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Published August 2011 | Supplemental Material
Journal Article Open

Brearleyite, Ca_(12)Al_(14)O_(32)Cl_2, a new alteration mineral from the NWA 1934 meteorite

Abstract

Brearleyite (IMA 2010-062, Ca_(12)Al_(14)O_(32)Cl_2) is a Cl-bearing mayenite, occurring as fine-grained aggregates coexisting with hercynite, gehlenite, and perovskite in a rare krotite (CaAl_2O_4) 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%) Al_2O_3 48.48, CaO 45.73, Cl 5.12, FeO 0.80, Na_2O 0.12, TiO_2 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))Σ_(11.97)Al_(13.89)Fe_(0.16)Ti_(0.01))Σ_(14.06)O_(31.89)Cl_(2.11). The Raman spectrum of brealryeite indicates very close structural similarity to synthetic Ca_(12)Al_(14)O_(32)Cl_2. Rietveld refinement of an integrated TEM-SAED ring pattern from a FIB section quantifies this structural relationship and indicates that brearleyite is cubic, ⌈43d; a = 11.98(8) Å, V = 1719.1(2) Å3, 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.

Additional Information

Supplementary data available a CIF text file. SEM, EBSD, and EPMA analyses were carried out at the Caltech GPS Division Analytical Facility, which is supported, in part, by grant NSF EAR-0318518 and the MRSEC Program of the NSF under DMR-0080065. FIB and TEM work was performed at the Naval Research Laboratory. This research was also supported by NSF REU grant AST 0851362 and NASA OSS Grant NNX09AB86G (to H.C.C. Jr.), NNX09AG40G (to E.M. Stolper), and NNH09AK541 (to T.J.Z.), and NSF grant EAR-0337816 (to G.R.R.), and NNSA Cooperative Agreement DE-FC88- 01NV14049 (to O.T.). We thank F. Colombo, C.M. Gramaccioli, and J.C. Melgarejo for constructive reviews of this manuscript.

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