Hutcheonite, Ca_3Ti_2(SiAl_2)O_(12), a new garnet mineral from the Allende meteorite: An alteration phase in a Ca-Al-rich inclusion

Abstract

Hutcheonite (IMA 2013-029), Ca_3Ti_2(SiAl_2)O_(12), is a new garnet mineral that occurs with monticellite, grossular, and wadalite in secondary alteration areas along some cracks between primary melilite, spinel, and Ti,Al-diopside in a Type B1 Fractionation and Unidentified Nuclear effects (FUN) Ca-Al-rich inclusion (CAI) Egg-3 from the Allende CV (Vigarano type) carbonaceous chondrite. The mean chemical composition of type hutcheonite by electron probe microanalysis is (wt%) CaO 34.6, TiO_2 25.3, SiO_2 20.9, Al_2O_3 15.7, MgO 2.1, FeO 0.7, V_2O_3 0.5, total 99.8, giving rise to an empirical formula of Ca_(2.99)(Ti^(4+)_(1.53)Mg_(0.25)Al_(0.17)Fe^(2+)_(0.05)V^(3+)_(0.03))(Si_(1.68)Al_(1.32))O_(12). The end-member formula is Ca_3Ti_2(SiAl_2)O_(12). Hutcheonite has the Ia3̄d garnet structure with a = 11.843 Å, V = 1661.06 Å^3, and Z = 8, as revealed by electron backscatter diffraction. The calculated density using the measured composition is 3.86 g/cm^3. Hutcheonite is a new secondary phase in Allende, apparently formed by iron-alkali-halogen metasomatic alteration of the primary CAI phases like melilite, perovskite, and Ti,Al-diopside on the CV chondrite parent asteroid. Formation of the secondary Ti-rich minerals like hutcheonite during the metasomatic alteration of the Allende CAIs suggests some mobility of Ti during the alteration. The mineral name is in honor of Ian D. Hutcheon, a cosmochemist at Lawrence Livermore National Laboratory, California, U.S.A.

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