Group · Stony-iron meteorites

Pallasites

Gem-like olivine crystals set in iron-nickel metal.

Classification: Stony-iron meteorites › Pallasites
Share of witnessed falls: about 0.4%
In our catalogue: 94 meteorites, 4 seen to fall
Heaviest: Brenham, 4.3 t
Earliest recorded fall: Mineo, 1826
Most recent fall: Omolon, 1981

Pallasites contain large olive-green crystals of olivine embedded in iron-nickel metal. The olivine may be single crystals, rounded or angular clusters, or veins running through the metal. Cut and polished slices glow like stained glass when light shines through them, making pallasites among the most sought-after meteorites.

They are named after the German naturalist Peter Simon Pallas, who described the Krasnojarsk iron-and-crystal mass found in Siberia in 1749. There are three groups: the main group (most pallasites), the Eagle Station grouplet and the pyroxene pallasites, each from a different parent body.

The classic idea is that pallasites come from the boundary between the metal core and the olivine mantle of a melted asteroid. Others argue there are too many pallasites and too few olivine-rich meteorites for that, and suggest that molten metal from one body was injected into the mantle of another during a collision. Only four pallasites have been seen to fall.

Mineralogy and chemistry

A pallasite is typically about half olivine by volume, with crystals usually 5 mm to a few centimetres across, set in iron-nickel metal. The metal is kamacite and taenite, often with plessite, and contains troilite, schreibersite and chromite. Phosphate minerals, some found only in pallasites, occur at the olivine margins. The olivine is magnesium-rich, around 11–14 mol% fayalite in the main group, gem-quality where it is unfractured; peridot from pallasites has been cut as a gemstone.

Textures vary. Some pallasites, such as Esquel and Imilac, have angular, broken olivine fragments; others, such as Brenham and Springwater, have rounded crystals; a few, such as Seymchan, have large areas of metal with no olivine at all.

Parent bodies and formation

For many years pallasites were pictured as samples of the boundary between an asteroid's metal core and its olivine mantle. Several findings complicate that. Main-group pallasite metal resembles the most evolved IIIAB irons, the last liquid of a crystallising core, yet the two do not share a parent body in detail. Their olivine cooled at rates that vary from one pallasite to another, and magnetic signals in tiny metal grains within the olivine record a magnetic field generated by a liquid core after the pallasites had cooled. That fits pallasites forming in pockets nearer the surface, where molten metal from an impactor or from the core was injected among mantle olivine, while the core below remained liquid.

The three groups, main group, Eagle Station and pyroxene pallasites, have different oxygen isotopes and must come from at least three parent bodies, and some ungrouped pallasites add more.

Abundance and famous examples

Around a hundred pallasites are known, about 0.4% of falls. The four seen to fall are Mineo (Sicily, 1826), Zaisho (Japan, 1898), Marjalahti (Karelia, 1902) and Omolon (Russia, 1981), whose main mass of about 250 kg was found two years after the fall. Krasnojarsk, a mass of about 700 kg found in Siberia in 1749 and described by Peter Simon Pallas in the 1770s, gave the group its name; Ernst Chladni argued in 1794 that it must have fallen from space. Imilac (Atacama Desert), Esquel (Argentina), Fukang (China) and Brenham (Kansas) are celebrated finds.

How to recognise one

A pallasite is unmistakable when cut: translucent yellow-green to amber crystals in shining metal. Weathered surfaces are rusty brown with dark, glassy olivine showing through. They are very heavy and strongly magnetic. Never try to cut or polish one with water-based methods at home; the metal rusts quickly, and the olivine can crack.

Groups

Famous examples

Imilac Pallasite, PMG, Chile, 1822 · 920 kg
Esquel Pallasite, PMG, Argentina, 1951 · 755 kg
Fukang Pallasite, PMG, China, 2000 · 1 t
Brenham Pallasite, PMG-an, United States, 1882 · 4.3 t
Springwater Pallasite, PMG-an, Canada, 1931 · 67.6 kg

Related groups

Main-group pallasites Nine in ten pallasites: magnesium-rich olivine in metal related to the IIIAB irons.
Eagle Station pallasites A tiny grouplet of pallasites with iron-rich olivine and nickel-rich metal, from a separate parent body.
Pyroxene and ungrouped pallasites Pallasites that fit neither main group nor Eagle Station, among them the pyroxene-bearing pair Vermillion and Yamato 8451.
IIIAB irons The most common iron meteorites: medium octahedrites from one asteroid core.
Mesosiderites Jumbled breccias of metal and basalt, made in a violent collision.

See all 94 Pallasites

Photos shared under Creative Commons licences (Wikimedia Commons).