CV Chondrites

Meteorite class named after the Vigarano meteorite that fell in Italy in 1910. They have abundant large, well-defined rimless (?) chondrules of magnesium-rich olivine (~0.7 mm diameter; 40-65 vol. %), often surrounded by iron sulfide. They also contain 7-20 vol. % CAIs. The often dark-gray matrix is dominated by Fe-rich olivine (~60 vol. %). The Allende meteorite is a very famous and well-studied CV meteorite that fell in 1969 in Chihuahua, Mexico.

The more detailed description below is an updated and modified version based on Systematics and Evaluation of Meteorite Classification by Weisberg, McCoy and Krot 2006.

The CV (Vigarano-like) chondrites have a high abundance of matrix, large chondrules with a mean diameter (based on Allende study) of ~0.9 mm +/- 0.7 mm1, and a high abundance of large CAIs and AOAs. Nearly all the CV chondrites are classified as type 3 with Mundrabilla 012 classified as a CV2 and NWA 3133 recently reclassified as a CV7. They are divided into three subgroups including two oxidized subgroups (CVoxA for Allende-like and CVoxB for Bali-like) and one reduced (CVred) subgroup. Differences between the three subgroups are based largely on petrological characteristics. Matrix/chondrule ratios increase in the order of CVred (0.5–0.6), CVoxA (0.6–0.7) and CVoxB (0.7– 1.2), whereas metal/magnetite ratios tend to decrease in the same order. The whole-rock O-isotopic compositions of the CV chondrites plot along the CCAM line, with the CVoxB chondrites being slightly depleted in 16O relative to the CVred and CVoxA chondrites.

Although nearly all CV chondrites have been classified as type 3, the CVoxB chondrites contain abundant (hydrated) phyllosilicates and could be classified as type 2. Further significant mineralogical differences between the CV subgroups are the result of varying degrees of late-stage alteration and may reflect secondary characteristics superimposed onto the members of this meteorite group. The CVoxB chondrites experienced aqueous alteration and contain hydrous phyllosilicates, magnetite, Fe,Ni-sulfides, Fe,Ni-carbides, fayalite, Ca,Fe-pyroxenes (Fs10–50Wo45–50), and andradite. The CVoxB matrices consist of very fine-grained (<1–2 μm) ferrous olivine (~Fa50), concentrically zoned nodules of Ca,Fe-pyroxenes and andradite, coarse (>10 μm) grains of nearly pure fayalite (Fa>90), and phyllosilicates (Fig. 6). The CVoxA chondrites are more extensively altered than the CVoxB, but contain very minor phyllosilicates. The major secondary minerals include ferrous olivine (Fa40–60), Ca,Fe-pyroxenes, andradite, nepheline, sodalite, Fe,Ni-sulfides, magnetite, and Ni-rich metal.

The CVoxA matrices are coarser-grained than those in the CVoxB chondrites and largely consist of lath-shaped ferrous olivine (~Fa50), Ca, Fe-pyroxene ± andradite nodules, and nepheline. Some oxidized CVs such as Meteorite Hills (MET) 00430 are mineralogically intermediate between the CVoxB and CVoxA chondrites. For example, the matrix in MET 00430 contains ferrous olivine with grain sizes intermediate between those in the matrices of the CVoxA and
CVoxB chondrites and with inverse compositional zoning (Fa80–50). The reduced CV chondrites Efremovka and Leoville experienced smaller degree of alteration than CVoxA meteorites. Both meteorites virtually lack phyllosilicates and contain nepheline, sodalite, and Ca,Fe-pyroxenes; however, these minerals are much less abundant than in the CVoxA meteorites. The CV chondrite Mokoia contains clasts of the CVoxA and CVoxB materials. Vigarano is a breccia that contains CVred, CVoxB, and CVoxA materials. These observations may indicate that the CV subgroups represent different lithological varieties of the CV asteroidal body that experienced complex, multistage aqueous alteration.

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