What is a pallasite and how did it form?
A pallasite is a stony-iron meteorite of olivine crystals in iron-nickel metal. Most likely formed where core metal and mantle olivine mixed, perhaps after an impact.
Pallasites are among the most beautiful meteorites, with golden-green olivine crystals set in shining metal. They are named after the German naturalist Peter Simon Pallas, who described a large mass found in Siberia in the 1770s, now called the Krasnojarsk meteorite.
What they contain. About half olivine and half metal by volume, with minor troilite, schreibersite, chromite and phosphates. The olivine crystals can be angular fragments or rounded grains, sometimes several centimetres across.
Groups. Most pallasites belong to the main group, whose metal resembles IIIAB irons. Smaller groups include the Eagle Station pallasites and the pyroxene pallasites, which come from different parent bodies.
How did they form? The classic idea was that pallasites formed at the boundary between a molten iron core and an olivine-rich mantle inside a differentiated asteroid. However, this raises problems: core-mantle boundaries would be deep and unlikely to be sampled in large amounts, and the metal of main-group pallasites does not match a simple core. Alternative ideas include molten metal from an impacting body being injected into the mantle of another, or mixing at shallower depths after a collision. Rounded olivine grains suggest the material stayed hot for a long time after mixing.
Magnetic evidence. Tiny grains in pallasite metal recorded magnetic fields long after they formed, suggesting their parent body still had a molten core producing a dynamo.
Famous pallasites include Brenham, Imilac, Esquel, Fukang and Seymchan. Slices are prized by collectors and museums.