Diogenites
Pyroxene-rich rocks from deep inside Vesta.
- Classification
- Stony meteorites › Achondrites › Asteroidal achondrites › HED meteorites: pieces of Vesta › Diogenites
- Share of witnessed falls
- about 1.1%
- In our catalogue
- 265 meteorites, 11 seen to fall
- Heaviest
- Johnstown, 40.3 kg
- Earliest recorded fall
- Manegaon, 1843
- Most recent fall
- Bilanga, 1999
Diogenites are coarse-grained rocks made mostly of orthopyroxene, sometimes with olivine. They crystallised slowly, deep in Vesta's lower crust or upper mantle, and were later dug up by impacts.
They are named after the Greek philosopher Diogenes of Apollonia, one of the first people to suggest that meteorites come from space. Tatahouine in Tunisia (1931) fell as thousands of bright green fragments.
Mineralogy and chemistry
Most diogenites are orthopyroxenites, made of 85% to almost 100% magnesium-rich orthopyroxene (about 20–30 mol% ferrosilite), with minor chromite, olivine, plagioclase, troilite, silica and metal. Olivine-bearing diogenites, sometimes called harzburgitic diogenites, contain up to about 40% olivine, and a few dunite-like rocks are nearly all olivine. Most diogenites are breccias of crushed pyroxene crystals, some originally several centimetres across.
Trace elements vary from stone to stone in ways that suggest diogenites did not all crystallise from one magma, but from many separate intrusions into Vesta's crust.
Parent body and formation
Diogenites crystallised slowly at depth, in the lower crust or upper mantle of Vesta, as pyroxene accumulated in magma chambers. Impacts, especially the giant Rheasilvia impact near the south pole, excavated them. Dawn found diogenite-rich areas in and around the Rheasilvia basin, as predicted, but surprisingly little olivine, suggesting Vesta's crust is thicker or its mantle less olivine-rich than once thought.
Diogenites sample deeper levels of Vesta than any other HED. Their trace elements and short-lived isotopes show that Vesta's lower crust was built from many pulses of magma over a few million years, rather than crystallising from one global magma ocean all at once. Olivine-rich diogenites may come close to Vesta's largely hidden mantle.
Name, abundance and famous examples
Diogenites are named after Diogenes of Apollonia, a Greek thinker of the 5th century BC who suggested that stones fall from the sky. They are about 1% of witnessed falls. Johnstown fell in Colorado on 6 July 1924, during a funeral service; about 40 kg was recovered. Tatahouine fell in southern Tunisia on 27 June 1931 and shattered into thousands of green fragments, about 12 kg in total. Shalka (India, 1850), Ibbenbüren (Germany, 1870), Ellemeet (Netherlands, 1925) and Bilanga (Burkina Faso, 1999) are other falls.
How to recognise one
A diogenite is pale yellowish green to grey-green inside, made of coarse, glassy-looking pyroxene grains, often crumbly. The fusion crust is black and can be glossy. It contains very little metal and is weakly magnetic or not at all. Similar terrestrial rocks exist, so laboratory analysis is needed.
Famous examples
- Johnstown Diogenite, United States, 1924 · 40.3 kg
- Tatahouine Diogenite, Tunisia, 1931 · 12 kg
- Shalka Diogenite, India, 1850 · 4 kg
- Ellemeet Diogenite, Netherlands, 1925 · 1.47 kg
- Bilanga Diogenite, Burkina Faso, 1999 · 25 kg
- Ibbenbüren Diogenite, Germany, 1870 · 2 kg
Related groups
- Howardites Broken rubble from the surface of asteroid Vesta, mixing crust and mantle rocks.
- Eucrites Volcanic basalt from the crust of Vesta.
- HED meteorites: pieces of Vesta The howardites, eucrites and diogenites: crust and mantle rocks from the giant asteroid 4 Vesta. The meteorites this site is named after.
- Mesosiderites Jumbled breccias of metal and basalt, made in a violent collision.
Photos shared under Creative Commons licences (Wikimedia Commons).