Stony-iron meteorites

Roughly equal parts metal and silicate. Rare, and among the most beautiful meteorites.

Classification

Stony-iron meteorites

Share of witnessed falls

about 1.1%

In our catalogue

281 meteorites, 11 seen to fall

Heaviest

Brenham, 4.3 t

Earliest recorded fall

Mineo, 1826

Most recent fall

Dong Ujimqin Qi, 1995

Stony-iron meteorites contain roughly equal amounts of iron-nickel metal and silicate minerals. Only about 1 in 100 meteorites seen to fall is a stony-iron.

There are two quite different kinds. Pallasites are olive-green crystals of olivine set in metal, and may come from deep inside melted asteroids. Mesosiderites are jumbled breccias of metal and basalt-like rock, thought to come from violent collisions.

Two families, two stories

Pallasites and mesosiderites both contain roughly equal amounts of metal and silicate, but almost everything else about them differs. In a pallasite the silicate is olivine, the main mineral of planetary mantles, in large, clean crystals. In a mesosiderite the silicate is a jumble of basalt, gabbro and pyroxenite, the rocks of an asteroid's crust, broken and mixed with metal. Pallasites suggest a mantle meeting a core; mesosiderites suggest a crust smashed into molten metal.

The metal in both is kamacite and taenite, like that of iron meteorites, with troilite, schreibersite and, in pallasites, phosphates and chromite. Cut and etched, the metal of many stony-irons shows a Widmanstätten pattern, proof that it cooled slowly over millions of years.

Origins and parent bodies

Oxygen isotopes are central to the story. Main-group pallasites and mesosiderites both have values close to those of the HED meteorites from Vesta, while the rare Eagle Station pallasites have oxygen like carbonaceous chondrites. Zircon dating published in 2019 suggested that mesosiderites formed on Vesta itself after a huge collision about 4,525 million years ago; main-group pallasites appear to come from a separate body whose metal resembles the IIIAB irons. Magnetic studies of pallasite olivine show that their parent body had a liquid, churning core that generated a magnetic field for millions of years, so the pallasite rock must have lain above, not at, the boundary of a still-molten core.

Abundance and recognition

Stony-irons are about 1% of falls. Only four pallasites and about seven mesosiderites have been seen to fall, though many large finds are known. A stony-iron is very heavy, strongly magnetic and shows metal on any cut or broken face. Pallasites reveal yellow-green to brown olivine in a metal network; mesosiderites show irregular metal lumps in dark grey rock. Both rust readily, and pallasite olivine can crack as the metal around it corrodes, so specimens are best kept dry. Anyone hunting for them needs the landowner's permission and should check local law before collecting or exporting.

Their beauty makes them prized by collectors and museums. Polished pallasite slices are often displayed against light so the olivine glows like stained glass, and mesosiderite slices show a speckled mosaic of silver metal and grey rock. Both are rarer than iron meteorites in collections, and much rarer than chondrites.

Why they matter

Stony-irons are among the few samples of the deep interiors and violent histories of melted asteroids. Pallasites test how metal and olivine separate inside small bodies, and how long their cores stayed molten. Mesosiderites record one of the most violent collisions in the early asteroid belt and the extraordinarily slow cooling that followed, deep under a blanket of debris. Together with iron meteorites and achondrites, they let scientists rebuild whole differentiated worlds from the fragments that reach Earth.

Classes and groups

Pallasites

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

94 in the catalogue

Mesosiderites

Jumbled breccias of metal and basalt, made in a violent collision.

187 in the catalogue

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
• Vaca Muerta Mesosiderite-A1, Chile, 1861 · 3.83 t
• Estherville Mesosiderite-A3/4, United States, 1879 · 320 kg

Related groups

• Pallasites Gem-like olivine crystals set in iron-nickel metal.
• Mesosiderites Jumbled breccias of metal and basalt, made in a violent collision.
• Iron meteorites Almost pure iron-nickel metal, mostly from the cores of asteroids that melted and were later shattered.
• 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.

Photos shared under Creative Commons licences (Wikimedia Commons).