Getting Stoned

1-2% HCl solution in a syringe.

Figuring out rocks?
Use hydrochloric acid.
Fizz! Oh, a limestone…

Acid testing is crucial for identifying various stone types in the field. Often, rocks look the same, but are quite different. Let’s use limestone as an example. Dolomitic fossiliferous limestone and fossiliferous limestones look rather similar. However, a chemical difference exists. Fossiliferous limestone, as we define in our stone maps, has a high percentage of calcite, or calcium carbonate (CaCO₃), where as dolomitic fossiliferous limestone, or CaMg(CaCO₃)₂, has more magnesium present, replacing some of the calcium. Due to the slight variation in chemical properties, fossiliferous limestone will react instantaneously with hydrochloric acid (HCl), releasing carbon dioxide (CO₂). This reaction can be seen in the form of a fizz; bubbles will start forming on the stone. In contrast, dolomitic fossiliferous limestone will have a delayed reaction when presented with HCl, and won’t fizz as violently. Therefore, HCl is a great way to test for dolomitic qualities in limestones.

The reaction of calcite with hydrochloric acid:

CaCO₃ (s) + 2HCl (aq) –> CO₂ (g) + H₂O (l) + Ca⁺ (aq) + 2Cl^– (aq)

Daniel testing stones in the Sanctuary.