UM-StL Scanned Tip & Electron Image Lab's
Jan'96 Image of the Month

Since the early 1960's, noble gas (i.e. xenon, argon, neon) anomalies in primitive carbonaceous chondrite meteorites have pointed toward the existence of materials, unhomogenized with the rest of the meteorite, containing atoms nucleosynthesized in the interior of different types of stars. In the past decade, such noble gas isotopic anomalies have been tracked to nanometer-sized diamonds, and to micron-sized silicon carbide crystals and graphite particles, in these meteorites. The graphite particles so identified appear with two morphologies, reminescent respectively of the vegetables cauliflower and onions. We focus here on the onions.

The onions are so named because they are spherical, and their outer layers show up in high resolution electron microscope images as concentric, atom-thick shells of <001> graphite. Some of them contain carbide grains in their interiors, which help constrain the conditions under which they formed. In addition, these layers are typically built around distinct spherical cores of a material which is not 3D graphite at all, but which in diffraction shows fine graphite rings, with sharp onsets and "tails" out to higher spatial frequencies consistent with 2D graphite plates only 1 atom thick and mean coherence widths of 4nm. Plasmon peak energies in electron energy loss spectra of this material are consistent with this interpretation of the diffraction data as well. The isotopic signatures, as well as structure and chemistry of the particles, point strongly to orgins in the outer atmosphere of carbon-rich red-giant stars (cf. Bernatowicz, Gibbons, Amari & Lewis, "On the nature of carbon cores in interstellar graphite", Lunar Planet. Sci. Conf. XXVI, 1995).

One of the projects that we've undertaken with these particles is to look for evidence concerning the structure of the platelets in these "2D graphite" cores with electron phase contrast imaging. Questions like: Are these atom-thick flakes flat, or wrinkled? Are they euhedral (i.e. faceted) along <100>, circular in shape, or irregular? A cursory look (cf. our Jun'95 image of the month) at HREM contrast from atom-thick ("2D") crystals suggests that we might see informative contrast from edge-on crystals, as well as (in the rarer event that the specimen were thin enough) contrast from individual crystals face on.

In our images for this month, we post an inset from a HREM image of a sectioned graphite onion, showing both 3D graphite <001> spots and a fine (100) graphite ring in the image power spectrum. This image was obtained with our Philips EM430ST. The two types of periodicity are distributed differently in the image, suggesting that we may be seeing rare signs of alternating growth between the 2D and 3D phases. Since the image includes fine (hk0) rings like those associated with the 2D cores, this image may also contain clues to the questions above. You are invited to examine the image inset provided here, for clues of this sort yourself.

In the meantime, work continues toward the acquisition of data on this and related materials, using this and other microscopes. Suggestions on ways to proceed are invited.{KB,DD,QW,PF}

Questions, and clues from your own analysis of the image here, as well as ideas on how we might learn more about these novel and ancient materials, are invited. Phil Fraundorf