Angrites

The geochemistry of angrites is characterized by strong silica undersaturation, by which we mean that there is not enough SiC 2 to combine with various cations to form common silicate minerals. The result is the formation of silica-poor minerals like kirschsteinite and nepheline. These meteorites also show strong depletions in moderately volatile elements. They are thought to have formed as partial melts of a chondritic source under oxidizing conditions. 

Iron meteorites offer the unique opportunity to examine metallic cores from deep within differentiated bodies. Most of these samples were exposed and dislodged when asteroids collided and fragmented. Although irons constitute only about 6% of meteorite falls, they are well represented in museum collections. Most iron meteorites show wide variations in siderophile-element abundances, which can be explained by processes like fractional crystallization in cores that mimic those in achondrites. However, some show perplexing chemical trends that may be inconsistent with their formation as asteroid cores. 

Once the chemical compositions of enough irons had been measured, it became clear that, although the various structural classes were reflected in chemical differences, chemical 

Widmanstatten pattern composed of intergrown plates of kamacite and taenite, in the Tres Castillos iron meteorite. The black phase is graphite, surrounded by troilite (dark gray), and the lighter gray phase is schreibersite. 

Siderophile element diagrams used to classify iron meteorites. Modified from Scott and Wasson (1975). 

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Direct measurements of the initial abundance of 53Mn in the early solar system have proven unreliable. CAIs, the first solids to form in the early solar system, are highly depleted in both manganese and chromium, both of which are moderately volatile. Operationally, the relative 53Mn-53Cr timescale has been anchored to an angrite, LEW 86010 (see above). Age differences have been calculated relative to LEW 86010 from... 

Lugmair, G. W. and Galer, S. J. G. (1992) Age and isotopic relationships among the angrites Lewis Cliff 86010 and Angra dos Reis. Geochimica et Cosmochimica Acta, 56, 1673-1694. 

Sahara 99555 Angra dos Reis Angrite parent body... 

Angrites Mesosiderites pallasites Aubrites Brachinites Ureilites HEP... 

Differentiated (planetary) Achondrites Angrites Aubrites Brachinites HED meteorites Eucrites Howardites Diogenites Ureilites Stony-irons Pallasites... 

The angrites are medium- to coarse-grained (up to 2-3 mm) igneous rocks of generally basaltic composition and consist mainly of Ca-Al-Ti-rich... 

McKay G., Crozaz G., Wagstaff J., Yang S.-R., and Lundberg L. (1990) A petrographic, electron microprobe, and ion microprobe study of mini-angrite Lewis Cliff 86010. In Lunar Planet. Sci. XXL The Lunar and Planetary Institute, Houston, pp. 771-772. 

Yanai K. (1994) Angrite Asuka-881371 priliminary examination of a unique meteorite in the Japanese collection of antarctic meteorites. Proc. NIPR Symp. Antarct. Meteorit. 7, 30-41. 

Differentiated, or evolved, achondrite types are rare. Other than the HED group, there are only two other known asteroidal sources, one for the angrites (six known meteorites) (Mittlefehldt et al., 2002) and one for the unique basalt NWA Oil (Yamaguchi et al., 2002). Oxygen isotopic... 

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