Diogenites
Synonyms: chladnites, bronzite achondrites, hypersthene achondrites
General: The diogenites are named for a Greek philosopher of the 5th century B.C., Diogenes of Apollonia. Diogenes was actually the first to suggest that meteorites have their origin in space, a fundamental assumption that was subsequently forgotten for the next 2,000 years.
Description: Due to the fact that they are Ca-poor achondrites, the diogenites exhibit a dull black fusion crust, unlike the glossy crusts of howardites, and eucrites. In fresh individuals their medium to coarse-grained matrix is light gray to greenish, often exhibiting large green, translucent orthopyroxene crystals, while weathered desert finds are mostly dominated by brownish colors, due to terrestrial weathering. Brecciation is common.
Mineralogy: Mineralogically, the diogenites are composed primarily of coarse-grained Mg-rich orthopyroxene, with only minor amounts of olivine and plagioclase, and other accessory phases. Besides these monomict diogenites, two polymict diogenites have been described from Northwest Africa, as well as a few rare olivine diogenites. While polymict diogenites are primarily composed as all the other diogenites, except of the fact that they also contain up to 10% of eucritic clasts, the olivine diogenites represent a different rock type. Opposed to the conventional diogenites they are no orthopyroxenites but peridotites, mainly composed of olivine, with only minor orthopyroxene, and plagioclase.
Formation history: Their coarse-grained textures suggest a cumulate origin for the diogenites in magma chambers within the deeper regions of Vesta's crust. They are intrusive igneous rocks similar to plutonic rocks found on Earth, and they experienced much lower cooling rates than did the eucrites, which allowed the pyroxene to form sizeable crystals. Olivine diogenites are thought to represent even deeper layers of Vesta, and future research will have to find out what they have to tell about the formation history of their parent body.
Origin: Asteroidal. As the howardites and eucrites, the diogenites are thought to have their origin on the main belt asteroid Vesta - have a look at our introductory text about the HED group. As we have seen for the eucrites, it might very well be that some "diogenites" will have to be reclassified. A famous example of another meteorite being mistaken for a diogenite is ALH 84001, a now well-known Martian meteorite that had been misidentified due to the fact that it's also an orthopyroxenite. However, most diogenites in our collections are well documented, and it is pretty sure that they actually represent genuine samples of 4 Vesta.
Members: The diogenite group consists of just about 57 members if we exclude all probable pairings, especially those from Antarctica. Classical diogenites are e.g. Tatahouine, a unique member from Tunesia, with large green pyroxene crystals, Bilanga, a more recent fall from Burkina Faso, and our own find, Sahara 98111. Only two polymict diogenites are available to the private collectors community, NWA 1239, and NWA 1648, plus a few of their respective pairings. The same counts for the ultra-rare olivine diogenites - just two distinct members from Africa have been fully classified, and published so far, NWA 1459, and NWA 1877, although a few paired stones have been reported for the latter find.