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D’Orbigny angrite

Alcide d’Orbigny was a French naturalist who explored South America from 1826 to 1833. The 16.55kg meteorite was found near the Argentine settlement that bears his name, about 500 km southwest of Buenos Aires.

Angrites are very old achondrites and probably come from a large parent body. They have a wide distribution of cosmic ray exposure ages suggesting that the parent body or fragments of it might still exist. Candidates include the asteroids 289 Nenetta, 3819 Robinson, 5261 Eureka and the planet Mercury. Eureka is a Mars trojan orbiting in its trailing Lagrangian point. Other smaller bodies at L5, might be rubble detached from Eureka by its fast spin. Rotational period is just 2.69 hours.

Angrites are igneous rock consisting of laths of plagioclase, augite, a pyroxene containing calcium and, in this case, some titanium and aluminum and olivine.

 

 

 

 

Salle d’Orbigny in the National Museum of Natural History in Paris welcomes visitors and houses the explorer’s collection.

 

 

All photos are from this one 27mm long thin section. Cross-polarized light (XPL).

 

 

The circular feature in the top left is one of the vacuoles for which d’Orbigny is famous. This one is empty. The pattern is an artifact from the thin section’s manufacture. FOV=8.3mm, XPL.

 

 

Colorful mineral grains are augite and olivine. White and gray skeletal laths are plagioclase. FOV=3mm, XPL.

 

 

Some skeletal vacancies are filled with olivine or augite. FOV=0.5mm, XPL.

 

 

Interesting radial pattern. FOV=3mm, XPL.

 

 

In plane polarized light (PPL) the elements are dark, telling us they are augite.

 

 

Closer. FOV=0.5mm, XPL.

 

 

Maybe the dark grain in the center was the nucleation point for all the leaves. FOV=0.5mm, PPL.

 

 

A six sided dark grain embedded in augite. FOV=0.3mm, PPL.

 

 

Titanium colors the augite pinkish-tan. Ti levels are higher toward the outside of crystals. FOV=3mm, PPL.

 

 

Symplectic textures, intergrowths between augite and probably spinel. FOV=0.2mm, PPL.

 

 

FOV=0.2mm, PPL.

 

 

FOV=0.3mm, PPL.

 

 

Likely iron sulfide. Oblique incident light. FOV=0.4mm.

 

 

Oblique incident light. FOV=0.4mm.
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