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Treating Rocks and Fossils with Hydrochloric Acid

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Limestone with Coral Fossils
Specimen before treatment
Photo (c) 2013 Andrew Alden, licensed to About.com (fair use policy)
Acid can sometimes be a good tool for cleaning fossils or crystals, or for freeing them from their rock matrix.

This is a specimen of Arizona limestone from the Martin Formation that displays fossils of Thamnopora, a tabulate coral found throughout rocks of Paleozoic age. Tabulate corals are extinct, having been replaced by today's scleractinian corals, but both organisms are colonies of tiny animals that live in honeycombs of calcium carbonate minerals (calcite or aragonite). Also visible at the right-hand edge is a bit of a shell. The left end is a disorganized mass of mineral material.

However, these fossils have been replaced with silica, otherwise I would have left them alone. I could tell that they were silicified in three ways: natural weathering had partially dissolved the limestone while leaving the fossils alone, they scratched steel, and they did not fizz when I dropped a few bits into a 10 percent hydrochloric acid solution. That meant I could try putting this rock into a bath of HCl. Professional fossil preparators are extremely careful about the acids they use and seldom do what I will show you here, but many geologists and rockhounds have good reasons to follow this procedure.

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