[What is this? Perhaps abstract expressionism or a new kind of carbon fiber, you might think. Would you believe me if I told you it's the inside of a meteorite? Photo by Kevin Walsh.]
Walking through the geology hall in the National Museum of Natural History in Washington, D.C., a few weeks ago, I was struck by what looked like a Jackson Pollock painting on the face of a rock. In fact, it wasn't a rock at all but instead part of an iron meteorite and what had looked like artwork was actually its interior. Beautiful!
Some meteorites are stony but others, like the one that caught my eye, are made of iron and nickel alloys (mixtures of two or more elements). Not all iron meteorites look like abstract paintings on the inside, but most do.
[Two iron meteorites are on display at the National Museum of Natural History in Washington, D.C.]
Originally, these meteorites were made of only one very hot alloy, called taenite. As the taenite cooled very slowly over time - as little as 100 degrees Celsius per million years in some meteorites - another alloy called kamacite formed and grew alongside it. The two alloys formed bands next to one another, producing the beautiful pattern on display at the museum.
The criss-crossing bands of alloys are known as Widmanstätten, or sometimes Thomson, patterns. Count Alois von Beckh Widmanstätten of Austria discovered the pattern when he heated a slice of a meteorite in 1808. Because the two alloys oxidized at different rates when heated, bands of different colors appeared on the slice.
An Italian named G. Thomson also independently discovered the bands four years earlier but his discovery wasn't as widely promoted as Widmanstätten's. Some suggest that because Thomson was the first to discover them, the bands should bear his name instead, but the Widmanstätten name remains more common.
Just slicing off a piece of iron meteorite, like the small one pictured at right, isn't enough to expose the Widmanstätten pattern. Typically, it is exposed when a meteorite slice is given a bath. After being ground, polished and cleaned, the slice is dipped into nitric acid which reveals the bands in about a minute.
[This iron meteorite, about 12 cm wide and weighing 1.7 kg, landed in Siberia during a 1947 meteorite shower.]
The resulting pattern depends on the angle of the slice. Just like cutting an onion in one direction produces rings while another produces strips, the way an iron meteorite is sliced dictates the angle between the iron-nickel alloy bands.
Finding iron meteorites on Earth is more common than finding stony meteorites because little of their bulk burns up when they enter our atmosphere and also because they look out of place in most parts of the world. The larger of the two meteorites in the photo at left was found in 1896 in Sonora, Mexico.