What Is the Acid Test in Geology?

Science

By

Andrew Alden

Updated on February 25, 2019

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Calcite in Hydrochloric Acid

Every serious field geologist carries a small bottle of 10 percent hydrochloric acid to perform this quick field test, used to distinguish the most common carbonate rocks, dolomite, and limestone (or marble, which may be composed of either mineral). A few drops of the acid are put on the rock, and limestone responds by fizzing vigorously. Dolomite fizzes only very slowly.

Hydrochloric acid (HCl) is available in hardware stores as muriatic acid, for use in cleaning stains from concrete. For geological field use, the acid is diluted to 10 percent strength and kept in a small strong bottle with an eyedropper. This gallery also shows the use of household vinegar, which is slower but suitable for occasional or amateur users.

Calcite making up a chip of marble fizzes vigorously in the typical 10 percent solution of hydrochloric acid. The reaction is immediate and unmistakable.

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Dolomite in Hydrochloric Acid

Andrew Alden

Dolomite from a chip of marble fizzes immediately, but gently, in a 10 percent HCl solution.

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Calcite in Acetic Acid

Andrew Alden

Bits of calcite from a geode bubble vigorously in acid, even in acetic acid like this household vinegar. This acid substitute is suitable for classroom demonstrations or very young geologists.

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Mystery Carbonate

Andrew Alden

We know this is a carbonate by its hardness (about 3 on the Mohs scale) and either calcite or dolomite by its color and excellent cleavage. Which is it?

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Calcite Test Fails

Andrew Alden

The mineral is put in acid. Calcite bubbles readily in cold acid. This is not calcite.

The most common white minerals in the calcite group react differently to cold and hot acid, as follows:

Calcite (CaCO₃): bubbles strongly in cold acid
Magnesite (MgCO₃): bubbles only in hot acid
Siderite (FeCO₃): bubbles only in hot acid
Smithsonite (ZnCO₃): bubbles only in hot acid

Calcite is by far the most common in the calcite group, and is the only one that typically looks like our specimen. However, we know it isn't calcite. Sometimes magnesite occurs in white granular masses like our specimen, but the main suspect is dolomite (CaMg(CO₃)₂), which is not in the calcite family. It bubbles weakly in cold acid, strongly in hot acid. Because we're using weak vinegar, we will pulverize the specimen to make the reaction faster.

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Crushed Carbonate Mineral

Andrew Alden

The mystery mineral is ground in a hand mortar. The well-formed rhombs are a sure sign of a carbonate mineral.

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Dolomite in Acetic Acid

Andrew Alden

Powdered dolomite bubbles gently in cold hydrochloric acid and in hot vinegar. Hydrochloric acid is much preferred because the reaction with dolomite is otherwise very slow.