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

摘要

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 mu m 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-mu m 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 similar to 5 mu m in size of moissanite (SiC); (iii) Ni-phosphide compound grains up to 60 mu m 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 th