Dolomite is a common calcium magnesium carbonate mineral found in sedimentary rocks. While similar to calcite, its distinctive chemical makeup gives it unique properties. Geologists often use a classic field test involving applying acid to a rock sample to identify its mineral composition. How dolomite reacts to acid reveals an important difference in its chemistry.
Dolomite’s Reaction to Acid
Applying cold, dilute hydrochloric acid (HCl) to a solid piece of dolomite typically results in a very weak reaction, or sometimes none at all. Unlike other carbonate minerals that exhibit a rapid burst of bubbles, a solid specimen of dolomite will only show a few slow-growing bubbles of carbon dioxide gas (\(\text{CO}_2\)) on its surface. This weak effervescence is a defining characteristic of the mineral under standard field conditions.
A noticeable fizz will occur only when specific conditions accelerate the chemical reaction. One method is warming the dilute acid, which provides the necessary energy for the reaction to proceed faster. Alternatively, powdering the dolomite significantly increases the surface area exposed to the acid, leading to a visible reaction.
The Chemical Reason for the Slow Fizz
The chemical interaction between a carbonate mineral and acid involves a reaction that releases carbon dioxide (\(\text{CO}_2\)) gas. All carbonate minerals contain the carbonate ion (\(\text{CO}_3^{2-}\)), which reacts with hydrogen ions (\(\text{H}^+\)) from the acid to form water and \(\text{CO}_2\). Dolomite’s chemical formula is \(\text{CaMg}(\text{CO}_3)_2\), indicating it is a calcium magnesium carbonate.
The presence of both calcium (\(\text{Ca}^{2+}\)) and magnesium (\(\text{Mg}^{2+}\)) ions within the crystal structure inhibits the reaction rate. Dolomite forms a double carbonate structure where these ions are arranged in alternating layers within the crystal lattice. This ordered structure creates a stronger, more tightly bonded crystal compared to simpler carbonate structures.
This tighter arrangement makes the dolomite structure less soluble in acid. Hydrogen ions have a harder time attacking and freeing the carbonate groups. At lower temperatures, the reaction is limited by the rate at which surface atoms can break away, known as surface-reaction limitation. The slow fizz results directly from this structural stability, requiring extra energy from heat or increased surface area to overcome the chemical bonds.
Using the Acid Test to Distinguish Dolomite from Calcite
The slow reaction of dolomite is a fundamental tool geologists use to distinguish it from calcite, the most common carbonate mineral. Calcite, which is pure calcium carbonate (\(\text{CaCO}_3\)), reacts immediately and vigorously when cold, dilute hydrochloric acid is placed on its surface. This quick, sustained bubbling clearly indicates calcite, which lacks the tight structural ordering found in dolomite.
A weak or absent reaction on a solid sample suggests the presence of dolomite or another less-reactive carbonate mineral. Geologists then perform a secondary test by scratching the specimen to create a small pile of powder. When acid is applied to this powder, the resulting fizz confirms the mineral is dolomite, differentiating it from non-carbonate minerals like quartz that do not react.
This difference in reactivity is a direct consequence of the minerals’ internal structure. The acid test provides a quick, reliable field method to tell apart dolostone (rock made of dolomite) from limestone (rock made of calcite). Dolomite’s resistance to acid also influences its weathering patterns, making it a more durable rock type.