3.9: Reaction to Dilute Hydrochloric Acid

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3.83.jpg — 3.83 Hydrochloric acid effervescing on calcite

One chemical property, the reaction of minerals to dilute (5%) hydrochloric acid (HCl), is included here because it is diagnostic for calcite, one of the most common minerals of the Earth’s crust. Drops of dilute acid placed on coarse samples of calcite cause obvious bubbling or fizzing, called effervescence, seen in the photo here (Figure 3.83).

Dolomite, a closely related carbonate mineral, effervesces when finely powdered but not when coarse. Other carbonate minerals, such as smithsonite (ZnCO₃), aragonite (CaCO₃), and strontianite (SrCO₃), effervesce to different degrees. They are usually distinguished, however, by crystal form, color, and other properties. Although acid tests have limited use, most mineralogy labs are equipped with small bottles of HCl and eyedroppers to aid in carbonate identification. Many geologists carry a small bottle of dilute hydrochloric acid when they go in the field so they may distinguish between rocks that contain calcite and rocks that do not.