File:Angrite (NWA 2999 Meteorite) (4.558-4.562 Ga) 5.jpg

English: Angrite (NWA 2999 Meteorite) (cut slice, 2.4 centimeters across)

Angrites are a rare group of achondrite meteorites (= meteorites lacking rounded structures called chondrules) with a distinctive chemistry and mineralogy. This is a slice from an angrite discovered in 2004, the NWA 2999 Meteorite (NWA = "Northwest Africa"). It is reportedly paired with the NWA 3164, NWA 4662, and NWA 4931 Meteorites. The rock is a very dark brown peridotite (ultramafite). "Peridotite" is an igneous-sounding name, and it is ultimately plutonic in origin, but is has been altered and metamorphosed since it originally crystallized from melt.

Available mineral analysis data has indicated that the NWA 2999 angrite contains ~65% olivine, ~23% clinopyroxene, ~8% metallic iron, ~4% spinel, ~1% anorthite (calcium plagioclase feldspar), plus trace amounts of other minerals.

Dates on some of the 30 known angrites are between 4.557 to 4.562 billion years. Published dates on the NWA 2999 angrite range from 4.558 to 4.562 billion years.

Angrites have been interpreted as originating from the mantle of Planet Mercury. This idea was based on, for one thing, the lack of volatile elements in the rocks. The full reasoning behind angrites having a Mercurian origin is summarized in Irving et al. (2005) and Kuehner et al. (2006).

Other meteorite researchers (e.g., Ruzicka & Hutson, 2006; R. Malcuit, pers. comm.) have not accepted the Mercurian provenance interpretation. Data from Planet Mercury that has been acquired by the MESSENGER spacecraft in 2011 & 2012 now show that angrites are not from Mercury (Irving et al., 2013). [Note: an olivine gabbro meteorite, designated Northwest Africa 7325, has recently been identified as being more consistent with the known geochemistry of Mercury rocks - see Irving et al., 2013]

The lack of volatile elements in angrites does argue for an origin in proximity to the Sun, possibly in the vulcanoid belt. The term “vulcanoids” refers to the hypothetical existence of several bodies (how many?) that used to orbit the Sun closer than Mercury. They have left their original orbits by one or more planetary migration events.

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Additional info. on this particular meteorite can be found in Gellissen et al. (2007) and Humayun et al. (2007).

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Info. mostly synthesized from:

Irving et al. (2005) - American Geophysical Union Fall Meeting Abstracts

Kuehner et al. (2006) - 37th Lunar and Planetary Science Conference Abstracts

Ruzicka & Hutson (2006) - 69th Annual Meteoritical Society Meeting Abstracts

Gellissen et al. (2007) - 38th Lunar and Planetary Science Conference Abstracts

Humayun et al. (2007) - 38th Lunar and Planetary Science Conference Abstracts

Markowski et al. (2007) - Earth and Planetary Science Letters 262(1-2): 214-229.

Amelin & Irving (2007) - Workship on Chronology of Meteorites abstract 4061.

Shukolyukov & Lugmair (2008) - 39th Lunar and Planetary Science Conference Abstracts

Adam Hupé (pers. comm.)