Ferrihydrite in Unmelted and Melted Micrometeorites

About 20% of the fine-grained unmelted micrometeorites collected at Cap-Prudhomme - their constituent sulfides have been leached out - still show a bulk residual amount of sulfur of about 0.2%. It was shown that this residual sulfur is hosted at high concentration levels of ~10% in a minor constituent phase of micrometeorites made of a variety of "dirty" hydrous iron oxide (ferrihydrite), which also contains comparable amounts of carbon and phosphorus (Maurette et al., 2001; Maurette, 1998; Maurette et al., 2003a). We even found a micrometeorite only made of this dirty variety of ferrihydrite.

Unexpectedly, ferrihydrite was also observed on the external surface of about 10% of the completely melted micrometeorites, e.g., cosmic spherules.

There, it protrudes as either a tiny "nugget" or a disgorged spread of material (Fig. 3.7). It is very likely that they result from a very fast process of density segregation in the tiny droplet of silicate melt resulting from the "pulse" heating of micrometeorites upon atmospheric entry. These microscopic concentrations of ferrihydrite were centrifuged to the external surface of the spherules during their fast rotation. They were probably ejected most of the time from the "melt," thus feeding micrometeoritic "smoke" with the ferrihydrite ashes of the "dead" micrometeorites.

Gero Kurat and Mike Zolensky cautioned that ferrihydrite (that they call "rust") could have been formed by cryogenic weathering of a primary nugget of FeNi metal. However, the external magnetite rim of the nugget reported in Fig. 3.7 (top) has not been destroyed, whereas the volume change expected during the transformation of a FeNi nugget into a hydrous iron oxide should have flaked off the rim. When large flower-efflorescences of ferrihydrite are observed on top of a primary ferrihydrite nugget coated with a magnetite rim, these nuggets were probably behaving as seed nuclei for the growth of much larger volumes of ferrihydrite during cryogenic weathering!

Several evidences indicate that ferrihydrite could have formed microscopic centers of prebiotic chemistry in waters, while being either a component of unmelted micrometeorites or after ejection from their parent-melted micrometeorites. Graciela Matrajt synthesized two varieties of ferrihydrite to investigate the characteristics of this mineral as an adsorbent of amino acids and catalyst of their polymerization in small bits of proteins (oligopeptides). This ferrihydrite, composed of "ordinary" ~10-|m-sized crystals, already behaved as one of the best adsorbent of amino acids and catalyst of their polymerization into oligopeptides (Maurette, 1998; Matrajt et al., 2001; Matrajt et al., 2002; Matrajt et al., 2003; Maurete et al., 2003a). As this mineral is also used as an efficient drug to remove excess phosphates from the blood, it could have extracted at least phosphates from early seas, and probably other dissolved salts such as sulfates.

It can be predicted that the micrometeoritic variety of ferrihydrite was probably much more reactive than the single crystals of the synthetic variety (see also Sect. 10). It concentrates in a small volume all major biogenic elements (C, H, O, N, P, S), as well as Fe, an important oligoelement. When ferrihydrite is imbedded into the fine-grained matrix of an unmelted micrometeorite, such bioelements are in contact on a nanoscale with both: (i) minerals that have well-established catalytic properties (e.g., saponites and iron sulfides); (ii) organics, which can be detected above detection threshold in a 100-|m-size grain, such as the most abundant amino acid (AIB) found in HCCs (Brinton et al., 1998); (iii) a rich mixture of PAHs-moieties (including reactive vinyl-PAHs) desorbed from the major carbonaceous component of micrometeorites, kerogen (Clemett et al., 1998); (iv) molecules bearing the carbonyl group; and (v) a mixture of complex organics still to be identified, which yield a rather similar and typical smooth raise in fluorescence observed

Fig. 3.7. SEM micrographs of polished sections of two cosmic spherules showing either a nugget (top) or a "disgorged" spread (bottom) of ferrihydrite on their external surfaces (Courtesy C. Engrand).

on Raman spectra of both micrometeorites and all HCCs, between -1000 and -1300 cm (Matrajt, 2002).

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