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Types of Meteorites: Stony, Iron and Stony-Iron

How scientists sort meteorites, what chondrites and achondrites say about the early Solar System, and which well-known specimens represent each type.

MeteorsBy the duorolt.com editorial teamPublished Facts checked 7 min read

More than 50,000 meteorites have been found on Earth, NASA says, and nearly all of them fit one of three families, sorted by a single question: how much metal do they hold? A stony meteorite is mostly rock, an iron meteorite is almost entirely metal, and a stony-iron meteorite is roughly half and half.

That simple split hides a deeper one. Some meteorites are leftovers that never melted, close to the raw material that built the planets. Others come from asteroids that melted and separated into a core, a mantle and a crust. Where a meteorite sits on that divide tells scientists a lot about the body it came from.

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Three families sorted by metal

The numbers below come from the Center for Meteorite Studies at Arizona State University (ASU). Shares refer to falls, meaning meteorites seen to land, not to every rock ever picked up.

Family Make-up Share of observed falls Examples
Stony About 75 to 90% silicate minerals and 10 to 25% nickel-iron About 94% Chelyabinsk, Murchison, Allende
Iron Almost entirely nickel-iron alloy Less than 5% Hoba (Namibia)
Stony-iron Roughly equal parts silicate and nickel-iron Rare Pallasites and mesosiderites

ASU notes that iron meteorites are rare among falls but are the easiest type to recognize, which matters when someone is looking at a strange rock in a field. Where the rocks came from is a separate question. NASA says 99.8% of the meteorites found on Earth come from asteroids, and the remaining 0.2% is split roughly equally between Mars and the Moon. For the basic vocabulary, see meteor, meteoroid and meteorite.

One melted asteroid can explain several of these groups, and ASU's descriptions fit that picture. When a small asteroid heats up and melts, dense metal settles toward the center to form a core, and lighter rock rises to form a mantle and a crust. Chondrites come from bodies that never reached that stage. Achondrites come from the crust and mantle of bodies that did. Irons come from cores. Stony-irons come from the boundary where core and mantle meet, or from collisions that mixed metal-rich and rock-rich material. Each family, in other words, is a different piece of the same layered structure, or of a body that never built one.

Chondrites: leftovers that never melted

Chondrites are the largest group of stony meteorites. ASU puts them at about 86% of recovered meteorites. The name comes from chondrules, small round grains rich in silicate minerals. ASU states that chondrites have never undergone melting, and that they preserve some of the earliest solids of the Solar System, including tiny calcium-aluminum-rich inclusions.

There are three main subgroups. Carbonaceous chondrites have a magnesium-to-silicon ratio close to the Sun's, and ASU lists Murchison and Allende among its examples. Ordinary chondrites have chondrules that make up much of their volume and only 10 to 15% fine-grained matrix. Enstatite chondrites formed under conditions with very little oxygen, and their minerals show it.

The Chelyabinsk meteorite of 2013 is an LL5 ordinary chondrite, according to a NASA-hosted conference abstract. The same abstract says fragments totaling much more than 100 kg were collected, from an object about 20 meters (66 feet) across before it entered the atmosphere.

Achondrites: rocks from melted worlds

Achondrites have no chondrules. ASU says they come from bodies that were differentiated, with cores and crusts, and that they formed from molten fragments knocked loose by collisions. That makes them samples of crust and mantle rock from small worlds that once melted. ASU also lists a bridge group, the primitive achondrites, which keep chondrite traits despite recrystallization or partial melting.

The group includes several well-known families. The howardite-eucrite-diogenite meteorites, or HEDs, are thought to come from the asteroid Vesta, and ASU says they crystallized between 4.43 and 4.55 billion years ago. The large asteroid Vesta and the Dawn mission show where that rock came from. Lunar meteorites were blasted off the Moon by impacts, and Martian meteorites were blasted off Mars. NASA says more than 60 Martian meteorites are known, and nearly 80 lunar ones that resemble Apollo Moon rocks.

Irons and stony-irons: cores and boundaries

ASU says iron meteorites most likely came from the cores of large asteroids. They are nearly pure nickel-iron, the same main ingredients as Earth's core.

Stony-irons come in two kinds. Mesosiderites are brecciated, meaning they are made of broken fragments cemented together, and ASU says they likely formed in collisions between metal-rich and silicate-rich asteroids. Fewer than 150 have been identified. Pallasites are believed to form where a differentiated asteroid's mantle meets its metal core. Their main silicate is green olivine, and gem-quality peridot has been found in several pallasites.

Notable falls and finds

  • Chelyabinsk, Russia, February 15, 2013. An LL5 ordinary chondrite whose airburst injured many people. The Chelyabinsk airburst of 2013 is covered in detail elsewhere.
  • Meteor Crater, Arizona. NASA's Earth Observatory describes the impactor as almost entirely nickel-iron, with more than 30 tons of meteoritic iron found scattered over an area 12 to 15 km across. For the crater itself, see how impact craters form and are confirmed.
  • Hoba, Namibia. ASU calls it the largest known individual meteorite. It is an iron measuring just under 2.7 meters (9 feet) wide. NASA's StarChild page estimates its present weight at 66 tons and says part of it has rusted away.
  • Murchison and Allende. Both appear on ASU's list of carbonaceous chondrites, the subgroup that sits closest to the Sun's own chemistry and that preserves very early Solar System material.
  • Nakhla, NWA 5000 and NWA 7611. ASU gives Nakhla as a Martian example, described as crustal rock ejected from Mars, and the other two as lunar examples, rock ejected from the Moon by impacts.

Common mistakes and myths

  • "All meteorites are metal." Stony meteorites are about 94% of observed falls, according to ASU, and ASU describes them as roughly 75 to 90% silicon-based minerals. Iron meteorites are the minority, even though they are the easiest to spot.
  • "A magnet picks up every meteorite." ASU says most meteorites attract a magnet, but ASU's identification guidance adds that lunar meteorites, made only of rocky minerals, will not. Weathering can also weaken the pull. A checklist for testing a possible meteorite explains why a magnet alone settles nothing.
  • "Meteorites come only from asteroids." Almost all do, but NASA counts Martian and lunar meteorites too.
  • "Meteorites are usually big chunks." Giants like Hoba are exceptional. NASA gives the typical size as a pebble to a fist.

Frequently asked questions

What is the most common type of meteorite?

Stony meteorites make up about 94% of observed falls, according to ASU, and chondrites are the biggest part of that group at about 86% of recovered meteorites.

How do scientists tell the types apart?

They look at metal content, texture and minerals. Chondrules point to a chondrite, a lack of them points to an achondrite, and a mostly metal sample points to an iron or stony-iron meteorite. Final classification takes laboratory work.

Which meteorites come from Mars and the Moon?

NASA says more than 60 known Martian meteorites and nearly 80 known lunar meteorites have been found. Impacts blasted them off their home worlds. The counts grow as new finds are classified, so check a current database for the latest numbers.

What do meteorites tell scientists?

Chondrites preserve some of the earliest solids of the Solar System, according to ASU. Achondrites and irons record melting and layering inside small bodies, and HED meteorites link back to Vesta.

Sources