Main-group pallasites
Nine in ten pallasites: magnesium-rich olivine in metal related to the IIIAB irons.
- Classification
- Stony-iron meteorites › Pallasites › Main-group pallasites
- In our catalogue
- 49 meteorites, 3 seen to fall
- Heaviest
- Brenham, 4.3 t
- Earliest recorded fall
- Zaisho, 1898
- Most recent fall
- Omolon, 1981
The main-group pallasites (PMG) are the classic pallasites: centimetre-sized crystals of yellow-green olivine set in iron-nickel metal. They are by far the largest pallasite group and include almost every famous example.
Mineralogy and chemistry
Main-group pallasites (PMG) contain magnesium-rich olivine, mostly 11–14 mol% fayalite, set in metal with about 8% to 13% nickel. Their metal is chemically similar to the most nickel-rich, evolved IIIAB irons: high in nickel and gold, low in iridium. Their oxygen isotopes lie below the terrestrial line, with a Δ17O of about −0.2 to −0.3 per mil, close to the HED meteorites, though careful measurements now separate the two.
A few main-group pallasites are called anomalous (PMG-an) because their olivine is more iron-rich or their metal has unusual chemistry. Brenham, Springwater, Huckitta and Krasnojarsk are examples.
Parent body and formation
The main group comes from a single parent body that melted and formed a metal core and olivine mantle. Cooling rates measured from the metal, a few degrees per million years, and from the olivine show slow cooling, deep but not at the very centre. Studies of Imilac and Esquel found magnetic signals in nanometre-scale metal grains, recording a magnetic field created by a liquid core well after the pallasite metal had solidified. This suggests the pallasites sat in the mantle or near the surface, perhaps where molten metal from an impact was injected into mantle rock, while the core remained molten below.
Lead and other dating methods show the metal and olivine formed within the first ten million years or so of the Solar System.
Abundance and famous examples
Main-group pallasites make up the large majority of known pallasites, and all four witnessed pallasite falls are main group or close to it. Imilac, from the Atacama Desert of Chile, was known from 1822, and about 920 kg has been collected; its olivine is angular and its metal is often skeletal after weathering. Esquel, a 755 kg mass found in Argentina in 1951, has gem-quality olivine. Fukang, found in Xinjiang, China, in 2000, weighed about a tonne. Brenham in Kansas was found from 1882 around a small impact crater. Seymchan, found in Russia in 1967 and first classed as an iron, revealed olivine-bearing parts in 2004. Huckitta in Australia's Northern Territory, about 2.3 tonnes, is heavily weathered.
How to recognise one
Main-group pallasites show olivine crystals evenly scattered through metal, or clusters of crystals, with bright metal on cut faces. Their olivine is clear yellow-green, unlike the darker, more iron-rich olivine of the Eagle Station pallasites. Fresh slices need sealing, as moisture causes rust that can stain and crack the crystals.
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
- Seymchan Pallasite, PMG, Russia, 1967 · 323.3 kg
- Omolon Pallasite, PMG, Russia, 1981 · 250 kg
- Marjalahti Pallasite, PMG, Russia, 1902 · 45 kg
- Krasnojarsk Pallasite, PMG-an, Russia, 1749 · 700 kg
- Springwater Pallasite, PMG-an, Canada, 1931 · 67.6 kg
- Admire Pallasite, PMG, United States, 1881 · 180 kg
Related groups
- 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.
- 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.
- Pallasites Gem-like olivine crystals set in iron-nickel metal.
Photos shared under Creative Commons licences (Wikimedia Commons).