Calcite is one of the most common minerals found across Earth’s crust. It is significant in various geological formations, including limestones, marbles, and hydrothermal veins. Identifying calcite is useful for geologists, mineral collectors, and in industrial applications.
Observable Physical Characteristics
One of the first steps in identifying calcite involves observing its physical characteristics. Calcite ranks 3 on the Mohs scale of mineral hardness, meaning it is relatively soft. It can be easily scratched by a copper coin (hardness 3.5), but not by a fingernail (hardness 2.5).
Calcite commonly forms distinctive rhombohedral crystals, appearing as six-sided, slanted blocks. It exhibits perfect rhombohedral cleavage, a tendency to break along smooth, flat planes. When a piece of calcite breaks, it typically forms smaller fragments that retain this characteristic rhombohedral shape. This consistent breakage pattern is a strong indicator.
Calcite’s luster, or how light reflects off its surface, typically ranges from vitreous (glassy) to dull. It displays a wide variety of colors, including clear, white, yellow, pink, blue, green, or brown, often due to impurities. Despite its varied colors, the streak of calcite, the color of its powder, is consistently white.
The Definitive Acid Test
The acid test is the most reliable method for identifying calcite. Calcite, or calcium carbonate (CaCO3), reacts with acids to produce carbon dioxide gas. This reaction results in a distinctive fizzing or effervescence, making it a definitive indicator.
To perform this test, apply a small drop of dilute hydrochloric acid directly to the mineral’s surface. Household vinegar, a weaker acid, can also be used, though the reaction might be less vigorous. Observing immediate and vigorous bubbling indicates the presence of calcite.
Wear eye protection to prevent accidental splashes. Use only a small amount of acid for the test.
Differentiating Calcite from Similar Minerals
Distinguishing calcite from similar minerals requires observing all its characteristics, especially the acid test. Quartz, for example, can be mistaken for calcite due to its common occurrence and crystalline forms. However, quartz is significantly harder than calcite, ranking 7 on the Mohs scale, and cannot be scratched by a copper coin or steel knife. Unlike calcite, quartz does not react with acid.
Dolomite, another carbonate mineral, can appear similar to calcite. While calcite fizzes vigorously with a cold drop of dilute acid, dolomite reacts much more slowly or may only show a noticeable reaction if the acid is warmed or the mineral is powdered. Dolomite is also slightly harder than calcite, typically ranging from 3.5 to 4 on the Mohs scale.
Gypsum is another mineral that can be confused with calcite, particularly its transparent varieties. Gypsum is considerably softer than calcite, ranking only 2 on the Mohs scale, and can be scratched with a fingernail. Unlike calcite, gypsum does not react with acid. Gypsum also exhibits different cleavage patterns, often breaking into tabular or platy fragments rather than the distinct rhombohedral shapes characteristic of calcite.