Discovery of a New Cl-Rich Silicate Mineral, Ca_(12)(Al_2Mg_3Si_7)O_(32)Cl_6: An Alteration Phase in Allende

Abstract

During a nanomineralogy investigation of the Allende CV3 carbonaceous chondrite, a new silicate mineral, Ca_(12)(Al_4Mg_3Si_7)O_(32)Cl_6 with the I 4 3d wadalite structure, was identified in the Type B1 Ca-Al-rich inclusion (CAI) Egg-3. Field-emission SEM with EDS and electron back-scatter diffraction and electron microprobe were used to characterize its composition, structure, and associated phases. Synthetic Ca_(12)(Al_4Mg_3Si_7)O_(32)Cl_6 are not reported. The Allende Ca_(12)(Al_4Mg_3Si_7)O_(32)Cl_6 is the first natural occurrence of this phase. Here we discuss its origin and significance for understanding alteration processes on the CV chondrite parent asteroid. The mineral is currently under review by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA 2014-028). It is a new member of the wadalite group.

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