Magnetite Crystals and Chains. The typical setting of magnetite crystals in the rim region of a carbonate globule is shown in Fig. 4 A and B. This specimen was fractured and carbon coated without resin embedding, and imaging was through the intact surface. The wide iron-rich rim stretches approximately vertically, evident as a zone of small granules overlying the substrate, orthopyroxene between green and blue lines and carbonate between blue and purple (Fig. 4B). Elemental analysis (below) indicates that the granules are rich in Fe and O, with only traces of S. Previous studies (1, 2) showed that these Fe- and O-rich rims are composed mainly of magnetite crystals, with a few grains possibly of iron sulfides, embedded in carbonate matrix. The stereo pair in Fig. 1 A and B shows the surface of the area in SEM-SE, whereas Fig. 1 D and E, in SEM-BSE, shows the underlying structures up to ca. 1 µm deep. Comparison of these images reveals that the entire area is covered by a low-electron-density (i.e., low-atomic-number) substance (LEDS) of unknown (perhaps organic?) chemical composition, with a granular surface visible in SEM-SE but transparent in the BSE mode. Fig. 1C is an SEM-BSE image of a magnetosome chain in a freeze-dried preparation of Magnetospirillum magnetotacticum, shown for comparison. The areas indicated in Fig. 1F are shown in BSE stereo pairs in Fig. 1 G and H and Fig. 1 J and K. In these stereo images the magnetite crystals embedded in transparent LEDS appear as though floating, whereas others are embedded in the large carbonate crystals. In the large crystal conglomeration in the upper left corner of Fig. 4A, the individual crystals are not resolved, because of superposition of the BSE signals. In other, less compacted clusters, individual crystals are distinguishable, and among these, many appear to form chains. In Fig. 4A a conspicuous chain of 7 or 8 elongated crystals arranged along their long axes is indicated by an arrow. In the stereo image pairs in Fig. 1 G and H and Fig. 1 J and K, several chains of various lengths, clarity, and state of imaging can be distinguished, as indicated in the auxiliary Fig. 1 I and L. The SEM-BSE micrographs in Fig. 3 A-F and Fig. 4C are of resin-embedded specimens. In the sectioned specimen shown in Fig. 3A the small carbonate structures (probably lobes of a carbonate globule) are embedded in plagioclase glass and orthopyroxene. The arrow indicates the site of the chain shown in Fig. 3E. Fig. 3B is a chain of six crystals clearly separated by gaps. Fig. 3 C and D show crystal chains with strong bends; in Fig. 3D the apparent S-shaped structure (arrows) may be composed of two chains. The bent chain of about 15 large isodiametric crystals in Fig. 3E and the chain of 6 small elongated crystals arranged in the direction of their long axes in Fig. 3F illustrate the wide range of crystal forms and sizes in ALH84001 (note different scales). Significantly, the size and shape of crystals within each chain are uniform. Many or perhaps most magnetite crystals, both single and in chains, seem to be surrounded, at least in part, by LEDS, which appears in SEM-BSE as a dark area. Because such areas are evident both in unprocessed (i.e., freshly fractured, Fig. 4A) and in resin-embedded (Figs. 3 B-F and 4C) specimens, they are not voids but filled with LEDS. A chain in Fig. 1 G and H (marked by an arrow in Fig. 1I) is surrounded by a dark halo, evident against the white background of carbonate in which the chain is embedded. Similar halos, if present around other chains, may not be visible against the dark LEDS background. Fig. 4C is a resin-embedded specimen with numerous chains and chain fragments. One chain is marked by an arrow, others are inside squares. The chain in the rightmost square is shown in Fig. 3B.
The number of magnetite crystals in the chains varied greatly. The frequency distribution of 154 magnetite chains consisting of at least four crystals (Table 1
) shows a non-Gaussian pattern, which suggests that the chains are not representative of a natural population but probably are disrupted fragments of originally longer chains.
Composition of Electron-Dense Particles. The nanometer-size electron-dense particles studied here are visible only in SEM-BSE, and no analytical method with sufficient resolution exists for this type of microscope to determine in situ the exact chemical composition of single crystals, except that they contain a heavy element. We suggest, however, that the following evidence very strongly, if not conclusively, indicates that the electron-dense particles are magnetite crystals: (i) The Fe-rich rim of carbonate globules, where the particles occur, contains large numbers of magnetite crystals of similar size range (2). (ii) After dissolution of the carbonate in 20% acetic acid, magnetite crystals are the only particulate structures left in the residue as identifiable in the transmission electron microscope (2) [with the exception of the rather rare iron sulfides (greigite and perhaps pyrrhotite), which have similar magnetic properties and are also known to be present in bacterial magnetosomes]. We repeated this test for confirmation of the results in ref. 2. (iii) The chains, as shown here, show a series of morphological features characteristic of magnetosome chains of bacteria that could not be explained by nonbiological processes. (iv) We used the two microanalytical techniques that come closest in resolution to our goal. Energy-dispersive x-ray spectrometry analyzes three-dimensional volumes, which we were able to reduce to 200 nm in diameter by working with a very low (5-kV) accelerating potential. Analyses at 24 points in the area of Fig. 4A (shown in Fig. 4B) showed the presence of excess Fe in some areas, compared with average ALH84001 orthopyroxene and rim carbonates, both of which have a narrow compositional range (2, 17). The iron level is highest in the area of dense conglomeration of particles (see details and spectra in Appendix 2, which is published as supplemental data on the PNAS web site, www.pnas.org). Auger electron spectroscopy, a method for surface compositional analysis, was used for the resin-embedded and polished specimen in Fig. 4C. Although the diameter of the incident beam was was estimated as Fe/Mg and O/Mg ratios were measured at five points in the carbonate substrate without particles present (triangles) and at five points with short chains (squares). At these points, the Fe and O contents were elevated (Fig. 4D). On the basis of these considerations and analyses, we conclude that the electron-dense particles are identical with the magnetite (Fe3O4) crystals described by Thomas-Keprta et al. (2).