achondrite

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A Eucrite achondrite from the Millbillillie meteorite shower.

An achondrite (from the Greek a-, meaning without) 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. As a result, achondrites have distinct textures and mineralogies indicative of igneous processes. ^[1]

Achondrites account for about 8% of meteorites overall, and the majority (about two thirds) of them are HED meteorites, originating from the crust of asteroid 4 Vesta. Other types include Martian, Lunar, and several types thought to originate from as-yet unidentified asteroids other than Vesta. These groups have been determined on the basis of e.g. the Fe/Mn chemical ratio and the ¹⁷O/¹⁸O oxygen isotope ratios, thought to be characteristic "fingerprints" for each parent body.^[2]

Achondrites are classified into the following groups:

• Primitive achondrites (PAC group)
  • Acapulcoites
  • Lodranites
  • Brachinites
  • Winonaites
  • Ureilites
• Asteroidal achondrites (evolved achondrites)
  • HED (Vesta) meteorites
    • Howardites
    • Eucrites
    • Diogenites
    • Dunite diogenite
    • Olivine diogenites
  • Angrites
  • Aubrites
• Lunar meteorites (Lunanites)
  • Impact breccias
  • Lunar Basalts
• Martian meteorites (SNC)
  • Shergottites
  • Nakhlites
  • Chassignites
  • OPX martian meteorites (ALH 84001)

References

1. ^ B. Mason, Meteorites. John Wiley, New York 1962.
2. ^ Mittlefehldt, McCoy, Goodrich and Kracher: Non-chondritic Meteorites from Asteroidal Bodies, In: Reviews in Mineralogy Vol. 36, Planetary Materials, Mineralogical Society of America, 1998.

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