Breccia is a rock composed of angular fragments cemented together. Whether it reacts with acid depends entirely on its specific mineral makeup, which varies dramatically. Some breccias will effervesce vigorously, while others show no reaction at all. Understanding the components and the chemistry of the acid test is necessary to predict its behavior.
Understanding Breccia’s Variable Composition
Breccia is a textural term describing a rock made of large, sharp, angular fragments, called clasts, embedded in a finer-grained material known as the matrix or cement. The angular shape of the clasts indicates they have not been transported far from their source, unlike the rounded fragments found in a conglomerate. The rock’s composition is determined by the material making up both the clasts and the cement.
The clasts can originate from virtually any rock type, including limestone, basalt, granite, or volcanic fragments. The matrix, which fills the spaces between the fragments, can be composed of fine rock flour, clay, or a chemical cement. Common cementing materials include silica, iron oxides, or carbonate minerals like calcite or dolomite. Because the source material and the cement differ widely, breccia has a highly variable chemical composition.
The Chemistry Behind Rock Acid Tests
The acid test used by geologists involves applying a drop of dilute hydrochloric acid, typically 5% to 10% concentration, to the rock’s surface. The purpose of this test is to determine the presence of carbonate minerals. The reaction, called effervescence, occurs when the acid reacts with the carbonate ion, releasing carbon dioxide gas.
The most common carbonate mineral is calcite (calcium carbonate), which reacts strongly and produces a vigorous fizz when cold, dilute acid is applied. Another common carbonate mineral is dolomite (calcium magnesium carbonate).
Dolomite reacts much less vigorously than calcite when exposed to cold, dilute acid, often producing only a weak fizz or a few slow-growing bubbles. This difference is due to dolomite’s stronger atomic bonding. To achieve a noticeable reaction, it may be necessary to warm the acid or scratch the rock to create a powder, increasing the surface area available for the chemical reaction.
When Breccia Reacts (And When It Doesn’t)
A breccia will react with acid only if it contains a sufficient amount of carbonate material, either in its clasts or its cement. The most reactive breccias are those where the matrix is primarily composed of calcite cement. In these cases, the entire rock may fizz vigorously when the acid is applied to the cementing material between the fragments.
A breccia may also react if the angular clasts themselves are made of a carbonate rock, such as limestone or dolostone. If the clasts are limestone, they will fizz strongly, even if the surrounding matrix is non-reactive, like silica or clay. Conversely, if the clasts are dolostone, they will produce a weaker, more subtle effervescence.
Many breccias show no reaction at all because they are composed of minerals that do not contain the carbonate ion. For example, a volcanic breccia, consisting of fragments of lava or other volcanic rock lithified by a silica cement, will not fizz. Similarly, a breccia composed of quartz, feldspar, or granite fragments held together by silica or iron oxide cement will be non-reactive. The presence of these silicate minerals, which are chemically stable in acid, prevents effervescence.
Interpreting the Significance of the Reaction
Observing a reaction when testing breccia provides immediate and valuable information about its composition and likely origin. If the rock fizzes, it confirms the presence of carbonate material, allowing geologists to classify the rock as a calcareous breccia. This compositional knowledge is useful because carbonate-cemented rocks, unlike those cemented by quartz, are more susceptible to chemical weathering over time.
The location of the fizzing offers further insight into the rock’s formation history. If the reaction is confined to the clasts, it indicates the fragments were sourced from a carbonate rock like limestone. A reaction occurring only in the matrix suggests that carbonate-rich fluids later flowed through the rock and precipitated as a cement. Distinguishing the location of the reaction helps determine whether the rock originated from the fragmentation of a pre-existing limestone or was cemented by later carbonate deposition.