Hibonite-(Fe),(Fe,Mg)Al_(12)O_19, a new alteration mineral from the Allende meteorite

Abstract

Hibonite-(Fe),(Fe,Mg)Al_(12)O_19, is the Fe^(2+)-dominant analog of hibonite CaAl_(12)O_19, 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)Al_(2)O_4, adjacent to nepheline, ilmenite, ferroan spinel, perovskite, and hibonite. The mean chemical composition determined by electron microprobe analysis of hibonite-(Fe) is Al_(2)O_3 90.05 wt%, FeO 3.60, SiO_2 2.09, MgO 1.61, Na_(2)O 0.55, CaO 0.28, TiO_2 0.04, V_(2)O_3 0.02, sum 98.25, corresponding to an empirical formula of (Fe^(2+)_(0.34)Mg_(0.27)Na_(0.12)Al_(0.11)Ca_(0.03))_(Σ0.87) (Al_(11.77)Si_(0.23))_(Σ12.00)O_(19). Hibonite-(Fe) is hexagonal, P6_(3)/mmc; a = 5.613 Å, c = 22.285 Å, V = 608.0 Å^3 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.

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