This very 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 composed primarily of phyllosilicate aggregates consisting of intergrown serpentine and saponite measuring tens of nm to hundreds of µm in size and exhibiting significant chemical heterogeneity. 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 permeated by aqueous fluids over a period of ~15 m.y. to produce a fine-grained (<1 µm) aggregate (FGAa) matrix, which is enriched in Fe-bearing ferrihydrite comprising Fe dissolved from sulfides and magnetites (King et al. (2015). Ultimately, continued alteration of this fine-grained material led to the elimination of most remnant mineral inclusions resulting in increased abundances of Fe and other elements (FGAb). Other lithologies which are present include a CGAFGA transitional lithology, a phosphate lithology (P possibly derived from precursor chondrules), and some anomalous lithologies.
Other end products resulted from extensive aqueous dissolution processes. 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 breunneritethese 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). An isochron was resolved spanning ~10 m.y., corresponding to a very early solar timeframe with an absolute age of 4.5635 (±0.0007) b.y. ago ending ~4.553 b.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 IXe age was determined to be 4.5604 b.y. (Hohenberg et al, 2000).
Chondrules are not present in Orgueil, although the small abundance of olivine that is observed likely represents material from disaggregated remnant chondrules. This absence of chondrules reflects either a characteristic at the time of formation or is a result of later aqueous alteration (Macke et al., 2011). Only a single aqueously altered CAI has been identified in the least altered CI chondrite 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 MgAlCr mineral. Utilizing Raman spectroscopy and a microprobe, the latter measuring 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 C-, Ca-, and Ti-isotopic compositions. These findings suggest a derivation of the graphite and kerogen-type carbon grains from stellar objects such 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 including Orgueil that contain 54Cr-rich grains, likely carried by nanoscale spinel particles (Qin et al., 2010). This 54Cr isotope anomaly was heterogeneously distributed throughout the protoplanetary disk, and evidence favors an injection of these grains as well as others through a late Type II supernova event. 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 present in Orgueil is also found in the other CI meteorites. It has been suggested that low-temperature alteration of primary troilite produced both pyrrhotite and magnetite. It was reported that Orgueil and Ivuna contain higher abundances of both magnetite and carbonate as well as Ni-enriched pyrrhotite, and that the pyrrhotite contains less pentlandite, all features which are consistent with a higher degree of alteration than that in other CI-group members. The finer-grained phyllosilicate present in Orgueil suggests that it experienced the highest degree of aqueous alteration (King et al., 2015). Contrariwise, the presence of ferrihydrite in both Orgueil and Alais but not in Ivuna may indicate a lower degree of aqueous progression in Ivuna. These consistently distinct petrographical and mineralogical features found among the CI members have prompted Bullock et al. (2003, 2005) to propose a division of the CI 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.
A study of CI chondrites representing both falls (Orgueil, Alais, and Ivuna) and finds (Y-82162 and Y-980115) was conducted by King et al. (2014) in which they measured modal abundances and employed X-ray diffraction and IR spectroscopy analyses. Compared to the CI falls, they found that the CI finds contain less phyllosilicate with higher olivine and sulfide abundances (see diagram below). In addition, significant differences were observed in the IR spectra for the two groups, and they attributed all of these differences to greater degrees of thermal metamorphism among the CI finds. Temperatures for the finds reached 500750°C compared to only ~150°C for the falls. Moreover, the heavier O-isotopic composition of the finds was found to be consistent with mass-dependent fractionation associated with thermal metamorphism. They argue that the duration of heating on the CI parent body, on the order of hours to several years, is more consistent with an orbital perigee very close to the Sun (<0.1 AU) rather than radiogenic or impact heating.
Diagram credit: King et al., National Institute of Polar Research, Fifth Symposium on Polar Science - Antarctic meteorites (2014)
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:
Fo100 -------------- 2.1
Fo80 ---------------- 2.6
Fo60 ---------------- 1.1
Troilite ---------------------- 1.2
Pyrrhotite ------------------ 2.7
Magnetite ------------------ 5.1
Serpentine ----------------- 7.7
Saponiteserpentine -- 73.8
Ferrihydrite -------------- 3.7
TOTAL ----------------- 100.0
grain density = 2.43 g/cm³
bulk density = 1.58 g/cm³
porosity = 35 vol%
The composition for Orgueil (along with Ivuna and Alais) was also determined by King et al. (2015) utilizing X-ray diffraction. They found the average modal composition of Orgueil to be 83% phyllosilicate (serpentine/saponite), 6.7% magnetite, 5.7% sulfide, 3.3% ferrihydrite, 1% olivine, and 0.3% gypsum (of probable terrestrial origin), and that it may also contain <1% dolomite.
Bland et al. (2004) found that an inverse correlation exists for the olivine and the phyllosilicate phases. It is presumed that the saponiteserpentine 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 likely 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 carbonphyllosilicate 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 both cometary dust and meteorites should be produced from the disruption of Jupiter-family comets which originate in the Kuiper belt. Studies have shown that Antarctic micrometeorites have a similar carbonaceous chondrite:ordinary chondrite ratio ((~7:1) as the composition of zodiacal dust (M.M.M. Meier, 2014). Based on observational evidence and current modeling, it is thought that comets should be dark in color and 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 low cosmic-ray exposure age (<10 m.y.). An early formation beyond the snowline would be consistent with accumulated ices and radiogenic elements such as 26Al, which would produce heating and aqueous alteration features.
Orbital data obtained from several carbonaceous chondrites (e.g., CI Orgueil [eyewitness plotting]; CMs Maribo and Sutter's Mill [instrument recording]) are a good match to the orbits expected from the disruption of Jupiter-family comets, but are unlike the orbits of ordinary chondrites and most other asteroidal objects (M.M.M. Meier, 2014). Both the orbital eccentricity and semimajor axis for Maribo is nearly identical to those of Comet Encke and the associated Taurid swarm of objects (Haack et al., 2011). Both the CI and CM groups of meteorites exhibit characteristics that are consistent with the descriptions in the previous paragraph. Consistent with a cometary origin, Orgueil contains only a few amino acids, mainly alpha- and beta-alanine, glycine, and gamma-amino-n-butyric acid (the smallest gamma-amino acid found in meteorites). These amino acids are considered to be products of very low temperature conditions (<150°C) such as might be found on an extinct comet in the asteroid belt (Botta et al., 2007), or having 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 high abundance of aliphatic amines (primarily methylamine and ethylamine) present in Orgueil may be the result of decomposition (decarboxylation) of their corresponding amino acids during extensive aqueous alteration (Aponte et al., 2015). The type specimen of the CI group, Ivuna, was also found to contain similar extraterrestrial amino acids in similar abundances to those of Orgueil. The CRE age of Orgueil is also quite short, calculated to be ~5 m.y., and is within the expected survivability window for a meteoroid derived from a Jupiter-family comet which was inserted into an Earth-crossing orbit.
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 (150200°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, 2015). 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 (H56), 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.
Notably, based on mineralogical analyses by King et al. (2015), three thermally metamorphosed (Stage III of Nakamura, 2005) CI-like chondritesY-82162, Y-86029, and Y-980115are considered to possibly represent a separate parent body, perhaps one of the known NEOs such as 3200 Phaethon. 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.