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

Abstract

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⁺_0.34Mg_0.27Na_0.12Al_0.11Ca_0.03)_Σ0.87 (Al_11.77Si_0.23)_Σ12.00O₁₉. Hibonite-(Fe) is hexagonal, P6₃/mmc; a = 5.613 Å, c = 22.285 Å, V = 608.0 ų 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.