Iron Meteorites
- Introduction
- Comets
- Asteroids
- Meteorites
- Meteor Showers
- Rock Types
- Identification
- Origins
- Destruction
The cores of large rocky bodies that are warm enough to become molten, or partially molten, will see all their iron sink to their cores. How then, do we find iron meteorites? The early Solar System was a dangerous place to be, with impacts and collisions a frequent occurrence. Over time some relatively large asteroids were chipped away, impacts smashing off their crusts and excavating their mantles to expose their metal core, which was also pulverised, sending shards of iron–nickel alloy spinning through the inner Solar System.
Iron is a somewhat more rare material than carbonaceous or silicate minerals, meaning that iron meteorite falls only account for five percent, and yet they are a popular member of meteorite collections. This is for a number of reasons; they actually look and feel like the stereotypical idea of a meteorite – dark, heavy and pitted – and for that reason they stand out more than stony meteorites when explorers are searching for them. Because of their density, they can also withstand atmospheric entry better than stony meteorites, and arrive in bigger lumps – all the largest meteorites found have been iron meteorites. They are also highly resistant to weathering, barring the odd spot of rust that can be easily cleaned off, and hence can survive for much longer on the surface of Earth than other types of meteorite.
Slice an iron meteorite open and pour a nitric oxide/alcohol solution over the exposed innards, and you will be treated to an intricate pattern of criss-crossing bands known as a Widmanstätten pattern. These interlaced ribbons (called lamellae) occur when one of the two dominant minerals of iron–nickel meteorites, taenite, begins to cool and lose some of its nickel atoms, forming lanes of the second dominant material, kamacite, instead. This transformation takes place when the iron is solid but hot, from about 700°C down to about 450°C . When elements move from one mineral to another in solid rock, it is known as diffusion. The thicker the lamellae are, the slower the rock cooled.
