Meteorite classification

The ultimate goal of meteorite classification is to group together all meteorite specimens that share a common origin on a single, identifiable parent body. This could be a planet, asteroid, Moon, or other current Solar System object, or one that existed some time in the past (e.g. a shattered asteroid). However, with a few exceptions, this goal is beyond the reach of current science, mostly because there is inadequate information about the nature of most solar system bodies (especially asteroids and comets) to achieve such a classification. Instead, modern meteorite classification relies on placing specimens into "groups" in which all members share certain key physical, chemical, isotopic, and mineralogical properties that are consistent with a common origin on a single parent body, even if that body is not identified. It is possible that several meteorite groups classified in this way may come from a single, heterogeneous parent body or that a single group may contain members that came from a variety of very similar but distinct parent bodies. As such information comes to light, the classification system will most likely evolve.

Terminology

Beyond the assignment of meteorites into such groups, which is essentially universally accepted, there is no consensus among researchers as to what hierarchy of classification terms is most appropriate. For chondrites, groups may be divided into subgroups where there are features that distinguish certain meteorites from the others in the group, but it is thought that all still come from a single parent body. It is also fairly common for groups that seem to be closely related to each other to be referred to as clans. In turn, groups or clans that appear to be loosely related are often referred to as chondrite classes (e.g., carbonaceous chondrites, enstatite chondrites, and ordinary chondrites). But higher order terms for aggregating groups of meteorites tend to be somewhat chaotic in the scientific and popular literature. There is little agreement on how to fit nonchondritic meteorite groups into an overall scheme.

Several other classification terms are in widespread use. Anomalous meteorites are members of well-established groups that are different enough in some important property to merit distinction from the other members. Ungrouped meteorites are those which do not fit into any known group, although they may fit into a clan or class (e.g., the meteorite Acfer 094 is in an ungrouped member of the CM-CO clan of carbonaceous chondrites).

Traditional classification scheme

Meteorites are often divided into three overall categories based on whether they are dominantly composed of rocky material (stony meteorites), metallic material (iron meteorites), or mixtures (stony-iron meteorites). These categories have been in use since at least the early 19th century but do not have much genetic significance; they are simply a traditional and convenient way of grouping specimens. In fact, the term "stony iron" is a misnomer as currently used. One group of chondrites (CB) has over 50% metal by volume and contains meteorites that were called stony irons until their affinities with chondrites were recognized. Some iron meteorites also contain many silicate inclusions but are rarely described as stony irons.

Nevertheless, these three categories sit at the top of the most widely used meteorite classification system. Stony meteorites are then traditionally divided into two other categories: chondrites (groups of meteorites that have undergone little change since their parent bodies originally formed and are characterized by the presence of chondrules), and achondrites (groups of meteorites that have a complex origin involving asteroidal or planetary differentiation). The iron meteorites were traditionally divided into objects with similar internal structures (octahedrites, hexahedrites, and ataxites), but these terms are now used for purely descriptive purposes and have given way to modern chemical groups. Stony-iron meteorites have always been divided into pallasites (which are now known to comprise several distinct groups) and mesosiderites (a textural term that is also synonymous with the name of a modern group).

Below is a representation of how the meteorite groups fit into the more traditional classification hierarchy:

• I) Stony meteorites
  • Chondrites
    • Carbonaceous chondrite class
      • CI chondrite (Ivuna-like) group
      • CM-CO chondrite (mini-chondrule) clan
        • CM chondrite (Mighei-like) group
        • CO chondrite (Ornans-like) group
      • CV-CK chondrite clan
        • CV chondrite (Vigarano-like) group
          • CV-oxA chondrite (oxidized, Allende-like) subgroup
          • CV-oxB chondrite (oxidized, Bali-like) subgroup
          • CV-red chondrite (reduced) subgroup
        • CK chondrite (Karoonda-like) group
      • CR chondrite clan
        • CR chondrite (Renazzo-like) group
        • CH chondrite (Allan Hills 85085-like) group
        • CB chondrite (Bencubbin-like) group
          • CBa chondrite subgroup
          • CBb chondrite subgroup
    • Ordinary chondrite class
      • H chondrite group
      • L chondrite group
      • LL chondrite group
    • Enstatite chondrite class
      • EH chondrite group
      • EL chondrite group
    • Other chondrite groups, not in one of the major classes
      • R chondrite (Rumuruti-like) group
      • K chondrite (Kakangari-like) grouplet (a grouplet is a provisional group with <5 members)
  • Achondrites
    • Primitive achondrites
      • Acapulcoite-lodranite clan
        • Acapulcoite group
        • Lodranite group
      • Brachinite group
      • Winonaite group
      • Ureilite group
    • HED meteorite clan (possibly from asteroid 4 Vesta)
      • Howardite group
      • Eucrite group
      • Diogenite group
    • Lunar meteorite group
    • Martian meteorite group (sometimes called "SNC meteorites")
      • Shergottites
      • Nakhlites
      • Chassignites
      • Other Martian meteorites, e.g., ALH84001
    • Angrite group
    • Aubrite group (enstatite achondrites)

• II) Stony-iron meteorites
  • Pallasites
    • Main group pallasites
    • Eagle station pallasite grouplet
    • Pyroxene pallasite grouplet
  • Mesosiderite group

• III) Iron meteorites
  • Magmatic iron meteorite groups
    • IC iron meteorite group
    • IIAB iron meteorite group
    • IIC iron meteorite group
    • IID iron meteorite group
    • IIF iron meteorite group
    • IIG iron meteorite group
    • IIIAB iron meteorite group
    • IIIE iron meteorite group
    • IIIF iron meteorite group
    • IVA iron meteorite group
    • IVB iron meteorite group
  • Non-magmatic or primitive iron meteorite groups
    • IAB iron meteorite "complex" or clan (formerly groups IAB and IIICD)
      • IAB main group
      • Udei Station grouplet
      • Pitts grouplet
      • sLL (low Au, Low Ni) subgroup
      • sLM (low Au, Medium Ni) subgroup
      • sLH (low Au, high Ni) subgroup
      • sHL (high Au, Low Ni) subgroup
      • sHH (high Au, high Ni) subgroup
    • IIE iron meteorite group

Alternative schemes

Two alternative general classification schemes were recently published by Krot et al. (2003) and Weisberg et al. (2006), illustrating the lack of consensus on how to classify meteorites beyond the level of groups. In the Krot et al. scheme, the following hierarchy is used:

• Chondrites
• Nonchondrites
  • Primitive
  • Differentiated
    • Achondrites
    • Stony irons
    • Irons

And, in the Weisberg et al. scheme, meteorites groups are arranged as follows:

• Chondrites
• Primitive achondrites
• Achondrites

where irons and stony-irons are considered to be achondrites or primitive achondrites, depending on the group.

See also

• Asteroid spectral types

External links

• Meteorite frequencies by group - National History Museum
• Meteorite "family tree" - National History Museum