Fig. 6: SEM, TEM and XANES data on C-bearing materials.
From: Mineralogical evidence for hydrothermal alteration of Bennu samples
a, High-resolution Ca and Mg X-ray map of OREX-800045-103 with larger dolomite (D) and Fe–Mn-rich magnesite (M) crystals in fine-grained sheet-silicate matrix. Some grains display a core-rim texture, with either dolomite or Fe–Mn-rich magnesite forming the core and the other mineral overgrowing (red arrows). b, BSE image of a large carbonate from OREX-800045-103. c, Panchromatic CL image of the same grain revealing a core-shell texture, with Fe–Mn-rich magnesite (M) non-luminescent crystals forming the core and epitaxial overgrowths of dolomites. d, High-resolution X-ray chemical map of a micrometre-sized open pore in the matrix of OREX-800045-106 with several euhedral calcite grains, magnetite framboids of various size and rare Fe–Mn-rich magnesite. A thin serpentine layer lines the pore. e, Panchromatic CL image of the same area showing the net luminescence of the calcite grains. f–h, BF-STEM and EDS maps of OREX-501005-101 revealing a zoned euhedral magnesite aggregate containing a siderite core and chemical zoning. i,j, BF-TEM image and EDS map of a C nanoglobule in OREX-501005-101 surrounded by and including sheet silicates. k, C-K XANES map of the C nanoglobule and surrounding sheet silicates in i,j. l, XANES spectra of C-K edge showing that C is spatially correlated with fine-grained sheet silicates and nanoglobules. Numbered spectra correspond to areas indicated in k. Varied functional groups are identified in the C-K edge. The peak at 284.8 eV is characteristic of aromatic C=C bonds. The peaks at 286.5, 287.4 and 288.3 eV are due to various functional groups, probably bonded to O and H, for example, enol, phenol, vinyl ketone, alkyl and carboxyl.
