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Calcite vs Dolomite: The Safe Acid-Reaction Test

Calcite and dolomite can both be white, gray, buff, crystalline, massive, and rhombohedrally cleaved. Their most useful field difference is reaction to cool dilute acid. Calcite reacts readily on a clean surface; dolomite normally reacts weakly unless a small area is powdered. A drop of cool dilute acid bubbles readily on calcite. Dolomite usually shows little reaction on an intact surface but reacts when a small scratched area is powdered. Both have rhombohedral cleavage, so cleavage alone may not separate them. Inspect a clean, unweathered surface and note whether the specimen is a single crystal or a mixed carbonate rock. If trained and equipped, apply one drop of cool dilute acid. Immediate bubbling supports calcite. If the intact surface is quiet, scratch a tiny inconspicuous patch into powder and retest. A powder-only reaction supports dolomite. Use hardness as supporting evidence: calcite is Mohs 3, while dolomite is slightly harder at 3.5 to 4. Report a mixed or uncertain carbonate when different areas react differently. Wear eye protection and follow laboratory or site procedures for dilute acid. Do not heat acid in the field. Natural limestone, dolostone, veins, and altered rocks may contain both minerals, producing uneven reactions. Do not create airborne carbonate dust; powder only a tiny dampened or controlled test area when appropriate.

  • Quartz vs Calcite — Tell quartz from calcite with hardness, cleavage, fracture, dilute-acid response, and crystal habit using a safe field workflow.
  • Gypsum vs Calcite — Tell white or clear gypsum from calcite with fingernail hardness, cleavage, dilute-acid response, and non-destructive checks.
  • Fluorite vs Calcite — Distinguish fluorite from calcite with four-way versus three-way cleavage, Mohs hardness, dilute-acid response, and UV limitations.
  • Marble vs Limestone — Tell marble from limestone using recrystallized texture, fossils, bedding, porosity, calcite reaction, and geologic context.
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