Angrite
A rare, extremely old basaltic achondrite rich in calcium and aluminium and strongly depleted in volatile elements.
Angrites are a small group of basaltic achondrites, numbering only a few dozen named specimens. Their mineralogy is distinctive: aluminium- and titanium-bearing calcium-rich clinopyroxene (once called fassaite), almost pure anorthite plagioclase, and calcium-rich olivine, sometimes including kirschsteinite. They are severely depleted in moderately volatile elements such as sodium and potassium, and they formed under relatively oxidising conditions.
Their importance lies in time-keeping. Some angrites crystallised within a few million years of the first solids of the Solar System and then cooled quickly with little later disturbance. That makes them ideal anchors for cross-calibrating dating systems, including lead–lead, aluminium–magnesium, manganese–chromium and hafnium–tungsten. The group is named after Angra dos Reis, which fell in Brazil in 1869. D'Orbigny, found in Argentina, is one of the best studied, with quenched textures and vesicles that point to rapid cooling of lava near the surface.
Angrites cannot be identified by eye with confidence; they are dark, fine- to medium-grained basalts. Classification relies on oxygen isotopes, which plot on their own line, and on the unusual pyroxene and olivine compositions measured by electron microprobe.
Related terms
Level: Expert Advanced