Brearleyite, Ca12Al14O32Cl2, a new mineral from the NWA 1934 meteorite: SAED ring pattern and Rietveld refinement


Chi Ma
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125

Harold C. Connolly, Jr.
Department of Physical Sciences, Kingsborough Community College of CUNY, Brooklyn, NY 11235 and Earth and Environmental Sciences, The Graduate Center of CUNY, New York, NY 10024, USA;
Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024, USA;
Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA

John R. Beckett, George R. Rossman
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125

Anthony R. Kampf
Mineral Sciences Department, Natural History Museum of Los Angeles County, Los Angeles, CA 90007

Thomas J. Zega
Materials Science and Technology Division, Naval Research Laboratory, Washington, DC 20375, USA

Oliver Tschauner
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125
Departemnt of Physics, University of Nevada, Las Vegas, NV 89154, USA

Stuart A. Sweeney Smith
Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024, USA

 Devin L. Schrader
Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA
Department of Geology, Carleton College, Northfield, MN 55057, USA

Abstract

Brearleyite (IMA 2010-062, Ca12Al14O32Cl2) is Cl-bearing manyenite, occurring as fine-grained aggregates along with hercynite, gehlenite and perovskite in an uncommon krotite (CaAl2O4) -dominant refractory inclusion from the Northweat Africa 1934 CV3 carbonaceous chondrite. It is the first occurrence of Ca12Al14O32Cl2 in a meteorite, identified by FE-SEM, TEM-SAED, micro-Raman and EMPA. It is potentially an extraterrestrial alteration mineral. The mean chemical composition determined by electron microprobe analysis of this 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. Its empirical formula calculated on the basis of 34 O+Cl atoms is (Ca11.91Na0.06)∑11.97(Al13.89Fe0.16Ti0.01)14.06O31.89Cl2.11. Rietveld refinement of an integrated TEM-SAED ring pattern from a FIB section revealed that brearleyite is cubic, I4bar3d; a = 11.9794(5) Å, V = 1719.12(22) Å3 and Z = 2. It has a framework structure in which AlO4 (aluminate) 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 0.374 occupancy and Ca is disordered on two partially occupied sites. It has a dark green color and a calculated density of 2.797 g/cm3. Brearleyite is not only a new meteoritic Ca-, Al-phase, but also a new Cl-rich phase in a primitive meteorite. It likely formed by reaction of krotite and Cl-bearing hot gas or fluids.


A reflected light photograph of the krotite-dominant CAI containing brearleyite.

A backscattered electron image showing brearleyite with hercynite, perovskite and gehlenite in the krotite host in two areas near the CAI center.


last updated 24-Nov-2010