CM chondrites are a group of meteorites that contain small, round particles called chondrules. They look similar to the Mighei meteorite, which is their example. CM chondrites are the most often found group within the carbonaceous chondrite class. However, they are less common in collections compared to ordinary chondrites.

Overview and Taxonomy

Meteorites are mainly divided into two main groups: Ordinary chondrites and Carbonaceous chondrites. Fewer meteorites belong to other groups, such as Enstatites and Ureilites. The word "chondrite" refers to meteorites that contain (or may have contained) small, round pieces of minerals called chondrules, which are surrounded by a matrix. Chondrules formed before the meteorites themselves. The term "carbonaceous" was used to describe Carbonaceous chondrites compared to Ordinary chondrites. Some Enstatite and Ureilite meteorites may contain more carbon than Carbonaceous chondrites. However, all Carbonaceous chondrites are different from Ordinary chondrites because they have a small amount of carbon (which makes them dark), along with other volatile substances, giving them a lower density. Later, scientists found a more precise definition: Carbonaceous chondrites have a higher proportion of magnesium compared to Ordinary chondrites.

Carbonaceous chondrites are further split into smaller groups, including CI, CM, CO, CV, CK, CR, and others (CH, CB, and ungrouped C-meteorites). These groups are named based on the rock and chemical features of the meteorites, with the group name coming from a well-known example. For example, CI stands for Ivuna-like, CM for Mighei-like, and CO for Ornans-like. The CM group is most similar to CI and CO chondrites, and sometimes a CM–CO type is described. All three groups contain unusual amounts of titanium and chromium isotopes.

Although Carbonaceous chondrites are much rarer than Ordinary chondrites, the CM group is the most common type of Carbonaceous chondrite. According to the latest Meteorite Catalogue (5th edition, 2000), there are 15 recorded CM falls (meteorites observed when they landed) and 146 CM finds (meteorites discovered later, possibly from ancient times). In comparison, the next most common group is CO, with 5 falls and 80 finds. These numbers are part of a larger group of 36 Carbonaceous chondrite falls and 435 finds. If the CM and CO groups are considered together as a single group, their combined presence is even more significant.

Petrologic types

CM chondrites, including CMs, have lower densities compared to other meteorites. CMs are slightly denser (~2.1 grams per cubic centimeter) than CIs but less dense than CO and other C-chondrites. This is because CMs often contain broken rock fragments with air spaces and materials that are naturally light. Some CMs, like Y-791198 and ALH81002, are not broken and remain whole.

Early scientists tried to classify meteorites based on their appearance. Rose called some meteorites "kohlige meteorite," and later, Tschermak created early systems for grouping them. In 1904, Brezina classified CM chondrites as "K" ("coaly chondrites"). In 1956, Wiik developed a modern system with three types (I, II, and III), placing CMs in Type II.

By 1967, CM chondrites were mostly classified as Type 2 on the petrographic scale, which measures changes caused by water. At this time, enough CM and CI chondrites were studied to define the "aqueous alteration" end of the scale. Type 1 on this scale (CI chondrites) matches Wiik’s Type I. Types 4 through 6 show increasing heat changes, while Type 3 is considered unaltered.

Van Schmus and Wood (1967); Sears and Dodd (1988); Brearley and Jones (1998); Weisberg (2006)

In 1969, Van Schmus divided Type 3 meteorites into groups called "V" and "O" (C3V and C3O). Wasson later added "C2M" in 1974, which was later shortened to "CM."

As Type 2 meteorites, CM chondrites have some chondrules (small, round rock pieces), while others have been changed or dissolved by water. CO chondrites have more chondrules, and CIs have only faint traces of them or none at all. Many CM chondrules are surrounded by layers of minerals or water-altered material.

CM chondrules are larger than those in CO chondrites, though both are smaller than average. This may be because water dissolves smaller chondrules, leaving larger ones. CMs also contain small amounts of CAIs (calcium–aluminum-rich inclusions).

The matrix (ground material between chondrules) in CMs is described as "sponge" or "spongy." CMs have fewer olivine and pyroxene grains than COs but more than CIs. These minerals and free metal are altered by water, with COs having more free metal and CIs having mostly oxidized metal. CMs fall between these extremes.

Most free metal and silicate grains in CMs have changed into matrix materials. CMs have more matrix than COs but less than CIs, which are mostly matrix.

In 1860, Wohler identified matrix as serpentinite. Later, Fuchs et al. (1973) called matrix a "poorly characterized phase" (PCP) because they couldn’t identify its minerals. Cronstedtite was named in 1975. In 1985, Tomeoka and Buseck identified cronstedtite and tochilinite, calling matrix "FESON" (Fe–Ni–S–O layers) and using "PCP" as a backronym. Later, "TCI" (tochilinite-cronstedtite intergrowths) was used. Other minerals in the matrix include chlorite, vermiculite, and saponite.

The CM group is large and varied. Scientists have tried to divide CMs further beyond the petrographic scale. McSween (1979) proposed adding suffixes to classify CMs, such as "CM2.9" (less altered, closer to COs) and "CM2.0" (more altered, closer to CIs). No true CM2.9 specimens have been found.

McSween measured matrix amounts and iron depletion to study alteration. Browning et al. (1996) created the "MAI" (Mineralogical Alteration Index) to measure unaltered silicate grains and chondrule changes. Rubin et al. (2007) studied carbonates, finding more dolomite and less calcite in more altered meteorites. Howard et al. (2009, 2011) measured phyllosilicate abundance. Alexander et al. (2012, 2013) used deuterium, C/H, and nitrogen isotope levels to study alteration.

Some meteorites, like Paris, are considered the least altered CMs, bridging the gap between CMs and COs. Others include ALHA77307, Adelaide, Acfer 094, MAC87300, and MAC88107. More altered examples are Bells, EET83334, ALH88045, Tagish Lake, and Dhofar 225.

CI and CM chondrites are "water-rich" meteorites, with CMs containing 3–14 weight percent water. This water is stored in minerals like tochilinite and cronstedtite. Scientists believe Earth’s oceans may have originated from this water, not comets, based on isotope studies (mainly deuterium).

Water inside meteorites

Chemistry

Carbonaceous chondrites contain significant amounts of carbon compounds. These include pure carbon, simple compounds like metal carbides and carbonates, organic molecules, and complex carbon structures called polycyclic aromatic hydrocarbons (PAHs).

The amounts of elements in some carbon chondrite groups (except for hydrogen, helium, and a few others) closely match the amounts found in the Sun. CI chondrites, in particular, match the Sun’s composition more closely than any other type of space or Earth material. This is considered very surprising. Only large planets like Jupiter and Saturn have enough mass to hold onto hydrogen and helium. This also applies to most noble gases and smaller amounts of nitrogen, oxygen, and carbon. Other elements, both volatile and refractory, match the Sun’s composition so well that CI chondrites are used as a standard in space science. Since the Sun makes up 99% of the Solar System’s mass, knowing the Sun’s composition is key to understanding other parts of the system.

CM chondrites also match the Sun’s composition, but the match is weaker. More volatile elements are less common in CM chondrites compared to CIs, and more refractory elements are more common.

A small amount of meteorite material consists of tiny presolar grains (PSGs). These are crystals that formed in space before the Solar System existed. PSGs include materials like silicon carbide, microscopic diamonds, and other hard minerals such as corundum and zircon. The chemical fingerprints of these grains do not match those in the Solar System; instead, they match those found in space between stars. Some PSGs may even contain smaller PSGs.

Like other meteorites, some carbon in carbon chondrites is found in carbides (such as Cohenite, a compound of iron and carbon) and carbonates like calcite and dolomite. Aragonite, a type of carbonate, is rare in CI chondrites.

The total amount of carbon in CM chondrites is less than in CI chondrites, but more of it is in complex, aromatic forms. Isotope studies show this carbon is from space, not Earth.

The organic materials in carbon chondrites are divided into two groups: soluble organic compounds and insoluble organic matter (IOM). Soluble compounds can be analyzed using older chemistry methods, revealing substances like paraffin, naphthene, and aromatics.

IOM makes up most of the organic material in these meteorites. In 1963, scientists could only describe it as having very high molecular weight. IOM itself splits into two types: one that breaks down easily when heated and one that is very stable.

Amino acids and other organic compounds were first found in meteorites, but early studies suggested they might have come from Earth. The 1969 fall of the Murchison meteorite, the largest CM chondrite ever found, provided over 100 kilograms of material. Scientists quickly collected samples from a dry area and used improved techniques to study them. These studies showed that sugars and amino acids, including some not found on Earth, existed in space. Isotope studies confirmed these materials are not from Earth.

Amino acids are more common in CM chondrites than in CI chondrites. Other related compounds, such as nitriles, cyanides, and heterocycles, are also found. These may form from the breakdown of other materials or act as building blocks for them.

Early studies did not detect a preference for left or right-handedness in meteorite organics, but later research found that some organic materials in meteorites show a preference for one direction. This includes both the soluble and insoluble organic parts.

• Kvenvolden et al. 1970; 2. Meierheinrich et al. 2004; 3. Martins et al. 2015; 4. Koga et al. 2017; 5. Rudraswami et al. 2018; 6. Pizzarello, Yarnes 2018

The first discovery of unusual gas in a carbon chondrite (Murray) was in 1960. Other types of meteorites that contain gas often store it in dark parts of their structure, similar to CM chondrites.

Gases in meteorites include gases from the early Solar System, gases from the Sun (including solar wind and a separate component from solar flares), gases formed by cosmic rays, and gases from radioactive decay. These gases are usually found in carbon-rich materials, including presolar grains like diamond, silicon carbide, graphite, and organic compounds.

Nogoya is a CM chondrite with a high amount of gas.

Micrometeorites lose much of their gas when entering Earth’s atmosphere due to heat, but they still deliver measurable amounts.

Studying isotope levels has helped scientists understand the history of materials in space. Oxygen, in particular, forms very stable compounds, so it takes major events or processes to separate its different forms.

CM and CI chondrites have measurable differences in oxygen isotope levels. This suggests they formed at different temperatures and locations in the young Solar System. However, CM and CO meteorites share similar oxygen isotope levels, showing they may have formed in related regions.

Provenance

CM chondrites, like other C-chondrites, face a serious observation bias. C-chondrites are easily broken because of large spaces between their parts and tiny layers of phyllosilicates, with many chondrules also containing phyllosilicate layers. These meteorites are sometimes called "tuff," which is a type of compacted volcanic ash.

For example, the Tagish Lake meteorite provided about 10 kilograms of samples from a meteorite that was estimated to weigh 60–90 tons before entering Earth’s atmosphere.

In contrast, many ordinary chondrite meteorites are tougher and more commonly found, and iron meteorites are even more common.

CI and CM chondrites, in particular, are affected by weathering after landing on Earth. Since much of C-chondrite material dissolves in water, ordinary chondrites and iron meteorites are more likely to be found and collected. More exploration in hot deserts and Antarctica has led to the discovery of many C-chondrite samples.

As carbonaceous meteorites, CM and similar groups are generally believed to come from carbonaceous asteroids. This includes the specific C-type asteroids and, to some extent, related G-, B- (including the outdated F-), D-, and P-type asteroids. Although carbonaceous types make up most asteroids, they are only a small percentage of found meteorites. This means that selection and filtering effects are very strong.

In addition to the variety of CM chondrites and the diversity of carbonaceous asteroid types, the origin of these meteorites remains uncertain. The Almahata Sitta meteorite was classified as a ureilite, a completely different type of meteorite. However, it came from asteroid 2008 TC 3. A rough spectrum taken before its arrival would have placed 2008 TC 3 as a F- or B-type asteroid.

Some space weathering is visible on carbonaceous asteroids, which makes it harder to match meteorites to their parent bodies using spectroscopy.

A theory suggests that all CM chondrites may have come from a single parent body.

Brecciated meteorites include monomict breccias (formed from fragments of one rock type) and polymict breccias (formed from different rock types). Polymict meteorites show evidence of exchanges between different locations. C-chondrite materials are often found in such meteorites.

• PRA 04401 – officially a HED meteorite, contains as much CM or CM-like material in its fragments as HED material
• Kaidun – a breccia made of many different types of materials
• Supuhee
• Plainview
• Jodzie

List of CM chondrites

• Mighei – 1889; the name of the group comes from this location
• Cold Bokkevelt – 1838; found in a dry area, and not changed much over time
• Nogoya – 1879;
• Boriskino – 1930;
• Murray – 1950;
• Murchison – 1969; 100 kg was found, leading to a lot of research
• Yamato 74662 – 1974; first meteorite found in Antarctica with type CM

• Aguas Zarcas – April 2019 fall, samples were collected quickly; more than 20 kg
• Winchcombe meteorite
• Mukundpura meteorite – June 6, 2017 fall, crashed and broke apart; 2.2 kg of pieces were found within hours

General References

• Mason, B. The Carbonaceous Chondrites. 1962 Space Sciences Reviews, Volume 1, page 621
• Meteorites and the Early Solar System, edited by Kerridge, J. and Matthews, M. 1988 University of Arizona Press, Tucson ISBN 9780816510634
• Planetary Materials, edited by Papike, J. 1999 Mineralogical Society of America, Washington DC ISBN 0-939950-46-4
• The Catalogue of Meteorites, edited by Grady, M. 2000 Cambridge University Press, Cambridge ISBN 0 521 66303 2
• Meteorites and the Early Solar System II, edited by Lauretta, D. and McSween, H. 2006 University of Arizona Press, Tucson ISBN 9780816525621