Carbonate Minerals: 18-Geol-A1 Mineralogy and Petrology, December 2018, Q12

Question 12: Carbonate Minerals (Part 2 – 10 marks)

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Both minerals below share the same rhombohedral cleavage and the $(\text{CO}_3)^{2-}$ anion group, so they are distinguished from one another primarily by their reaction to dilute hydrochloric acid, not by cleavage habit.

Two common carbonate minerals and their diagnostic properties
Mineral	Formula	Diagnostic property
Calcite	$\text{CaCO}_3$	Effervesces vigorously in cold, dilute HCl — the single fastest and strongest acid reaction of any common carbonate, immediately diagnostic in the field.
Dolomite	$\text{CaMg}(\text{CO}_3)_2$	Effervesces only weakly, or only when powdered or in warm HCl — the key test separating it from calcite in hand specimen, since the two share nearly identical rhombohedral cleavage and appearance; crystal faces are often visibly curved ("saddle-shaped").

Topic. Carbonate mineral group diagnostics: calcite vs. dolomite.

Key relations. Both share rhombohedral cleavage and the $(\text{CO}_3)^{2-}$ anion; acid reactivity to cold dilute HCl is strong (calcite) vs. weak (dolomite) because substituting the smaller, more strongly bonded $\text{Mg}^{2+}$ for half the $\text{Ca}^{2+}$ sites strengthens the dolomite lattice against acid attack.

Why this works. The dilute-HCl test works because reaction rate with acid depends on how readily the carbonate lattice is attacked, which is controlled by cation identity; $\text{Mg}^{2+}$'s smaller ionic radius and higher lattice energy in the ordered dolomite structure make it markedly more acid-resistant than the pure-Ca calcite structure, so this single field test lets a geologist distinguish the two without a hand lens or microscope.

Common pitfall. Relying on cleavage alone to separate the two; calcite and dolomite share essentially the same rhombohedral cleavage, so the acid test (cold dilute HCl, powdered if needed) is required.

Source. Boggs, Petrology of Sedimentary Rocks, 2nd ed., ch.9–10 (carbonate mineralogy and diagenesis); Klein & Dutrow, Manual of Mineral Science, 23rd ed., ch.13 (carbonate mineral group).

Question 12 of 12: Carbonate Minerals

Question 12: Carbonate Minerals (Part 2 – 10 marks)