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American Mineralogist

Journal of Earth and Planetary Materials

Ed. by Baker, Don / Xu, Hongwu / Swainson, Ian


IMPACT FACTOR 2018: 2.631

CiteScore 2018: 2.55

SCImago Journal Rank (SJR) 2018: 1.355
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1945-3027
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Volume 96, Issue 5-6

Issues

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

Chi Ma
  • Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A. chi@gps.caltech.edu
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/ Anthony R. Kampf
  • Mineral Sciences Department, Natural History Museum of Los Angeles County, Los Angeles, California 90007, U.S.A.
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/ Harold C. Connolly Jr.
  • Department of Physical Sciences, Kingsborough Community College of CUNY, Brooklyn, New York 11235 and Earth and Environmental Sciences, The Graduate Center of CUNY, New York, New York 10024, U.S.A.
  • Department of Earth and Planetary Sciences, American Museum of Natural History, New York, New York 10024, U.S.A.
  • Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721, U.S.A.
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/ John R. Beckett
  • Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A.
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/ George R. Rossman
  • Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A.
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/ Stuart A. Sweeney Smith
  • Department of Earth and Planetary Sciences, American Museum of Natural History, New York, New York 10024, U.S.A.
  • Department of Geology, Carleton College, Northfield, Minnesota 55057, U.S.A.
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/ Devin L. Schrader
Published Online: 2015-04-02 | DOI: https://doi.org/10.2138/am.2011.3693

Abstract

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 Ca1.02Al1.99O4. 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 > 4σF reflections. Krotite is colorless and transparent with a vitreous luster and white streak. Mohs hardness is ~6½. 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.

Keywords: Krotite; CaAl2O4; new mineral; refractory inclusion; NWA 1934 meteorite; CV3 carbonaceous chondrite; XRD; EBSD

About the article

Received: 2010-09-14

Accepted: 2011-01-29

Published Online: 2015-04-02

Published in Print: 2011-05-01


Citation Information: American Mineralogist, Volume 96, Issue 5-6, Pages 709–715, ISSN (Online) 1945-3027, ISSN (Print) 0003-004X, DOI: https://doi.org/10.2138/am.2011.3693.

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© 2015 by Walter de Gruyter Berlin/Boston.Get Permission

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