A shower of stones fell after sonic booms at 7:00 P.M. in Grosnaja, Mekensk, USSR. Only one stone of about 3.5 kg was recovered, the remainder falling into the river Terek.
Grosnaja has a high matrix abundance of 69.6 vol%, a virtual absence of metal, and a shock stage of S3 (Bonal et al., 2020 and references therein). The CV3 group has been historically subdivided into three subgroups (McSween, 1977; Weisberg et al., 1997):
Reduced subgroup: e.g., Arch, Efremovka, Leoville, Vigarano, and QUE 93429
Oxidized-Allende subgroup: e.g., Allende, Axtell, Tibooburra, and ALH 84028
Oxidized-Bali subgroup: e.g., Bali, Grosnaja, Kaba, and Mokoia
The CV-oxidized and CV-reduced subgroups were separated on the basis of metal abundances and the Ni content of sulfide (Howard et al., 2010). The previously used discriminator, magnetite abundance, has been shown to overlap between oxidized and reduced subgroups. The oxidized-Bali subgroup has a higher degree of aqueous alteration than oxidized-Allende. The subgroups reflect varying degrees of aqueous/oxidative alteration, which has been found to be correlated with the amount of ice-bearing matrix that was initially accreted (Ebel et al., 2009). For more mineralogical relationships, see Appendix I, Carbonaceous Chondrites).
Some investigators (e.g., Greenwood et al., 2003 and Wasson et al., 2013) have proposed that the CK chondrites could represent an extension of the CV group. This subgroup is considered to reflect varying degrees of metamorphism including impact-generated crushing, thermal alteration, and recrystallization processes (Wasson et al., 2013). In a subsequent study, Dunn et al. (2016) compared magnetite in a number of CK and CV chondrites, and presented geochemical, mineralogical, and petrographic evidence which is more consistent with separate CV and CK parent bodies; details of their study can also be found on the Dhofar 015 page.
A study was undertaken by Bonal et al. (2004, 2006) to refine the subtypes of several CV3 chondrites. They utilized several methods to obtain their data, including Raman spectrometry of organic material, a petrologic study of Fe zoning in olivine phenocrysts, presolar grain abundance, and a noble gas study. These methods are in contrast to that of TL sensitivity data of feldspar which is typically used to determine subtypes of ordinary chondrites, and which was previously applied to the CV3 chondrites. They suggest that TL sensitivity data are not applicable to aqueously altered carbonaceous chondrites because of loss of feldspars through dissolution, leading to an underestimate of the petrologic subtypes. They have redefined the petrologic subtypes of the common CV3 members as follows:
These differences in petrologic subtype are explained by Greenwood et al. (2009) in their study of CV and CK chondrite relationships. They assert that there is a decoupling between the silicate and organic components with respect to measurements involving thermal metamorphism.
The finding of Allende-like oxidized lithologies in the reduced Vigarano breccia, as well as in the Bali-like oxidized member Mokoia, led to the suggestion that all three CV3 subgroups derive from a common heterogeneous asteroid. However, based on a comparative study of 31 CV chondrites representing all three subgroups, which was published in GCA by Bonal et al. (2020), along with further advanced analyses including 23 additional CV chondrites, which was published in abstract form and in EPSL by Gattacceca et al. (2019 #6372, 2020), it was concluded that the CVoxA subgroup represents a more deeply buried, thermally metamorphosed stage of CVoxB. This inference is supported by the observation that in comparison to CVoxB, CVoxA is more metamorphosed, less hydrated, and depleted in ferromagnetic minerals but enriched in secondary awaruite. Furthermore, their analyses demonstrate that significant differences exist between the CVox and CVred subgroups. CVox can be clearly distinguished from CVred based on the average Ni content of sulfides and on the magnetic susceptibility (see diagram below). In addition, it was demonstrated with statistical significance that chondrules in CVred are on average larger than chondrules in CVox (860 µm vs. 768 µm), and that the average matrix abundance in CVox is greater than that in CVred (52.3 vol% vs. 40.3 vol%). Statistically significant differences are also evident in the δ18O values between CVox and CVred. Their data indicate that the CV parent body may be laterally heterogeneous to a significant degree, or perhaps more plausible, that these two subgroups derive from distinct parent bodies that formed in a similar location. They propose that the two bodies be designated CVox and CVred in keeping with historical terminology.
Ni Content of Sulfides vs. Magnetic Susceptibility for CV Chondrites
circles: hot desert finds; squares: Antarctic and falls
Diagram credit: Gattacceca et al., Earth and Planetary Science Letters, vol. 547, Article 116467 (2020)
'CV chondrites: More than one parent body'
The Bali-like matrix mineralogy was formed by one or more mechanisms; in particular, asteroidal aqueous alteration of precursor material at temperatures below 300°C, or re-condensation of vaporized, pre-accretionary, chondritic-rich dust. Grosnaja contains 51 vol% matrix component and exhibits a petrofabric of chondrule flattening (McSween, 1977); this is consistent within the CV group with a shock stage of ~S3 (Rubin, 2012).
The Bali-like mineralogy of Grosnaja includes the phyllosilicates saponite and sodium phlogopite replacing Ca-rich minerals in chondrules and CAIs. It is unique within its group for containing serpentine and chlorite group phyllosilicates, indicative of higher than normal temperatures during alteration. Other secondary minerals present include magnetite, fayalite, andradite, and CaFe-rich pyroxenes. In the Allende-like lithologies that are present in all CV3 subgroups, virtually no phyllosilicate or fayalite is found in the chondrules or CAIs. Instead, nepheline, sodalite, fayalitic olivine, and CaFe-rich pyroxenes are found indicating a higher temperature of alteration than that experienced in the Bali-like lithology on the CVox parent asteroid.
The high content of magnetite in the Bali-like subgroup was instrumental in paleomagnetism studies of type 3.0 Kaba by Gattacceca et al. (2013, 2016). Tentative results suggest a very early (~4.558 b.y. ago) parent body acquisition of a stable magnetic field, consistent with a core dynamo within a partially differentiated planetesimal (see the Allende page for further details on the hypothesis for the existence of an internal core dynamo on the CV parent body).
The K-class asteroid 599 Luisa has been identified as an asteroidal analog for the Bali-like meteorite Mokoia. Luisa has a diameter of ~65 km and is located near the 5:2 resonance at ~2.8 AU as well as the ν6 secular resonance, both of which supply fragments into Earth-crossing orbits on relatively short timescales (see diagram below).
Diagram credit: M. M. M. Meier et al., Earth and Planetary Science Letters, vol. 490 (2018)
'Cosmic history and a candidate parent asteroid for the quasicrystal-bearing meteorite Khatyrka'
On the other hand, the ~39 km diameter, C-type asteroid 495 Eulalia was found to have spectral characteristics very similar to Grosnaja, including a "featureless" spectrum, a slight negative slope, and a virtually identical albedo (Fieber-Beyer et al., 2008). Moreover, Eulalia is located at the 3:1 Kirkwood Gap (a mean motion resonance located at 2.487 AU) and is predicted to rapidly deliver almost half of its ejected material to Earth crossing orbits; the timing is consistent with the 1.7 m.y. CRE age of Grosnaja. Then too, using improved techniques to compare the reflectance spectra of meteorites and asteroids, Eschrig et al. (2019 #6336, 2021) determined that the CK group is most closely matched to the K-type asteroids of the Eos family, while the CO/CV chondrites are a good match to the EOS family, L-type, and Cb-type asteroids (see diagram below).
Comparison of Chondrites with Asteroids From Reflectance Spectra
click on image for a magnified view
Diagram credit: Eschrig et al., Asteroid Science, #2024 (2019)
See also the article by Eschrig et al. in Icarus, vol. 354 (2021)
In another study compiling reflectance spectra for a large number of rare meteorites, Burbine et al. (2020 #2235) demonstrated that the K class is the best asteroid analog for CV3 chondrites, CK chondrites of types 4-6, and a CM12 chondrite. This is supported by a reflectance spectra analysis conducted by Tanbakouei et al. (2021 #1456) in which ten CV and CK chondrites were compared to the three Eos family asteroids (221) Eos, (661) Cloelia, and (742) Edisona. A good match was obtained for the spectral slope and certain other features between the CV and CK meteorites and the three Eos family asteroids.
The specimen of Grosnaja shown above is a 1.5 g partial slice. The slice was cut from a 20.5 g specimen formerly in the Natural History Museum, Humboldt University, Berlin.