Carbonaceous Chondrites


CI1 and CM2 Groups

Carbonaceous chondrites are those that obviously contain carbon-bearing matter: elemental carbon, nano diamonds, abiotic organic compounds, fullerenes, and other rare, carbon forms. Because of this carbon-bearing matter, these meteorites are probably the most important type of meteorite for research, a genealogical root of the tree of life. As we mentioned earlier, most of the carbon in life forms, probably originated from sources outside of the solar system; carbon stars, giant molecular clouds, etc., although these organic compounds would not necessarily lead directly to life.

Instead of representing a thermal metamorphic grade, as in the case for ordinary chondrites, e.g., H3, H4, H5, the designations of 1 and 2, e.g., CI1 and CM2 indicate the degree of water processing or aqueous alteration. This alteration occurred in the parent body under very low temperatures, probably in the range of 20 to 50° C in a water-rich environment. This contrasts to the thermal metamorphism of ordinary chondrites that occurred in the range of 600 to 900° C under very dry conditions. Carbonaceous chondrites may contain up to 20 weight % water. Possibly, up to 50% of this water was absorbed during the meteorite's residence on Earth.

Murchison Meteorite. Image source: http://www.astrosurf.org/lombry/Images/meteorite-murchison.jpg

Ivuna is the type example of CI1 meteorites and Mighei is the type example of CM2 meteorites. CI1 denotes that the meteorite is the most primitive in terms of volatile/elemental retention (the highest known degree of primitivism) and is the most aqueously altered. Ivuna fell in Tanzania in 1938. Murchison fell near Victoria, Australia in 1969. Over 100 kg or 220 pounds were recovered over a 5 square mile strewnfield. Most CI1 meteorites contain elements in nearly the same abundances as in the sun, which means that they have not lost or gained anything since they were formed in the earliest stages of the solar system. Notice that these meteorites are very dark, caused by high carbon contents, the high amounts of Fe-rich clays or phyllosilicates, and very fine grain size. Also note that there is an absence of chondrules in Ivuna (they were all altered to clays and iron oxides), a network of tiny, web-like white veins, and the small, low number of chondrules in Murchison (the designation of 2 means that not everything changed. The white veins are fractures that are filled with water-soluble sulfate minerals that either formed on the parent body when water alteration was changing olivine and pyroxenes into clay minerals, with some leftover elements precipitating in fractures, or on the Earth during absorption of atmospheric water, or both.

Classification of Carbonaceous Chondrites

The large number of carbonaceous chondrite classes makes their identification a little challenging. The table below summarizes some of the prominent features of the various carbonaceous classes. The first thing someone would do on finding a stone they thought might be a carbonaceous chondrite, would be to look at the color (black to light gray). Then see if it has chondrules. With a magnifying glass or hand lens, estimate the number of chondrules versus the fine-grained matrix on a fractured or cut surface. Finally, look to see if shiny flecks of metal are evident, or shiny, buff to yellow specks of sulfides are present (look like "fool's gold"), or dull, dark gray grains of magnetite. Knowing these bits of information may help determine which carbonaceous chondrite it is. For the more ambitious person, especially those with equipment, more detailed information can be gathered to confirm the exact type. A summary of the diagnostic mineralogic and chemical characteristics of carbonaceous chondrites is given in the table below.

CI (C1)CM (C2)CO (C3O)CV (C3V)CKCRCH
Type meteorite (other examples)Ivuna, (Orgueil)Mighei, (Murray, Murchison)OrnansVigarano, (Allende)KaroondaRenazzo(ALH 85085)
Chemistry1Abundance
patterns
solar abundances of Z>8 (except gases) RLE = 1.15 x CI; VE = 0.5 x CI RLE = 1.13 x CI; VE = 0.2 x CI RLE = 1.35 x CI; lower in Fe, Na, K than CO; VE = 0.23 x CIRLE = 1.21 x CI; VE between CO and CVRLE = CI; VE = 0.24 x CIRLE = CI; high total Fe and other nonvolatile metals (1.4 x CI); VE = 0.1 x CI
Mean Mg/Si1.051.041.051.071.131.041.06
Mean Al/Si0.0860.0950.0940.1170.1110.0860.083
Mean Ca/Si0.0620.0690.0700.0850.0800.0630.060
Mean δ18O16.412.2-1.10-0.8 6.31.5
Mean δ17O8.84.0-5.1-4.0-4.6 2.3-0.7
Color2blackdarkmedium graymedium gray light gray (olive when weathered)light graylight gray
Approximate chondrule abundance (vol. %)absent~20 (highly variable) 50454550-60 70
ChondruleSize (mm)-0.30.151.0 0.80.70.05
Types 7-porphyritic(95), nonporphyritic(5)PO(8), POP(69), PP(18), BO(2), RP(2), CC(1)porphyritic (95), BO(6), RP(0.2), CC(0.1)porphyritic(>99)porphyritic(96-98), nonporphyritic(2-4)porphyritic(20), BO(1), CC(79)
Coarse-grained chondrule rims-rarerare Fo-rich common rarecommonnone
Ca-Al InclusionsVolume (%)none58-157-20 4<1<1
Types6 - a, H, p, S A, c, H, g, p, S A, B, C, H, S b, s A, b, c, g, h, S A, G, H, P, S
Amoeboid olivine aggregates (AOAs) - common common common common common common
Matrix (%)>95~70 (highly variable)3540 4030-505
Dominant matrix silicateclays3clays3 olivineolivineolivinesome clayspyroxene
Oxidation state (dominant martrix opaque4)highly oxidized, water (mag) oxidized, water; (mag > prh)less oxidized than CM, only traces of water; (mt with <1.0% Cr) 2 subgroups: oxidized (CVO), Allende; (prh); reduced (CVR), Vigarano; (mag) oxidized; (Cr-mag)reduced, contains phyllosilicates; (mag ≥ prh)reduced; (mt)
Ni-Fe metal (vol. %)none trace; metal inclusions in forsterite aggregates 1-50-5none5-820
Olivine FeO-variablevariable5 - - very low -
Fa (mol. %)--- range: 0-50 range: 1-47; mode: 29-331-3 range: 1-36; mode: 2
FeO/MnO --110-138>136105-1407-24-
Pyroxene composition (FeO & mol. %)-variablevariable variablemed/high (Fs 22-29)very low (Fs 4)low (Fs <10)

NOTES. 1 RLE = refractory lithophile elements, VE = volatile elements. 2Assuming low levels of terrestrial oxidation that colors the rock interior light to red brown. Dark to gray shades are consistent with total carbon content. For example, CIs have ~3.8 wt. % carbon; CMs, ~ 2.5; CVs, ~ 0.3; CKs, CRs, CHs, < 0.10%. 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. 5 Depends on the degree of equilibration (petrologic subtype), examples: CO3.0, Fa0-100; CO3.4 Fa48-60; CO3.6, Fa36-46; Cr2O3 also shows systematic variations (see histograms below). 6 CAI types (CAPITAL indicates common; lower case = rare): A,a = melilite-rich; B,b = Al,Ti-px-rich; C,c = plaioclase-pyroxene-rich; G,g = grossite-rich; H,h = hibonite±spinel-rich; P,p = pyroxene-hibonite spherules; S,s = spinel±pyroxene-rich. 7 Chondrule types (percentage): PO = porphyritic olivine; POP = porphyritic olivine-pyroxene; PP = porphyritic pyroxene; BO = barred olivine, RP = radial pyroxene, CC = cryptocrystalline.

Carbonaceous chondrite types based upon olivine composition (Grossman & Brearley, 2005).