Meteorites classification
Encyclopedia
The ultimate goal of meteorite classification is to group all meteorite
Meteorite
A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...

 specimens that share a common origin on a single, identifiable parent body. This could be a planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

, asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

, Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

, or other current Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

 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 comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...

s) to achieve such a classification. Instead, modern meteorite classification relies on placing specimens into "groups" in which all members share certain key physical, chemical
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....

, isotopic
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...

, and mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...

ogical properties consistent with a common origin on a single parent body, even if that body is unidentified. Several meteorite groups classified this way may come from a single, heterogeneous parent body or 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 chondrite
Chondrite
Chondrites are stony meteorites that have not been modified due to melting or differentiation of the parent body. They formed when various types of dust and small grains that were present in the early solar system accreted to form primitive asteroids...

s, 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 chondrite
Carbonaceous chondrite
Carbonaceous chondrites or C chondrites are a class of chondritic meteorites comprising at least 7 known groups and many ungrouped meteorites. They include some of the most primitive known meteorites...

s, enstatite chondrite
Enstatite chondrite
Enstatite chondrites are a rare form of meteorite thought to comprise only about 2% of the chondrites that fall on Earth. Only about 200 E-Type chondrites are currently known....

s, and ordinary chondrite
Ordinary chondrite
The Ordinary chondrites are a class of stony chondritic meteorites. They are by far the most numerous group and comprise about 87% of all finds...

s). 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 that do not fit any known group, though 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 meteorite
Iron meteorite
Iron meteorites are meteorites that consist overwhelmingly of nickel-iron alloys. The metal taken from these meteorites is known as meteoric iron and was one of the earliest sources of usable iron available to humans.-Occurrence:...

s), 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
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...

 inclusions
Inclusions
Inclusions are non-living substances that may or may not be present in a cell, depending on the cell type. Inclusions are stored nutrients, secretory products, and pigment granules...

 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 chondrule
Chondrule
Chondrules are round grains found in chondrites. Chondrules form as molten or partially molten droplets in space before being accreted to their parent asteroids...

s), and achondrite
Achondrite
An achondrite is a stony meteorite that does not contain chondrules. It consists of material similar to terrestrial basalts or plutonic rocks and has been differentiated and reprocessed to a lesser or greater degree due to melting and recrystallization on or within meteorite parent bodies...

s (groups of meteorites that have a complex origin involving asteroidal or planetary differentiation
Planetary differentiation
In planetary science, planetary differentiation is the process of separating out different constituents of a planetary body as a consequence of their physical or chemical behaviour, where the body develops into compositionally distinct layers; the denser materials of a planet sink to the center,...

). The iron meteorites were traditionally divided into objects with similar internal structures (octahedrite
Octahedrite
Octahedrites are a class of iron meteorites within the structural classification. They are the most common class of iron meteorites.They are composed primarily of the nickel-iron alloys: taenite - high nickel content, and kamacite - low nickel content....

s, hexahedrite
Hexahedrite
Hexahedrites are a type of iron meteorite.They are composed almost exclusively of the nickel-iron alloy kamacite and are lower in nickel content than the octahedrites. The nickel concentration in hexahedrites is always below 5.8% and only rarely below 5.3%....

s, and ataxite
Ataxite
Ataxites are a class of iron meteorites. They are composed mainly of the alloy taenite, and also contain plessite, troilite, and microscopic lamellae of kamacite. They have no visible Widmanstätten pattern. Ataxites are the most nickel-rich meteorites known; they usually contain over 18% nickel...

s), 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 pallasite
Pallasite
A pallasite is a type of stony–iron meteorite.-Structure and composition:It consists of cm-sized olivine crystals of peridot quality in an iron-nickel matrix. Coarser metal areas develop Widmanstätten patterns upon etching...

s (which are now known to comprise several distinct groups) and mesosiderite
Mesosiderite
Mesosiderites are a class of stony–iron meteorites consisting of about equal parts of metallic nickel-iron and silicate. They are breccias with an irregular texture; silicates and metal occur often in lumps or pebbles as well as in fine-grained intergrowths...

s (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
    • Chondrite
      Chondrite
      Chondrites are stony meteorites that have not been modified due to melting or differentiation of the parent body. They formed when various types of dust and small grains that were present in the early solar system accreted to form primitive asteroids...

      s
      • Carbonaceous chondrite
        Carbonaceous chondrite
        Carbonaceous chondrites or C chondrites are a class of chondritic meteorites comprising at least 7 known groups and many ungrouped meteorites. They include some of the most primitive known meteorites...

         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
              Allende meteorite
              The Allende meteorite is the largest carbonaceous chondrite ever found on Earth. The fireball was witnessed at 1:05 a.m. on February 8, 1969, falling over the Mexican state of Chihuahua. After breaking up in the atmosphere, an extensive search for pieces was conducted and it is often described as...

              -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
        Ordinary chondrite
        The Ordinary chondrites are a class of stony chondritic meteorites. They are by far the most numerous group and comprise about 87% of all finds...

         class
        • H chondrite
          H chondrite
          The H type ordinary chondrites are the most common type of meteorite, accounting for approximately 40% of all those catalogued, 46% of the ordinary chondrites, and 44% of the chondrites....

           group
        • L chondrite
          L chondrite
          The L type ordinary chondrites are the second most common type of meteorite, accounting for approximately 35% of all those catalogued, and 40% of the ordinary chondrites....

           group
        • LL chondrite
          LL chondrite
          The LL chondrites are a class of stony meteorites, the least abundant group of the ordinary chondrites, accounting for about 10–11% of observed ordinary-chondrite falls and 8–9% of all meteorite falls ....

           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)
    • Achondrite
      Achondrite
      An achondrite is a stony meteorite that does not contain chondrules. It consists of material similar to terrestrial basalts or plutonic rocks and has been differentiated and reprocessed to a lesser or greater degree due to melting and recrystallization on or within meteorite parent bodies...

      s
      • Primitive achondrites
        • Acapulcoite group
        • Lodranite group
        • Winonaite group
      • Asteroidal achondrites
        • HED meteorite
          HED meteorite
          HED meteorites are a subgroup of achondrite meteorites. HED stands for "Howardite–Eucrite–Diogenite".These achondrites came from a differentiated parent body and experienced extensive igneous processing not much different from the magmatic rocks found on Earth and for this reason they closely...

           clan (possibly from asteroid 4 Vesta
          4 Vesta
          Vesta, formally designated 4 Vesta, is one of the largest asteroids, with a mean diameter of about . It was discovered by Heinrich Wilhelm Olbers on March 29, 1807, and is named after the Roman virgin goddess of home and hearth, Vesta....

          , also called basaltic achondrites)
          • Howardite
            Howardite
            Howardites are achondritic stony meteorites that originate from the surface of the asteroid 4 Vesta, and as such are part of the HED meteorite group. There are about 200 distinct members known.-Characteristics:...

             group
          • Eucrite
            Eucrite
            Eucrites are achondritic stony meteorites, many of which originate from the surface of the asteroid 4 Vesta and as such are part of the HED meteorite group. They are the most common achondrite group with well over 100 distinct finds at present....

             group
          • Diogenite
            Diogenite
            Diogenites are a subgroup of the HED meteorite group, a type of achondritic stony meteorites.-Origin and composition:Diogenites originates from deep within the crust of the asteroid 4 Vesta, and as such are part of the HED meteorite group...

             group
        • Angrite
          Angrite
          Angrite is a rare achondrite consisting mostly of the mineral augite with some olivine, anorthite and troilite. It is named for the Angra dos Reis meteorite. Angrites are basaltic rocks, often having porosity, with vesicle diameters of up to 2.5 cm. Angrites are the oldest igneous rocks, with...

           group
        • Aubrite
          Aubrite
          Aubrites are meteorites named for Aubres, a small achondrite meteorite that fell near Nyons, France, in 1836. They are primarily composed of the orthopyroxene enstatite, and are often called enstatite achondrites...

           group (enstatite achondrites)
        • Ureilite
          Ureilite
          Ureilite is a rare type of stony meteorite that has a unique mineralogical composition very different from that of other stony meteorites. This dark grey or brownish meteorite type is named after the village Novy Urey , Mordovia Republic of Russia, where a meteorite of this type fell on 4 September...

           group
        • Brachinite group
      • Lunar meteorite
        Lunar meteorite
        A Lunar meteorite is a meteorite that is known to have originated on the Moon.-Discovery:In January 1982, John Schutt, leading an expedition in Antarctica for the ANSMET program, found a meteorite that he recognized to be unusual...

         group
      • Martian meteorite group (sometimes called "SNC meteorites")
        • Shergottites
        • Nakhlites
        • Chassignites
        • Other Martian meteorites, e.g., ALH84001
          ALH84001
          Allan Hills 84001 is a meteorite that was found in Allan Hills, Antarctica on December 27, 1984 by a team of U.S. meteorite hunters from the ANSMET project. Like other members of the group of SNCs , ALH 84001 is thought to be from Mars. However, it does not fit into any of the previously...



  • II) Stony–iron meteorites
    • Pallasite
      Pallasite
      A pallasite is a type of stony–iron meteorite.-Structure and composition:It consists of cm-sized olivine crystals of peridot quality in an iron-nickel matrix. Coarser metal areas develop Widmanstätten patterns upon etching...

      s
      • Main group pallasites
      • Eagle station pallasite grouplet
      • Pyroxene pallasite grouplet
    • Mesosiderite
      Mesosiderite
      Mesosiderites are a class of stony–iron meteorites consisting of about equal parts of metallic nickel-iron and silicate. They are breccias with an irregular texture; silicates and metal occur often in lumps or pebbles as well as in fine-grained intergrowths...

       group


  • III) Iron meteorite
    Iron meteorite
    Iron meteorites are meteorites that consist overwhelmingly of nickel-iron alloys. The metal taken from these meteorites is known as meteoric iron and was one of the earliest sources of usable iron available to humans.-Occurrence:...

    s
    • 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
        IIE iron meteorite
        The iron meteorites of the IIE chemical type are octahedrites of various coarseness, most of which contain numerous inclusions of recrystallized stony silicates....

         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
      • Achondrite
        Achondrite
        An achondrite is a stony meteorite that does not contain chondrules. It consists of material similar to terrestrial basalts or plutonic rocks and has been differentiated and reprocessed to a lesser or greater degree due to melting and recrystallization on or within meteorite parent bodies...

        s
      • Stony irons
      • Irons
        Iron meteorite
        Iron meteorites are meteorites that consist overwhelmingly of nickel-iron alloys. The metal taken from these meteorites is known as meteoric iron and was one of the earliest sources of usable iron available to humans.-Occurrence:...



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.

External links

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