Question

During a hike in Utah's Zion National Park, you pick up a sedimentary rock sample. When you examine the sample with your hand lens, you see that the rock consists mainly of rounded glassy particles that appear to be quartz. To be sure, you make two basic tests. When you check for hardness, the rock easily scratches glass, which is what quartz would do. However, when you place a drop of acid on the sample, it fizzes. Explain how a rock that appears to be rich in quartz could effervesce with acid.

Short answer

The rock contains both quartz and carbonate minerals, like calcite, causing it to fizz with acid.

Step-by-step solution

Analyzing Quartz Properties

Quartz is a hard, crystalline mineral composed of silicon and oxygen atoms. It is known for its glassy luster and hardness, as it scratches glass and is on the Mohs hardness scale at a level of 7. Quartz does not react with hydrochloric acid (HCl), so effervescence from acid is not due to quartz.

Identifying Effervescence Cause

The key observation here is the fizz or effervescence when the acid is applied. This reaction typically indicates the presence of a carbonate mineral, such as calcite (CaCO₃), which reacts with acids such as HCl to produce carbon dioxide (CO₂) gas.

Recognizing a Mixed Rock Composition

The presence of both glassy quartz and fizzing indicates that the rock is not pure quartz. Instead, it implies that the rock might contain a mixture of minerals including both quartz and carbonate minerals like calcite.

Key concepts

Quartz Properties

Quartz is one of the most common and abundant minerals found on Earth's crust. It is primarily composed of silicon dioxide (SiO₂), which gives it unique properties. Quartz appears glassy and clear or colored, depending on the impurities within it. Its notable hardness, rated at a 7 on the Mohs hardness scale, means that it can easily scratch glass. This characteristic is often used for initial identification.
Quartz is chemically stable and does not react with most acids, including hydrochloric acid (HCl). This means you will not observe any fizzing or bubbling when acid is applied to quartz. Thus, when identifying rocks, the absence of an acid reaction confirms the presence of quartz.

Calcite Reaction

Calcite is another mineral commonly found in sedimentary rocks. It is composed of calcium carbonate (CaCO₃) and differs significantly from quartz in its reactions to certain tests. When hydrochloric acid is applied to calcite, a notable effervescent reaction occurs. This is due to the release of carbon dioxide gas: \[ \text{CaCO}_3 + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{CO}_2 + \text{H}_2\text{O} \] The acid test is a reliable method to distinguish calcite from other minerals because few others have such a noticeable fizz when exposed to acids. This effervescence can help confirm the presence of calcite within a rock sample that otherwise might seem to be composed of other minerals.

Carbonate Minerals

Carbonate minerals are a group of minerals that contain the carbonate ion (CO₃²⁻) as the fundamental building block. These minerals include calcite, dolomite, and aragonite, among others. They are notably reactive with acids, making them easy to identify with simple acid tests.
Carbonate minerals are integral to sedimentary rock formations, including limestone and dolostone. They often form in marine settings where calcium-rich waters facilitate their deposition. In rock identification, the presence of carbonate minerals is indicated by the effervescence observed during an acid test. This is due to their chemical structure that allows for carbon dioxide release upon reacting with acids.

Acid Test for Minerals

The acid test is a straightforward method used to identify carbonate minerals in rocks. It involves applying a small amount of acid, typically hydrochloric acid, to a mineral sample.

• If the sample effervesces or fizzes, it is likely to contain carbonate minerals, such as calcite.
• If there is no reaction, the mineral might be quartz or another non-carbonate mineral.

The test exploits the chemical reaction where carbonate minerals interact with the acid to produce carbon dioxide gas, which causes bubbling. This simple yet effective test helps differentiate minerals in geological studies, aiding scientists and enthusiasts in identifying rock compositions even in the field.