Brearleyite, Ca₁₂Al₁₄O₃₂Cl₂, a new alteration mineral from the NWA 1934 meteorite

Abstract

Brearleyite (IMA 2010-062, Ca₁₂Al₁₄O₃₂Cl₂) is a Cl-bearing mayenite, occurring as fine-grained aggregates coexisting with hercynite, gehlenite, and perovskite in a rare krotite (CaAl₂O₄) dominant refractory inclusion from the Northwest Africa 1934 CV3 carbonaceous chondrite. The phase was characterized by SEM, TEM-SAED, micro-Raman, and EPMA. The mean chemical composition of the brearleyite is (wt%) Al₂O₃ 48.48, CaO 45.73, Cl 5.12, FeO 0.80, Na₂O 0.12, TiO₂ 0.03, -O 1.16, sum 99.12. The corresponding empirical formula calculated on the basis of 34 O+Cl atoms is (Ca_11.91 Na_0.06)_Σ11.97(Al_13.89Fe_0.16Ti_0.01)_Σ14.06O_31.89Cl_2.11. The Raman spectrum of brealryeite indicates very close structural similarity to synthetic Ca₁₂Al₁₄O₃₂Cl₂. Rietveld refinement of an integrated TEM-SAED ring pattern from a FIB section quantifies this structural relationship and indicates that brearleyite is cubic, I4̅3d; a = 11.98(8) Å, V = 1719.1(2) ų, and Z = 2. It has a framework structure in which AlO₄ tetrahedra share corners to form eight-membered rings. Within this framework, the Cl atom is located at a special position (3/8,0,1/4) with 0.4(2) occupancy and Ca appears to be disordered on two partially occupied sites similar to synthetic Cl-mayenite. Brearleyite has a light olive color under diffuse reflected light and a calculated density of 2.797 g/cm³. Brearleyite is not only a new meteoritic Ca-,Al-phase, but also a new meteoritic Cl-rich phase. It likely formed by the reaction of krotite with Cl-bearing hot gases or fluids.