Carbonaceous Chondrites Guidelines
Oxygen Isotope Chart


Classification

Table 3.  Summary of carbonaceous chondrite characteristics. Bold type denotes critical criterion.

Parameters CI CM CO CV CK CR CH C41
Matrix Color2  black dark med. gray med. gray light gray light gray light gray light gray
Chondrule abundance (vol. %) absent  10-40 60-70 40-65 15 50-60 5-10 65
Chondrule average size (mm) - < 0.5 < 1.0 > 0.7 0.7 0.4 0.05 0.6
Coarse-grained chondrule rims - rare rare common rare common none rare
Ca-Al Inclusions (vol. %) none 5 8-15 7-20 4 <1 <1 1

Dominant matrix silicate

clays3

clays3

olivine

~ 60%

olivine

~ 60%

olivine

~80%

some clays

pyroxene

~70 %

olivine

~60%

Dominant martrix opaque4 mag mag > prh mt with <1.0% Cr

R = prh

O = mag

Cr-mag mag ≥ prh mt tr
Ni-Fe metal (vol. %) none trace 1-6 0-5 none 6 20-25 2
Olivine composition (FeO & mol. %)

-

variable

variable5

high

(Fa  0-50)

med/high

(Fa 28-33)

very low

(Fa 3)

low

(Fa <10)

low

(Fa 10-14)

Pyroxene composition (FeO & mol. %) 

-

variable

variable

variable

med/high

(Fs 22-29)

very low

(Fs 4)

low

(Fs <10) 

low

Opague darkening no no no no yes no no no
wt % Carbon ~3.8 ~2.5   ~0.3 <0.10 <0.10 <0.10  

1 C4 is an old classification notation, given here for convenience. The new scheme is "Coolidge grouplet."
2 Assuming low levels of terrestrial oxidation that colors the rock interior light to red brown.
 Dark to gray shades are consistent with total carbon content.
3 Clay is short lay-term for phyllosilicates.
4
mag = magnetite; Cr-mag 1.5 to 14 wt. % Cr; prh = pyrrhotite; pt = pentlandite; tr = troilite; mt = Ni-Fe metal.
   R = reduced subgroup; O = oxidized subgroup.
 5
Dependent on degree of equilibration (subtype), examples: CO3.0, Fa  = 0-100; CO3.4 = 48-60; CO3.6,
   Fa = 36-46.        

    This chart courtesy of the Northern Arizona University  It has been modified to meet ARN needs .

Notes from Phil Duton
[Carbonaceous chondrites
; show alteration due to aqueous action completely unlike the alteration seen in ordinary chondrites. They are graded from "3" that indicates little alteration to "1" that indicates the chondrules have been obliterated due to metamorphic alteration. These primitive meteorites are rare with classifications named for the specimen that originally had the type.
Thus, "CI" meteorites are named for the Ivuna stone, which fell in Tanzania in 1938. It is a "CI 1" stone with 3-5% carbon, 20% water in silicates, and magnetite. There are also some amino acids present. The grade "1" indicates the chondrules have been obliterated by the action of the water.

The type "CM" group is named for the Mighei stone that fell in the Ukraine in 1889. It is a "CM 2" class stone with 0.6 to 2.9% carbon and 13% water, visible chondrules and some amino acids.

The type "CO" group is named for the Ornans stone that fell in Doubs, France in 1868. It is a class "CO 3.4" specimen with 0.2 to 1% carbon and 1% water. Chondrules, while present, are small.

The type "CV" group is named for the Vigarano stone that fell in Italy in 1910. It is a class "CV 3.3" stone with less than 0.2% carbon and less than 0.03% water. Chondrules are present. The Allende meteorite that fell in Mexico is one of this type. It also contains mysterious "CAI" inclusions, which are calcium, aluminum minerals not found on earth. These white, irregular shaped inclusions do not derive from the same process as the chondrules. Their origin is subject of current speculation.

Three recent additions have been added as new, rare, stones have been found.
These are "CK" from Karoonda, Australia that appear to be similar to "CO" and "CV" meteorites.
 "CR" for a stone from Renazzo, Italy formerly classified as a "CM" but shown to have visible metal.
"CH" which is similar to "CR" but with a very high metallic iron content.

NOTE: The carbonaceous meteorites are generally observed falls. This is because they are extremely fragile in our earth's environment and rapidly weather beyond recognition. Prompt recovery is essential to identification of these stony meteorites.

O/I