Clinopyroxene-rich achondrites
Angrites:
Named after Angra dos Ries in Rio de Janeiro, Brazil in 1869.
A small group of meteorites composed of 93% pyroxene (fassaite), olivine with a considerable proportion of calcium, and an anorthite rich plagioclase.
Accessory minerals include Mg-kirschsteinite, hercynitic spinel, merrillite, NiFe, troilite, celsian, baddelyite and Ti-magnetite.
That is, they have the components typically found in CAI and must have come from a body rich in high temperature condensates. Partial melting of a chondritic parent under slightly more oxidizing conditions than needed to produce the eucrites, could have formed the angrites.
They all have ages of 4.56 b.y.
Nakhlites:
Named after a town in Egypt where they fell in 1911, supposedly killing a dog in the process.
These are cumulates formed of 75% diopside with lesser amounts of olivine and accessory oligoclase, sanidine, Cr-Ti magnetite with exsolved ilmenite, pyrite with marcasite lamellae, troilite, chalcopyrite, and Fe-Cl apatite.
They formed under conditions of low oxygen fugacity of about 10-12 to 10-15.
Formed by igneous crystallization at 1.24 b.y.
Volatiles are similar to terrestrial ocean-ridge basalts.
The presence of hydrated minerals suggests to some that they were exposed to an environment in which water was present.
In same part of oxygen isotope diagram as shergottite, chassignites, ALH84001 and nakhlites.
Shergottites:
Named after Shergotty in India where they fell in 1865. Several different types have been recognized.
Basaltic shergottites consist primarily of plagioclase and pyroxenes (pigeonite and augite) which are relatively fine grained, suggesting formation as lava flows or shallow intrusions. Pyroxene cores contain amphiboles which of necessity indicates a magma containing water.
Lherzolitic shergottites are much coarser grained and are ultramafic cumulates consisting mostly of olivine and orthopyroxene with minor plagioclase and oxides.
One Antarctic meteorite contains clasts of both rock types.
They are monomict breccias which have been heavily altered by shock metamorphism at pressures to 300 kb producing maskelynite.
They were produced in a primary igneous event at 0.65 to 1.2 b.y, a shock event at 165 m.y. and have cosmic ray exposure ages of 2.5 m.y.
The following image shows a shergottite.
Eucrites:
The name is derived from the Greek eukritos meaning "easily distinguished".
The look like terrestrial basalts but they are not. Many are monomict breccias.
They consist primarily of a pigeonite (Ca-poor) pyroxene and a Ca-rich feldspar.
The pyroxenes exhibit exsolution, indicating that the rocks formed at shallow (0.5 to 12 km) depth.
Clasts include groups with
1) flat REE patterns, negligible Eu anomalies, REE x8 to x10 chondrites and Fe/Fe+Mg = 0.6. These could form as a 10% partial melt of a chondrite.
2) higher REE, negative Eu anomaly, higher Fe/Fe+Mg. Could be a residual liquid following fractional crystallization of 1).
3) cumulate of 4.56 and 4.41 b.y. age with adcumulate.
The following image is of a eucrite.
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