Broadly speaking, an achondrite is a stony meteorite that formed from a melt on its parent body. Thus achondrites have differentiated compositions, having lost a large fraction of their primordial metal content, and generally do not contain chondrules. There are many different groups of achondrites, some of which can be linked together to form associations allied with specific parents. The separate associations have little, if any, genetic relationship to each other.
Acapulcoite-Lodranite association: two groups of achondrites that show a range of properties that grade into each other, with similar oxygen isotopic compositions. They are thought to be partial melts of chondritic precursors, and have been described as primitive achondrites, suggesting that they are a bridge between chondrites and achondrites. Type specimens: Acapulco, Lodran.
Angrites: medium- to coarse-grained basaltic igneous rocks. Although the angrites have similar oxygen isotopic compositions to the HEDs, they are unrelated. Type specimen: Angra dos Reis.
Aubrites: or enstatite achondrites, are highly reduced meteorites with mineralogies and oxygen isotopic compositions similar to those of enstatite chondrites, leading to the suggestion that aubrites might have formed by partial melting of an enstatite chondrite precursor. Type specimen: Aubres.
Brachinites: olivine-rich igneous rocks with approximately chondritic bulk composition, and oxygen isotopic compositions similar to the HEDs. Like the acapulcoite-lodranite association, brachinites are considered as primitive achondrites. Type specimen: Brachina.
Howardite-Eucrite-Diogenite association (HED): a suite of generally brecciated igneous rocks ranging from coarse-grained orthopyroxenites (diogenites) to cumulates and fine-grained basalts (eucrites). The howardites are regolith breccias, rich in both solar wind gases and clasts of carbonaceous material. The HEDs all have similar oxygen isotopic compositions; one candidate for the HED parent body is the asteroid 4 Vesta. Examples are Stannern (eucrite), Johnstown (diogenite) and Kapoeta (howardite)
Ureilites: a group of carbon-rich igneous rocks, whose origin and genesis is uncertain. They have been described as partial melt residues, and as igneous cumulates. They show a wide range in oxygen isotopic compositions, and so have not achieved isotopic equilibration on a single parent body. Type specimen: Novo-Urei.
Winonaites: are closely related to the silicate inclusions in IAB irons, and may derive from the same parent body. They are also described as primitive achondrites. Type specimen: Winona.