Iron meteorites are, in my opinion, some of the coolest and most interesting specimens available for study. They have the distinction of being one of the more rare observed meteorite falls (roughly 5%), but, mass wise, being the most common (1). In fact the largest meteorite ever recovered in North America was the iron Willamette Meteorite. As the name suggests, it was found here in 1902 in the Willamette Valley in the town of West Linn. This monster weighs in at 15.5 tons and can now be found in the American Museum of Natural History.
The important thing to keep in mind is that iron meteorites are not chondrites. They are about as different as sandstone and basalt. Both may be rocks, but they’re petrologically unique. To continue with the comparison, chondrites are the result of accreted space dust and debris- ok, it’s actually more complicated than that, but for the purpose of this post, it’ll do. And just as basalt forms from magma that comes from the mantle of the earth, so to do iron meteorites come from the mantle of planetesimals. So, if you have the chance to see and handle an iron meteorite, you are actually interacting with a fragment of a failed planet’s core. Pretty cool, eh?
It becomes even more interesting when you see an iron meteorite with the lovely Widmanstatten pattern etched across its surface. This lattice like network is the result of mixing between its two primary iron-nickel minerals, kamacite and taenite, while molten in the core of the planetesimal. Taenite is the nickel rich (about 25%) mineral component of iron meteorites and is highly acid resistant. This is important because acid etching is used on the cut surface of an iron meteorite in order to expose the Widmanstatten pattern. Too much taenite and the structure doesn’t develop (2).
Gibeon Iron Meteorite with Widmanstatten pattern. Image from New England Meteoritical Services.
There is a lot more that can be said in regards to iron meteorites, but I’m going to end it here. I want to save the rest for future posts where I can get into more detail.
1. Hutchison, Robert. Meteorites: A petrologic, chemical and isotopic synthesis. Cambridge University Press. 2004. P. 322.
2. Benedix, Gretchen., Russell, Sara., Smith, Caroline. Meteorites. Firefly Books. 2009. P. 64-65.