October 7, 2012

Geology Word of the Week: W is for Widmanstätten Pattern

Posted by Evelyn Mervine

A slice of meteorite displaying a Widmanstätten pattern (silvery patches) in between oxidized or rusted sections (reddish brown patches). Photo courtesy of Lockwood Dewitt.

def. Widmanstätten Pattern:
An interweaving pattern of the extraterrestrial minerals kamacite (a low nickel content iron-nickel alloy, similar to the terrestrial mineral ferrite) and taenite (a high nickel content iron-nickel alloy, similar to the terrestrial mineral austenite) that appears in some iron-nickel meteorites when a cut section of the meteorite is etched with weak acid.

Widmanstätten patterns appear during acid etching because kamacite is more easily dissolved by weak acid than taenite. Widmanstätten patterns are believed to form in a few different ways (depending on the pressure and temperature conditions experienced; I won’t go into too much detail on this) as iron-nickel material separates into the high-nickel and low-nickel minerals as it cools. Whatever the formation pathway, Widmanstätten patterns can only form when there is very slow cooling in an environment such as the core of a planet. Therefore, Widmanstätten patterns are only found in meteorites, not in any naturally forming rocks on Earth’s surface. In fact, Widmanstätten patterns require such slow cooling that they cannot even be reproduced by scientists in a laboratory.

Widmanstätten patterns are named after Count Alois von Beckh Widmanstätten, an Austrian scientist who discovered the patterns in 1808 when he was flame heating a meteorite. Count Widmanstätten never published his discovery, but he orally communicated it to his scientific colleagues, and the pattern was named after him. Some scientists believe that the patterns should also be called Thomson patterns because a scientist named G. Thomson had previously, independently discovered the patterns when he was trying to use acid to clean some rust off of a meteorite. Thomson published his discovery in French in 1804. However, Thomson’s discovery was not widely spread throughout the scientific community because the Napoleonic wars interrupted Thomson’s communication with his scientific colleagues (Thomson was English) as he was living in Naples, Italy at the time. Thomson also died at an early age in 1806 before Widmanstätten made his own discovery of the patterns.

I, for one, always forget how to spell and pronounce “Widmanstätten.” Perhaps I’ll remember better after this post. Regardless, I’m happy to know that, due to some scientists trying to right some misfortunes and twists of history, I can always google “Thomson patterns” to find out the more popular name of “those pretty meteorite pattern thingies.”

Below are a few more pictures of Widmanstätten patterns. If anyone else has additional pictures of Widmanstätten patterns, I’d love to add them to this post. Just email them to georneys blog (AT) gmail.

Another view of the Widmanstätten pattern on Lockwood's meteorite. Picture courtesy of Lockwood DeWitt.

Widmanstätten pattern on the Cape York meteorite at the American Museum of Natural History in New York City. Picture courtesy of Patrick Donohue.


***Thanks to several of my twitter followers for suggesting this week’s word. Thanks to Lockwood DeWitt and Patrick Donohue for providing pictures.***