Quick answer: calcite is calcium carbonate, and acid breaks carbonate apart into water and carbon dioxide gas; the bubbles are CO2 escaping. That reaction is so dependable that geologists carry dropper bottles of dilute hydrochloric acid into the field. The famous exception is dolomite, a calcium-magnesium carbonate that barely reacts as a solid chunk but fizzes once it’s scratched into powder.
The reaction
CaCO3 + 2HCl → CaCl2 + H2O + CO2. With vinegar (acetic acid) the reaction is gentler, but the gas is the same. Mineralogy labs keep small bottles of roughly 5% HCl with eyedroppers for exactly this test, and field geologists use it to tell calcite-bearing rock from rock without calcite (Perkins, Mineralogy, LibreTexts).
The acid test, mineral by mineral
- Calcite: fizzes vigorously on a solid crystal.
- Aragonite: same formula as calcite (CaCO3), different crystal structure, and it fizzes too. See Polymorphs: aragonite and calcite.
- Dolomite: effervesces only when finely powdered (LibreTexts). Geologists scratch the rock with a knife and drop acid on the powder.
- Smithsonite and strontianite: carbonates that react to different degrees, usually identified by other properties instead (LibreTexts).
- Quartz, feldspar, agate, flint: no reaction at all. A drop of acid is a fast way to separate a white quartz pebble from a white calcite one.
Why dolomite is stubborn
In dolomite’s structure, layers of calcium alternate with layers of magnesium, and the magnesium-carbonate bonds dissolve much more slowly in cold dilute acid. Powdering the mineral vastly increases surface area, so enough reaction happens at once for visible bubbles.
The man dolomite is named for wrote his mineralogy in prison
Dolomite honors French geologist Déodat de Dolomieu, who first described the rock in the Alps; the Dolomites mountains are named after him too. Returning from Napoleon’s Egyptian expedition, Dolomieu was captured and held in a cell in Messina from 1799 to 1801. With almost nothing to write on, he composed his main treatise, Sur la philosophie minéralogique et sur l’espèce minérale, in the margins of a Bible (Britannica). He was freed in 1801 after Napoleon’s victory over Naples, published it that year, and died that November.
Do the test safely at home
- Use plain white vinegar, not lab acid.
- Test on an inconspicuous spot or a loose chip, never the face of a nice crystal; acid permanently etches calcite’s luster.
- Watch with a loupe: vinegar bubbles on calcite are small and slow but steady.
- Rinse with water afterwards.
Why this matters when cleaning specimens
Collectors use acid on purpose to dissolve calcite off quartz, which is how a lot of quartz and amethyst gets “cleaned.” The flip side: never use vinegar, lemon, or descaler on a calcite, aragonite, dolomite, or smithsonite specimen. See How to clean and store specimens without ruining them.
FAQ
Do pearls and seashells fizz too? Yes. They are calcium carbonate, as aragonite, calcite, or both.
My “calcite” didn’t fizz. Is it fake? Try a fresh scratch; weathered surfaces can be coated. If still nothing, it may be dolomite, quartz, or something else. See Reading a specimen without a label.
Related pieces in the shop
- Golden Calcite Crystal Point, Ohio
- Pink Dolomite Crystal Druse on Basalt
- Blue Aragonite, Rio Tinto, Spain
- Smithsonite with Calcite, Laurion, Greece
Sources: Perkins et al., Mineralogy, 3.09 “Reaction to Dilute Hydrochloric Acid” (LibreTexts); Encyclopaedia Britannica, Dieudonné Dolomieu.
