Mesosiderites
Jumbled breccias of metal and basalt, made in a violent collision.
- Classification
- Stony-iron meteorites › Mesosiderites
- Share of witnessed falls
- about 0.7%
- In our catalogue
- 187 meteorites, 7 seen to fall
- Heaviest
- Vaca Muerta, 3.83 t
- Earliest recorded fall
- Barea, 1842
- Most recent fall
- Dong Ujimqin Qi, 1995
Mesosiderites are breccias of roughly equal parts iron-nickel metal and silicate rock, with an irregular texture: metal and silicates occur as lumps, pebbles and fine intergrowths. The silicates are basalt- and pyroxenite-like rocks very similar to eucrites and diogenites. They are thought to have formed when the molten metal core of one asteroid and the crust of another were mixed together in a collision.
At Vaca Muerta in Chile's Atacama Desert, fragments totalling about 3.8 tonnes were found by prospectors in the 19th century, who mistook the shiny metal for silver ore. Mesosiderites seen to fall include Barea (Spain, 1842), Estherville (Iowa, 1879, about 320 kg), Veramin (Iran, 1880), Dyarrl Island (Papua New Guinea, 1933), Lowicz (Poland, 1935, about 59 kg), Patwar (India, 1935) and Dong Ujimqin Qi (China, 1995, about 129 kg).
Mineralogy and chemistry
Mesosiderites are about half iron-nickel metal and half silicate. The metal has about 7% to 8% nickel, occurs as grains, nodules and veins, and contains troilite and schreibersite. The silicate parts are breccias of basalt, gabbro and orthopyroxenite made of pyroxene, calcium-rich plagioclase, olivine, silica and phosphate, closely resembling eucrites and diogenites. Their oxygen isotopes are the same as the HED meteorites, with a Δ17O of about −0.24 per mil.
They are classified by their silicates: class A has more plagioclase (basaltic), class B more orthopyroxene, and class C almost all orthopyroxene. A number from 1 to 4 describes how strongly they were reheated and recrystallised. The name, from Greek for 'half iron', was given by Gustav Rose in 1863.
Parent body and formation
Mesosiderites record a collision in which molten metal, perhaps from the core of one body, was mixed with the crust of a differentiated asteroid. The silicates were then buried and cooled extraordinarily slowly: metallographic cooling rates are only a fraction of a degree per million years below about 500 °C, the slowest of any meteorites, suggesting burial under many kilometres of debris. A study of zircons published in 2019 found that the silicate crust formed about 4,558 million years ago and that the metal and silicate were mixed about 4,525 million years ago. Their matching oxygen isotopes, and the timing, led the authors to propose that mesosiderites formed on Vesta itself, after a giant impact.
Abundance and famous examples
Mesosiderites make up about 0.7% of falls; well over a hundred are known. Falls include Barea (Spain, 1842), Estherville (Iowa, 1879), Veramin (Iran, 1880), Dyarrl Island (Papua New Guinea, 1933), Łowicz (Poland, 1935), Patwar (Bangladesh, 1935) and Dong Ujimqin Qi (China, 1995). Estherville fell on 10 May 1879 after a loud fireball; about 320 kg was recovered, including one mass of about 200 kg that buried itself several metres deep. Vaca Muerta in the Atacama, Mount Padbury in Western Australia and Bondoc in the Philippines, about 890 kg, are large finds.
How to recognise one
A mesosiderite looks like a dark grey or brownish rock with irregular lumps and veins of metal, so it is heavy and strongly magnetic. Polished, it shows a speckled mixture of shiny metal and grey silicate. Weathered surfaces are rusty. They can be confused with stony-iron slag or with metal-rich chondrites; confirmation needs a laboratory.
Famous examples
- Vaca Muerta Mesosiderite-A1, Chile, 1861 · 3.83 t
- Estherville Mesosiderite-A3/4, United States, 1879 · 320 kg
- Dong Ujimqin Qi Mesosiderite, China, 1995 · 128.8 kg
- Bondoc Mesosiderite-B4, Philippines, 1956 · 888.6 kg
Related groups
- 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.
- Eucrites Volcanic basalt from the crust of Vesta.
- Diogenites Pyroxene-rich rocks from deep inside Vesta.
- Pallasites Gem-like olivine crystals set in iron-nickel metal.
- CB chondrites (bencubbinites) More than half metal, with chondrules made in a planet-sized collision.
Photos shared under Creative Commons licences (Wikimedia Commons).