The metallurgy of iron meteorites
Iron meteorites are pieces of asteroid cores that cooled over millions of years. Their crystal structures and nickel distributions record cooling rates and the size of the bodies they came from.
- Describe the body-centred and face-centred cubic structures of kamacite and taenite.
- Explain the growth of the Widmanstätten pattern using the Fe–Ni phase diagram.
- Explain how metallographic cooling rates are measured and interpreted.
Links to: A-level chemistry: metallic bonding and alloys; A-level geology: crystallisation and cooling rates. The bcc and fcc structures and diffusion modelling go beyond A-level.
Two structures
Iron–nickel alloys adopt two structures. Taenite is face-centred cubic (fcc) and can hold a lot of nickel. Kamacite is body-centred cubic (bcc) and holds at most about 7% nickel. At high temperature, metal with typical meteoritic compositions of 6 to 12% nickel is entirely taenite.
As it cools below roughly 700 to 900 °C, depending on composition, kamacite becomes stable. It nucleates as plates along the octahedral {111} planes of the parent taenite crystal and grows as nickel diffuses out of it into the surrounding taenite.
Widmanstätten patterns
The result, seen on an etched surface, is the Widmanstätten pattern: intersecting kamacite plates in up to four orientations, separated by thin taenite lamellae and fine-grained mixtures called plessite. The width of the kamacite bands is used to classify the meteorites, from coarse to fine octahedrites. Metal with less than about 6% nickel becomes almost entirely kamacite (a hexahedrite); metal with more than about 16% nickel develops only microscopic kamacite (an ataxite).
Diffusion of nickel in iron is very slow at these temperatures. Patterns millimetres wide could only grow over millions of years.
Cooling rates and core sizes
As kamacite grows, nickel piles up at the edges of the taenite, producing M-shaped nickel profiles across taenite lamellae. Measuring these profiles with an electron microprobe and comparing them with diffusion models gives the metallographic cooling rate, typically from a few to thousands of degrees per million years.
If a group of irons all came from one slowly cooling core surrounded by an insulating mantle, they should share one cooling rate. Some groups, such as the IVA irons, show a wide spread, suggesting they cooled in a metal body whose rocky mantle had been stripped away by an impact.
How long to cool?
An iron meteorite has a metallographic cooling rate of 50 °C per million years through the range in which the Widmanstätten pattern grew, from 700 °C to 400 °C. How long did this take?
- Temperature change = 700 − 400 = 300 °C.
- Time = temperature change ÷ cooling rate = 300 ÷ 50.
- Time = 6 million years.
Answer: About 6 million years. Cooling this slow requires a large, well-insulated body, kilometres to tens of kilometres beneath the surface.
Sketch unit cells of bcc and fcc structures and count the atoms per unit cell (2 and 4). Then draw a schematic M-shaped nickel profile across a taenite lamella between two kamacite plates and explain how its shape arises from slow diffusion of nickel.
Quiz
5 questions. Choose an answer to see whether you are right and why. Finish the quiz to complete the lesson.
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Question 1What is the crystal structure of kamacite?
Kamacite is bcc iron–nickel with low nickel; taenite is fcc.
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Question 2Along which planes of taenite do kamacite plates grow?
Kamacite nucleates along the {111} planes, giving the octahedral geometry of the pattern.
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Question 3Why do Widmanstätten patterns require slow cooling?
Growth of millimetre-scale kamacite plates needs nickel to diffuse over long distances, which takes millions of years.
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Question 4What is an ataxite?
With more than about 16% Ni, kamacite forms only microscopically, as in the Hoba meteorite.
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Question 5What does a wide range of cooling rates within one iron group suggest?
In a mantled core, all parts cool together at one rate; a spread implies an exposed metal body.