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 carbonate minerals in addition to quartz, causing it to fizz with acid.

Step-by-step solution

Identify the Problem

We need to understand why a rock sample that seems to be rich in quartz is fizzing when acid is applied, despite quartz not typically reacting with acid.

Analyze Properties of Quartz

Quartz is a mineral known for its hardness and ability to scratch glass, which aligns with one observation made about the rock. However, quartz does not react with dilute acids such as hydrochloric acid.

Consider Effect of Acid

Effervescence (fizzing) occurs when acid reacts with carbonate minerals, releasing carbon dioxide gas. This suggests the presence of a carbonate mineral within the rock, despite its quartz-like appearance.

Identify Potential Minerals

Common carbonate minerals like calcite and dolomite might be present in the rock. These minerals are known to effervesce when exposed to acid, which aligns with the observation made upon applying the acid to the sample.

Conclude Composition

The rock likely contains a significant amount of carbonate minerals mixed with quartz. The glassy particles may be quartz while the carbonate minerals are dispersed within the rock, explaining the fizzing reaction.

Key concepts

Quartz

Quartz is a widely abundant mineral found in the Earth's crust, known for its hardness. On the Mohs scale of mineral hardness, quartz ranks as a 7, meaning it can scratch substances like glass. This attribute makes quartz particularly recognizable when testing its hardness. Quartz is chemically composed of silicon dioxide (\( SiO_2 \)). It is glassy or vitreous in appearance, often found in a variety of colors, including clear, white, pink, and more.
Quartz's most distinct property, aside from its hardness, is that it is generally non-reactive with common acids such as hydrochloric acid. This means that if a rock fizzes when acid is applied, there must be something more than quartz in the composition.

Carbonate Minerals

Carbonate minerals are a group of minerals that contain the carbonate ion \( CO_3^{2-} \) as a basic structural and functional component. These minerals include well-known varieties such as calcite and dolomite. Carbonate minerals are typically formed in sedimentary environments through biological and chemical processes.
One of the hallmark features of carbonate minerals is their reaction with acids. When exposed to dilute hydrochloric acid, many carbonate minerals will effervesce or fizz because the acid reacts with the carbonate to release carbon dioxide gas. This is a useful test geologists use to identify carbonate-containing rocks.

• Effervescence is a direct indication of the presence of carbonate minerals within the rock sample.
• The acid test for carbonate minerals allows for quick field identification of potential mineral content.

Calcite

Calcite is one of the most common carbonate minerals, composed of calcium carbonate \( CaCO_3 \). It exhibits high sensitivity to dilute acids, which results in immediate and noticeable fizzing when an acid is applied. Calcite can appear in multiple crystal shapes and forms, often being found within sedimentary rocks as well as in stalactites and stalagmites in caves.
This mineral is particularly important for its wide range of uses in construction and manufacturing processes, forming the main constituent of limestone.
Calcite's immediate reaction with acids makes it an important mineral for identifying carbonate rocks, as its presence can signal other geological processes, such as mineral deposits formed from evaporating water or sediment layering.

Dolomite

Dolomite is a carbonate mineral composed of calcium magnesium carbonate \( CaMg(CO_3)_2 \). It is similar to calcite but contains magnesium, which affects its reactivity with acids. Dolomite does not react instantly with diluted acids, but if powdered or scratched, it starts to fizz due to the acid exposing more surface area to react with. This more subdued reaction allows for it to be differentiated from calcite in mineral testing.
In nature, dolomite often forms extensive rock formations known as dolostone, which is used in construction and chemical applications. Dolostone is characterized by its resistance to weathering in comparison to limestone, making it a valuable material in areas exposed to environmental elements.
Understanding the behavior of dolomite alongside calcite and quartz helps in diagnosing the mineral composition of rocks, especially when discerning between different types of sedimentary rocks where mixed mineral contents can be present.