Petrography of the carbonaceous, diamond-bearing stone “Hypatia” from southwest Egypt: A contribution to the debate on its origin

1Georgy A. Belyanin, 1Jan D. Kramers, 2Marco A.G. Andreoli, 1,3Francesco Greco, 1,4,5Arnold Gucsik, 1Tebogo V. Makhubela, 6,7Wojciech J. Przybylowicz, 8Michael Wiedenbeck
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.12.020]
1Department of Geology, University of Johannesburg, Auckland Park 2006, South Africa
2School of Geosciences, University of the Witwatersrand, PO Box 3, Wits 2050, South Africa
3Dipartimento di Scienze Biologiche, Geologiche ed Ambientali, Università di Bologna, Via Zamboni 67, 40126 Bologna, Italy
4Department of Nonlinear and Laser Optics, Wigner Research Institute for Physics, Hungarian Academy of Sciences, Konkoly-Thege Miklós út 29-33, Budapest, H-1121, Hungary
5Department of Mineralogy and Geology, Cosmochemistry Research Group, University of Debrecen, Egyetem tér 1., H-4032, Hungary
6iThemba Labs, National Research Foundation, P.O. Box 722, Somerset West 7129, South Africa
7AGH University of Science and Technology, Faculty of Physics & Applied Computer Science, 30-059 Kraków, Poland
8Deutsches GeoForschungsZentrum GFZ, D14473 Potsdam, Germany
Copyright Elsevier

The stone named “Hypatia” found in the Libyan Desert Glass area of southwest Egypt is carbon-dominated and rich in microdiamonds. Previous noble gas and nitrogen isotope studies suggest an extraterrestrial origin. We report on a reconnaissance study of the carbonaceous matrix of this stone and the phases enclosed in it. This focused on areas not affected by numerous transecting fractures mostly filled with secondary minerals. The work employed scanning electron microscopy (SEM) with energy-dispersive (EDS) and wavelength-dispersive (WDS) electron microprobe (EMPA) analysis, Proton Induced X-ray Emission (PIXE) spectrometry and micro-Raman spectroscopy. We found that carbonaceous matrices of two types occur irregularly intermingled on the 50-500 μm scale: Matrix-1, consisting of almost pure carbonaceous matter, and Matrix-2, containing Fe, Ni, P and S at abundances analyzable by microprobe. Matrix-2 contains the following phases as inclusions: (i) (Fe,Ni) sulphide occurring in cloud-like concentrations of sub-μm grains, in domains of the matrix that are enriched in Fe and S. These domains have (Fe+Ni)/S (atomic) = 1.51 ± 0.24 and Ni/Fe = 0.086 ± 0.061 (both 1SD); (ii) grains up to ∼5 μm in size of moissanite (SiC); (iii) Ni-phosphide compound grains up to 60 μm across that appear cryptocrystalline or amorphous and have (Ni+Fe)/P (atomic= 5.6. ± 1.7 and Ni/Fe = 74 ± 29 (both 1SD), where both these ratios are much higher than any known Ni-phosphide minerals; (iv) rare grains (observed only once) of graphite, metallic Al, Fe and Ag, and a phase consisting of Ag, P and I. In Matrix-2, Raman spectroscopy shows a prominent narrow diamond band at 1340 cm-1. In Matrix-1 the D and G bands of disordered carbon are dominant, but a minor diamond band is ubiquitous, accounting for the uniform hardness of the material. The D and G bands have average full width at half maximum (FWHM) values of 295 ± 19 and 115 ± 19 cm-1, respectively, and the D/G intensity ratio is 0.75 ± 0.09 (both 1SD). These values are similar to those of the most primitive solar system carbonaceous matter. The diamond phase is considered to be a product of shock. The (Fe,Ni) sulphide phase is probably pyrrhotite and a shock origin is likewise proposed for it. Moissanite is frequently associated with the Ni-phosphide phase, and a presolar origin for both is suggested. The lack of recrystallization of the Ni-phosphide phase suggests that the Hypatia stone did not experience long-lasting thermal metamorphism, in accord with the Raman D-G band characteristics.

A lack of silicate matter sets the stone apart from interplanetary dust particles and known cometary material. This, along with the dual intermingled matrices internal to it, could indicate a high degree of heterogeneity in the early solar nebula.

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