608.0 Å3 and Z = 2. Its electron backscatter diffraction pattern is a good match to that of the hibonite structure. Hibonite-(Fe) is apparently a secondary alteration product formed by iron-alkali-halogen metasomatism, whereas hibonite is a primary refractory phase.