Meteorites demystified: A beginner’s guide (Part 1)

Helen Gould UK)

What are meteorites?

Lumps of rock left over from the formation of the solar system or “chipped off” planets during major impacts can become trapped in the Earth’s gravitational field and fall as meteorites. The three main types are iron, stony and stony-iron. All of these are discussed in this article.

In particular, I consider two important questions:

Why are they so important? Because they represent the growth (accretion) of planets, they carry clues to our Solar System’s formation.
How do we know we are dealing with a meteorite? Like other rocks, meteorites record events. Most of their minerals are familiar but some have higher or lower concentrations than rocks found on Earth, suggesting an extra-terrestrial origin.

Irons

Most contain 7-15 wt % of Nickel (Ni) metal, with traces of other minerals. At room temperature, instead of a single mineral, this forms a Widmanstätten structure, whose intergrowth lamellae show two different minerals, one with about 40% Ni, the other with only about 5% Ni, and indicate slow cooling from greater than 700°C. Iron (Fe) meteorites have usually been completely melted, proving they formed in asteroid cores. So even asteroids are differentiated – like the major planets – with a core and mantle which solidified slowly.

Widmanstätten patterns
Also known as Thomson structures, these are figures of long nickel–iron crystals, found in the octahedrite iron meteorites and some pallasites. They consist of a fine interleaving of kamacite and taenite bands or ribbons called lamellae.

Stony-irons

Stony-iron meteorites probably came from large asteroids (for example, Vesta). Metal content varies; some cores contain sulphide, others water and “organic” compounds with nickel-iron metal, and traces of minerals, not found on Earth due to the oxygen in our atmosphere, may be present.

Pallasites

Pallasites contain olivine and nickel-iron metal. Olivine crystallises in molten rock at very high temperatures. In a pallasite, the olivines are surrounded by metal with a continuous Widmanstätten structure. Contraction during cooling probably allowed molten metal to fill cracks and solidify around the olivine crystals.

Mesosiderites

Mesosiderites contain metal veins and fragments, basaltic rock and glassy material. Each metal fragment developed separately, as it has its own Widmanstätten structure. The glass and metal veins imply high temperatures, followed by slow cooling and fragmentation. These meteorites probably formed during major impacts.

Stony

Texture (the relationship between grains) dictates whether a stony meteorite is classified as a chondrite or an achondrite.

Chondrites

Chondrites usually contain evenly distributed Ni-Fe and stony minerals. They cannot have melted since forming or this mixture would have separated out. If we ignore the Sun’s hydrogen and helium components, their chemistry is very like that of the Sun, and probably represents the primitive solar nebula.

Carbonaceous chondrites

These contain olivine and pyroxene, and sometimes rock fragments that show melting to the extent that they represent asteroid impacts. They also contain dust formed in stars’ atmospheres, keeping a record of when the Solar System was a gas-and-dust cloud.

Metamorphosed carbonaceous chondrites have a dark grey matrix with chondrules (olivines) and >20% volatiles and material which records the primitive solar nebula. They also contain speckles of iridium (Ir), the clue which allowed Walter Alvarez to suggest a giant meteorite caused the demise of the dinosaurs, following his discovery of an Ir spike at the Cretaceous-Tertiary boundary.

Achondrites

Achondrites look like terrestrial basaltic lavas produced by metallic cores separating from silicate crusts – further evidence of differentiation on the asteroids. Crystal sizes suggest slow cooling such as occurs inside planets and asteroids. Melting ended 4,400 million years ago on the asteroids.

Images courtesy NASA website.

Meteorites demystified: A beginner’s guide (Part 1)

What are meteorites?

Irons