This extremely rare carbonaceous chondrite fell in France a few minutes after 8:00 P.M.. A luminous meteor and sonic booms were followed by the fall of twenty stones; the largest stone was the size of a man's head, but most were only fist-sized. The fall covered an area of over two miles², and the total recovered weight of this low-density meteorite is ~13 kg. This is the most chemically primitive of the meteorite classes, with near-solar ratios of elemental abundances. However, volatile abundances may represent an enrichment that occurred during aqueous alteration processes, rather than a preservation of primitive solar abundances.
Orgueil is a micro-regolith breccia consisting primarily of phyllosilicate aggregates (serpentinite and smectite/saponite) measuring tens of nm in size to hundreds of µm in size, and showing a wide diversity of chemical compositions. An alteration sequence by which these aggregates may have formed was suggested by Morlok et al (2006). They believe the initial lithology was a magnesian, coarse-grained aggregate (CGA) of phyllosilicates, sulfides, magnetites, and carbonates. These components were then dissolved in aqueous fluids to produce a fine-grained aggregate (FGAa) matrix, which is enriched in Fe-bearing ferrihydrite comprising Fe dissolved from sulfides and magnetites. Finally, continued alteration of this fine-grained material eliminated most remnant mineral inclusions which resulted in increased abundances of Fe and other elements (FGAb). Other lithologies which are present include a CGA–FGA transitional lithology, a phosphate lithology (P possibly derived from precursor chondrules), and some anomalous lithologies.
Other end products resulted from this aqueous dissolution process. Sulfates formed from S-rich solutions and were readily mobilized to enter into veins. Carbonate grains were first dissolved, and then recomposed to form initially dolomite, and then breunnerite—these have been precisely dated through the radiogenic isotopes of Cr and Mn (Hoppe et al., 2004, 2007; Petitat et al., 2011; Fujiya et al., 2011) to resolve an isochron spanning ~10 m.y., corresponding to a very early solar timeframe with an absolute age of 4,563.5 (±0.7) m.y. ago ending ~4,553 m.y. ago. These two carbonates, plus calcite, have variable isotopic compositions, and they were likely precipitated from an evolving, low-temperature (26°C for dolomite to –6°C for breunnerite; Guo et al., 2007) aqueous fluid that was primarily mobilized by impact heating, but residual heat from the decay of radioactive 26Al and 60Fe was probably also a contributor. Magnetite was also formed through aqueous alteration processes at temperatures of up to 150°C. An I–Xe age was determined to be 4,560.4 m.y. (Hohenberg et al, 2000)
Chondrules are not present in Orgueil, and only a single aqueously altered CAI has been identified in Ivuna (Frank et al., 2011), but presolar grains of graphite, diamond (the highest content known), corundum, silicon-carbide, chromium-oxide, and FeNi-sulfide do occur, as well as a new Mg–Al–Cr mineral. Utilizing Raman spectroscopy and a NanoSIMS microprobe, of which the latter measures the isotopes of interstellar grains at a size range ≤500 atoms, it was discovered that some high-density crystalline graphite grains and some low-density, glassy, kerogen-type carbon grains present in Orgueil contain highly anomalous isotopic compositions in C, Ca, and Ti, suggesting that they are derived from such stellar objects as low-metallicity AGB stars and type II supernovae, respectively (Jadhav et al., 2007, 2010; Wopenka et al., 2011). Other regions have been identified in certain meteorites like Orgueil which contain 54Cr-rich grains, likely carried by nanoscale spinel particles (Qin et al., 2010). This 54Cr isotope anomaly is heterogeneously distributed throughout the protoplanetary disk, and evidence favors an injection of these grains, and probably other grains, through a late Type II supernova. Observed variability of 54Cr among the different meteorite classes suggests that the supernova dust contamination occurred during the active formation of the solar protoplanetary disk.
The low-Ni sulfide pyrrhotite presently makes up ~1 vol% of the Orgueil meteorite, and is similarly found in the other CI1 meteorites. It has been suggested that low-temperature alteration of primary troilite produced pyrrhotite with pentlandite inclusions, along with magnetite. Orgueil and Ivuna contain higher CI1 abundances of both magnetite and carbonates, as well as Ni-enriched pyrrhotite, consistent with a higher degree of alteration than that experienced by other CI1 members. By contrast, the pyrrhotite grains present in the CI1 meteorites Alais and Tonk are larger in size and contain pentlandite inclusions (Bullock et al., 2003). Moreover, Alais and Tonk contain larger phyllosilicates and a lower abundance of magnetite and carbonates than Orgueil and Ivuna.
These consistently distinct petrographical and mineralogical features found among the CI1 members have prompted Bullock et al. (2003, 2005) to propose a subgrouping of the CI1 group based on degree of aqueous alteration as follows: 1) Orgueil and Ivuna experienced an extended period of aqueous alteration in which acidic hydrothermal fluids completely dissolved pentlandite, and 2) Alais and Tonk (and probably Revelstoke) experienced a shorter alteration period resulting in the preservation of some pentlandite. Eventually, dissolved Ni was deposited within the matrix of Orgueil and Ivuna; some Ni was combined with Na to form the sodium nickel sulfate, Ni-bloedite, whereas other Ni formed ferrihydrite.
X-ray diffraction techniques and Mössbauer spectroscopy have been used by Bland et al. (2004) to determine the modal mineralogy of several carbonaceous chondrites including Orgueil. They were also able to quantify the compositional range of the olivine phases. In addition, the grain density can be readily estimated from the mode data, and therefore, in combination with the calculated bulk density, the porosity can be determined. The modal mineralogy (vol%) of Orgueil is as follows:
Olivine
Fo100 -------------- 2.1
Fo80 ---------------- 2.6
Fo60 ---------------- 1.1
Troilite ---------------------- 1.2
Pyrrhotite ------------------ 2.7
Magnetite ------------------- 5.1
Serpentine ----------------- 7.7
Saponite–serpentine -- 73.8
Ferrihydrite --------------- 3.7
TOTAL ------------------ 100.0
grain density = 2.76 g per cubic cm
An inverse correlation was found to exist for the olivine and the phyllosilicate phases (Bland et al., 2004). It is presumed that the saponite–serpentine was formed by the aqueous alteration of primary anhydrous olivine. White veins of Mg- and Ca-sulfate (epsomite and gypsum, respectively) were not present in Orgueil or the other CI1 meteorites when original examinations were performed shortly after they fell. Rather, the veins are the result of the hygroscopic nature of CI1 meteorites. Terrestrial atmospheric water absorbed during their residence on Earth has mobilized existing sulfates to produce the white veins, which, coincidentally, has affected both the measured porosity and total water content of these meteorites. On the other hand, organic carbon–phyllosilicate assemblages present in Orgueil and other CI chondrites have been shown to be primary constituents that are the product of aqueous alteration on the parent body; these assemblages may have served as catalysts for the formation of more complex organic molecules (Garvie and Buseck, 2007).
Current studies suggest that if meteorites can be derived from comets, the source of the comets would be from the Jupiter-family comets that probably originated in the Kuiper belt. Based on current understanding, cometary meteorites are expected to be very rare. In addition to their having a dark color, they should have a low density and strength, a high porosity, a solar ratio of elements, an elevated ratio of C, H, O, and N, a high interstellar grain content, anhydrous and highly unequilibrated silicates, few to no chondrules, and a cosmic-ray exposure history similar to asteroidal meteorites.
The CI meteorites may be the closest match to a meteorite group with the above characteristics, but only if extensive aqueous alteration occurred on the parent body. Consistent with a cometary origin for Orgueil is the finding of only a few amino acids, mainly beta-alanine, glycine, and gamma-amino-n-butyric acid (the smallest gamma-amino acid found in meteorites). These are considered to be products of very low temperature conditions such as might be found on an extinct comet in the asteroid belt (Botta et al., 2007), or which were derived from limited precursor components such as hydrogen cyanide, ammonia, and carbonyl compounds (e.g., formaldehyde, acetaldehyde, and acetone)—constituents identified in several comets. The type specimen of the CI group, Ivuna, was also found to contain similar extraterrestrial amino acids in similar abundances to those of Orgueil.
Analysis of seven fragments recovered from three aerogel tracks obtained through NASA's Stardust mission to comet Wild 2 revealed an average composition similar to the bulk composition of CI chondrites (Stephan et al., 2007). Examples of low-temperature (<~200°C) aqueous alteration phases found in both comet Wild 2 and Orgueil include the nickel-, copper-, and zinc-bearing iron sulfides cubanite, pyrrhotite, pentlandite, and sphalerite (Berger et al., 2011). Olivine in both Orgueil and Ivuna has a similar compositional range to that found in comet Wild 2 particles as well as in anhydrous IDPs, and less similar to other carbonaceous chondrite types (Le Gac et al., 2009). Besides the CI meteorites, there exists some C-rich aggregates in clasts and inclusions within some unequilibrated ordinary chondrite breccias that closely fit a cometary profile. Meteorites with such inclusions include Sharps (H3.4), Dimmitt (H4), Tsukuba (H5–6), and Krymka (LL3.1). In their research on micrometeoroid/microxenolith populations, originating from both comets and asteroids, Briani et al. (2011) determined that two distinct populations could not be resolved. Therefore, they concluded that a continuum may exist between carbonaceous asteroids and comets.
The CRE age of Orgueil is calculated to be ~5 m.y. The specimen of Orgueil shown above is a 0.162 g specimen measuring approximately 5 mm in diameter. A more representative photo of Orgueil can be seen on the website of the Muséum National d'Histoire de Paris.
