Krotite, CaAl2O4, a new refractory mineral from the NWA 1934 meteorite

Chi Ma, Anthony R. Kampf, Harold C. Connolly, John R. Beckett, George R. Rossman, Stuart A.Sweeney Smith, Devin L. Schrader

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Krotite, CaAl2O4, occurs as the dominant phase in an unusual Ca-,Al-rich refractory inclusion from the NWA 1934 CV3 carbonaceous chondrite. Krotite occupies the central and mantle portions of the inclusion along with minor perovskite, gehlenite, hercynite, and Cl-bearing mayenite, and trace hexamolybdenum. A layered rim surrounds the krotite-bearing regions, consisting from inside to outside of grossite, mixed hibonite, and spinel, then gehlenite with an outermost layer composed of Al-rich diopside. Krotite was identified by XRD, SEM-EBSD, micro-Raman, and electron microprobe. The mean chemical composition determined by electron microprobe analysis of krotite is (wt%) Al2O3 63.50, CaO 35.73, sum 99.23, with an empirical formula calculated on the basis of 4 O atoms of Ca1.02Al 1.99O4. Single-crystal XRD reveals that krotite is monoclinic, P21/n; a = 8.6996(3), b = 8.0994(3), c = 15.217(1) Å, β = 90.188(6), and Z = 12. It has a stuffed tridymite structure, which was refined from single-crystal data to R1 = 0.0161 for 1014 Fo > 4αF reflections. Krotite is colorless and transparent with a vitreous luster and white streak. Mohs hardness is ̃61/2. The mineral is brittle, with a conchoidal fracture. The calculated density is 2.94 g/cm3. Krotite is biaxial (-), α = 1.608(2), β = 1.629(2), γ = 1.635(2) (white light), 2Vmeas = 54.4(5)°, and 2Vcalc = 55.6°. No dispersion was observed. The optical orientation is X = b; Y a; Z c. Pleochroism is colorless to very pale gray, X < Y = Z. Krotite is a low-pressure CaAl2O4 mineral, likely formed by condensation or crystallization from a melt in the solar nebula. This is the first reported occurrence of krotite in nature and it is one of the earliest minerals formed in the solar system.

Original languageEnglish (US)
Pages (from-to)709-715
Number of pages7
JournalAmerican Mineralogist
Issue number5-6
StatePublished - 2011
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Geophysics
  • Geochemistry and Petrology


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