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. 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 Fig. 1. Iron meteorite. 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 patternsAlso 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, … Read More

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