If a small planetary body is sufficiently heated (to above 1200° C) it will melt with the heavy elements, such as iron and nickel descending to the molten interior. The original material composing the body forms layers with distinct compositions.
The core (center) region is metallic and composed primarily of iron and nickel with the addition of trace amounts of other elements such as iridium.
Above the core is the mantle, which is rich in olivine. Pallasites originate in the mantle and contain distinctive olivine crystals embedded in a nickel/iron matrix. Iron meteorites are fragments of the parent's core itself.
Above the mantle lies the crust. This is a basaltic type rock and is the region where the following meteorite types originate.
Eucrites are meteorites originating from the surface of such a body. They are low in the elements that tend to accumulate in the core and thus richer in the lighter minerals such as the silicates. Eucrites are similar to earthly basalts with the typical very fine crystal structure.
Diogenites are composed of minerals similar to the Eucrites but that have cooled slowly, well below the surface of the parent body. The larger crystal grain distinguishes Diogenites.
Howardites are samples of regolith composed of a mixture of Eucrite, Diogenite, and the remains of the impacting bodies that cratered the surface of the parent body.
Isotopic analysis indicates that the differentiated bodies formed between 4.56 and 4.45 billion years ago. The lava that formed the Eucrites dates from between 4.53 and 4.56 billion years ago. The heating that produced this differentiation occurred at the same time as the heating which altered the chondrite parent bodies, but to a much more severe degree.
Howardites, Eucrites, and Diogenites are known as HED meteorites. They are frequently referred to as achondrites.
There are other achondrites recognized as well. Their origin is poorly understood. These rare meteorites are called Aubrites, Angrites, and Urelites. Urelites are the only achondrites found to contain significant free metal.
Aubrites, also called enstatite achondrites, are composed mostly of the orthopyroxene Enstatite (MgSiO3). Aubrites are named for a meteorite that fell near Nyons, France in 1836. Aubrites have a white interior and a light-colored fusion crust. The suite of accessory minerals found in aubrites indicate their origin is magmatic with reducing conditions present. Most aubrites are brecciated indicating a violent collisional history. The light spectra of aubrites is similar to asteroid 44 Nysa and a small near-earth asteroid 3103 Eger. Eger is suspected of being the actual parent body of the aubrite meteorites.
Angrites are composed of augite with some olivine and troilite. Angrites are named for the Angra dos Reis meteorite which fell in Rio de Janerio, Brazil in 1869. Angrites are basaltic with small round vesicles to 2.6 centimeters in diameter. Angrites are the oldest igneous rocks known to date having formed about 4.55 billion years ago. Thought to have formed on one of the earliest differentiated planetesimals, two potential parents bodies have been identified, 289 Nenetta and 3819 Robinson.
Ureilites are named for Novo Urei, Morodava Republic, Russia where several meteorites fell in 1886. The origin of ureilites is enigmatic. Mineral and isotopic compositins suggest they represent primitive achondrites that have been subjected to at least partial melting. Other chemical characteristics suggest that ureilites come from a moderately differentiated C-class asteroid that had been in a collision and then re-cooled. Recent ureilite finds do not indicate they have been shocked so perhaps we are studying ureilites with differing origins and histories.
Meteorites; Their Impact on Science and History. Brigitte Zanda and Monica Rotaru Cambridge Press. 2001
Meteorites. Alain Carion