Afmite, Al3(OH)4(H2O)3(PO4)(PO3OH)·H2O, a new mineral from Fumade,
Tarn, France: description and crystal structure

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

Kampf AR
Mineral Sciences Department
Natural History Museum of Los Angeles County, Los Angeles, CA 90007, USA

Connolly HC Jr
Department of Physical Sciences
Kingsborough Community College of CUNY, Brooklyn, NY 11235

Beckett JR, Rossman GR
Division of Geological and Planetary Sciences
California Institute of Technology, Pasadena, CA 91125

Sweeney Smith SA
Department of Geology, Carleton College, Northfield, MN 55057, USA

Schrader DL
Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA


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 EMPA. 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) Å, b = 8.0994(3) Å, c = 15.2170(11) 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 approximately 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)°, 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 oldest minerals formed in the Solar System.

Krotite image
Krotite comprizes the off-white portion of egg-shaped inclusion photographed in reflected light

last updated 11-Sep-2011