Etching
Etching is treating a polished iron meteorite surface with weak acid so that its hidden crystal structure, such as the Widmanstätten pattern, becomes visible.
A freshly cut and polished iron meteorite looks like a mirror of plain steel. Within it, however, are interlocking crystals of two iron-nickel minerals, kamacite and taenite, which differ in nickel content. Etching takes advantage of this. A dilute solution of nitric acid in alcohol, often called nital, attacks the minerals at different rates, so the crystal boundaries appear as a pattern of light and dark bands.
In octahedrites, etching reveals the famous Widmanstätten pattern, which formed only through extremely slow cooling over millions of years inside an asteroid's core. In hexahedrites, which are almost pure kamacite, it shows fine Neumann lines instead, and nickel-rich ataxites show little visible structure. The width of the bands helps classify the iron. Because man-made iron and steel do not develop this pattern, etching is also a strong identification test.
Gibeon from Namibia and Muonionalusta from Sweden are famous for beautiful etched slices. Etching uses acid and must be done only by adults with gloves, eye protection and good ventilation; children should never attempt it. Etched slices then need careful drying and storage, as they rust easily.
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