Eucrites
Synonyms: pigeonite-plagioclase achondrites
General: The term eucrite is an old petrologic designation, derived from the Greek word "eukritos" meaning "easily distinguished". Although eucrites are in fact easily distinguished from chondrites, they closely resemble terrestrial basalts. Eucrites are extraterrestrial basalts, igneous rocks representing the crust of their parent body, the asteroid 4 Vesta.
Description: Due to the fact that all eucrites are Ca-rich rocks, they usually develop a glossy black fusion crust. In contrast to that crust, their interiors are white to light gray, showing a typical basaltic intergrowth of medium to fine-grained pyroxenes and feldspars. Most eucrites are brecciated rocks, and only very few members are unbrecciated, such as HAH 262, or the anomalous eucrite NWA 1240.
Mineralogy: Eucrites are primarily composed of the Ca-poor pyroxene, pigeonite, and the Ca-rich plagioclase, anorthite. Accessory minerals such as silica, chromite, troilite and nickel-iron metal are common, and many other minor phases have been identified in the various members of this group. Based on mineralogical and chemical differences, the eucrites have been further divided into three distinct subgroups: the non-cumulate group, the cumulate group, and the polymict group.
Formation history: The members of the non-cumulate group represent the upper crust of Vesta that solidified on a magma ocean after the core and the mantle of Vesta had already been formed by forces of igneous fractionation, gravitational segregation, and subsequent crystallization. Cumulate eucrites are the products of the gravitational settling of crystallized minerals, primarily pyroxene and plagioclase, within magma chambers trapped below Vesta's early crust. The members of the polymict group are closely related to the howardites, and they represent polymict breccias that contain more than 90% eucritic material and less than 10% diogenitic clasts. They formed through major impacts, mixing material from the deeper layers of Vesta, such as diogenitic clasts, into the eucritic matrix.
Origin: Asteroidal. As the howardites and diogenites, the eucrites are thought to have their origin on the main belt asteroid Vesta - have a look at our introductory text about the HED group. However, it's more than likely that several unusual eucrites will be reclassified in the future, as has been done for the now ungrouped basaltic achondrites Ibitira, and NWA 011. Both had initially been classified as eucrites until more recent studies showed that they are genetically unrelated to the HED group, and that they must have formed on another differentiaded asteroid, similar but not identical to 4 Vesta.
Members: Representing the most common class of achondrites, more than 130 eucrites have been identified so far, excluding all probable pairings. The non-cumulate group comprises a lot of historical falls, such as Juvinas, Stannern, Béréba, and Jonzac, as well as an abundance of hot deserts finds, such as our own find, Sahara 98110. Cumulate eucrites are more rare, and famous members, such as Moore County, are impossible to get. More recent cumulate eucrite falls, such as Talampaya, are sometimes for sale, but usually the collector will have to go with desert finds, such as Dhofar 007, to close that gap in in his collection. Polymict eucrites are also rare, and they are represented by famous falls, such as Pasamonte, USA, and Millbillillie, Australia, as well as by various desert finds, such as NWA 1109 from Northwest Africa.