CV Chondrites

MeteoriteWork in progress. A solid natural object reaching a planet’s surface from interplanetary space. Solid portion of a meteoroid that survives its fall to Earth, or some other body. Meteorites are classified as stony meteorites, iron meteorites, and stony-iron meteorites. These groups are further divided according to their mineralogy and class named after the Vigarano meteorite that fell in Italy in 1910. They have abundant large, well-defined rimless (?) chondrulesRoughly spherical aggregate of coarse crystals formed from the rapid cooling and solidification of a melt at  ~1400 ° C. Large numbers of chondrules are found in all chondrites except for the CI group of carbonaceous chondrites. Chondrules are typically 0.5-2 mm in diameter and are usually composed of olivine of magnesium-rich olivineGroup of silicate minerals, (Mg,Fe)2SiO4, with the compositional endpoints of forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Olivine is commonly found in all chondrites within both the matrix and chondrules, achondrites including most primitive achondrites and some evolved achondrites, in pallasites as large yellow-green crystals (brown when terrestrialized), in the silicate portion (~0.7 mm diameter; 40-65 vol. %), often surrounded by iron sulfide. They also contain 7-20 vol. % CAIsSub-millimeter to centimeter-sized amorphous objects found typically in carbonaceous chondrites and ranging in color from white to greyish white and even light pink. CAIs have occasionally been found in ordinary chondrites, such as the L3.00 chondrite, NWA 8276 (Sara Russell, 2016). CAIs are also known as refractory inclusions since they. The often dark-gray matrixFine grained primary and silicate-rich material in chondrites that surrounds chondrules, refractory inclusions (like CAIs), breccia clasts and other constituents. 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) chondritesChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such 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 oxidizedOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons,  while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants subgroups (CV_oxA for Allende-like and CV_oxB for Bali-like) and one reducedOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons,  while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants (CV_red) subgroup. Differences between the three subgroups are based largely on petrological characteristics. Matrix/chondrule ratios increase in the order of CV_red (0.5–0.6), CV_oxA (0.6–0.7) and CV_oxB (0.7– 1.2), whereas metalElement that readily forms cations and has metallic bonds; sometimes said to be similar to a cation in a cloud of electrons. The metals are one of the three groups of elements as distinguished by their ionization and bonding properties, along with the metalloids and nonmetals. A diagonal line drawn/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 CV_oxB chondrites being slightly depleted in ¹⁶O relative to the CV_red and CV_oxA chondrites.

Although nearly all CV chondrites have been classified as type 3, the CV_oxB chondrites contain abundant (hydrated) phyllosilicatesClass of hydroxyl-bearing silicate minerals with a sheet-like structure. They result from aqueous alteration are dominantly serpentine and smectite in meteorites; found in the matrixes of carbonaceous chondrites. Phyllosilicates consist of repeating sequences of sheets of linked tetrahedra (T) and sheets of linked octahedra (O). The T sheet consists of 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 CV_oxB chondrites experienced aqueous alteration and contain hydrous phyllosilicates, magnetiteFe oxide, FeFe2O4, containing oxidized iron (Fe) found in the matrix of carbonaceous chondrites and as diagnostic component in CK chondrites. In CK chondrites, magnetite is typically chromian, containing several wt. % Cr2O3., Fe,Ni-sulfides, Fe,Ni-carbides, fayalitePure* iron end-member (Fe2SiO4) of the olivine solid solution series and an important mineral in meteorites. When iron (Fe) is completely substituted by magnesium, it yields the the pure Mg-olivine end-member, forsterite (Mg2SiO4). The various Fe and Mg substitutions between these two end-members are described based on their forsteritic (Fo), Ca,Fe-pyroxenes (Fs10–50Wo45–50), and andradite. The CV_oxB 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 CV_oxA chondrites are more extensively altered than the CV_oxB, but contain very minor phyllosilicates. The major secondary mineralsMineral that forms through processes such as weathering, and in the case of meteorites can also include pre-terrestrial alteration. Secondary minerals in meteorites that formed during terrestrial weathering include oxides and hydroxides formed directly from metallic Fe-Ni by oxidation, phosphates formed by the alteration of schreibersite, and sulfates formed by include ferrous olivine (Fa40–60), Ca,Fe-pyroxenes, andradite, nepheline, sodalite, Fe,Ni-sulfides, magnetite, and Ni-rich metal.

The CV_oxA matrices are coarser-grained than those in the CV_oxB 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 CV_oxB and CV_oxA chondrites. For example, the matrix in MET 00430 contains ferrous olivine with grain sizes intermediate between those in the matrices of the CV_oxA and
CV_oxB chondrites and with inverse compositional zoning (Fa80–50). The reduced CV chondrites Efremovka and Leoville experienced smaller degree of alteration than CV_oxA meteorites. Both meteorites virtually lack phyllosilicates and contain nepheline, sodalite, and Ca,Fe-pyroxenes; however, these minerals are much less abundant than in the CV_oxA meteorites. The CV chondriteChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such Mokoia contains clasts of the CV_oxA and CV_oxB materials. Vigarano is a brecciaWork in Progress ... A rock that is a mechanical mixture of different minerals and/or rock fragments (clasts). A breccia may also be distinguished by the origin of its clasts: (monomict breccia: monogenetic or monolithologic, and polymict breccia: polygenetic or polylithologic). The proportions of these fragments within the unbrecciated material that contains CV_red, CV_oxB, and CV_oxA materials. These observations may indicate that the CV subgroups represent different lithological varieties of the CV asteroidal body that experienced complex, multistage aqueous alteration.