Angrites (Synonyms: fassaite achondrites)

General: The meteorites of this group are named for their type specimen, Angra Dos Reis, a fassaite-rich achondrite that fell near Rio de Janeiro, Brazil, in early 1869. During the last three decades, a few similar achondrites have been found, leading to the establishment of this most interesting group.

Description: The interior of most angrites in our collections is light to dark grey, often with a greenish hue, and they exhibit a black fusion crust. They are basaltic rocks with medium-grained, cumulate textures, often containing porous areas and abundant round vesicles with diameters up to 2.5 cm.

Mineralogy: Angrites are primarily composed of varying amounts of fassaite, i.e., a Ca-Al-Ti-rich pyroxene, anorthitic plagioclase, minor Ca-rich olivine, kirschsteinite, and other accessory minerals and phases.

Formation history: All angrites show rather ancient crystallization ages of about 4.56 billion years, suggesting their formation in the early days of our solar system. They are thought to have formed on one of the earliest differentiated asteroids from the igneous processing of CAI-rich chondritic matter, similar to carbonaceous chondrites of the CI or CM group. The vesicles in some angrites have been interpreted as remnants of gas-bubbles that formed prior to crystallization. However, current research suggests that the vesicles originally were solid spheres that have been exsolved in subsequent stages of rock-formation. Both theories are consistent with a magmatic origin of the angrites, making them the most ancient igneous rocks in our solar system.

Origin: Asteroidal. The reflectance spectra of the angrites resemble those of two main belt asteroids: 289 Nenetta, and 3819 Robinson. Further research will have to determine whether one of these asteroids actually represents the angrite parent body.

Members: This group comprises just 8 distinct members, and only two of them are widely available to private collectors: Our own find, the Sahara 99555, a single stone that was found in the Sahara in 1999, and D'Orbigny, an Argentinian find from 1979 that wasn't recognized as a meteorite until 1998.

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