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 conduct 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 likely contains quartz grains cemented by carbonate minerals, which react with acid.

Step-by-step solution

Understand the Composition of Quartz

Quartz is a hard mineral composed of silicon dioxide (SiO₂), and it does not react with dilute acid. Therefore, if the rock was made purely of quartz, it would not effervesce when acid is applied.

Consider the Presence of Carbonate Minerals

Many sedimentary rocks contain carbonate minerals like calcite (calcium carbonate) or dolomite. These carbonate minerals do react with acid, producing carbon dioxide gas in this process, hence causing effervescence.

Recognize the Possibility of Cementing Material

In sedimentary rocks, it is common for grains of resistant minerals, such as quartz, to be bound together by a 'cement' formed from the precipitation of minerals like calcite. The effervescence observed upon adding acid suggests the presence of such cementing carbonate minerals.

Combine Observations for Conclusion

The visual observation of glassy quartz-like particles and the hardness test suggest quartz as the main constituent. However, the acid test indicating fizzing points to carbonate minerals, likely as a binding or secondary component, causing the reaction.

Key concepts

Quartz

When you look at most sedimentary rocks, there’s a good chance you'll find quartz. This mineral is prized for its resilience and strength. Quartz is composed of silicon dioxide (\( \text{SiO}_2 \)), making it incredibly hard. That’s why it can scratch glass with ease. In fact, the hardness of quartz is a key indicator when trying to identify this mineral in the field. Its hardness is rated 7 on the Mohs scale.

Quartz is quite common because it is resistant to weathering. This durability means that many sedimentary rocks contain quartz as a primary component. It often appears as clear, glassy particles. Thus, despite its abundance, identifying whether you’re indeed looking at quartz involves checking its hardness. If a rock scratches glass, quartz is likely present.

• Quartz: Silicon dioxide (\( \text{SiO}_2 \))
• Hardness: 7 on the Mohs scale
• Durability: Weather resistant

Carbonate Minerals

Carbonate minerals are somewhat less durable than quartz and are significant in sedimentary rocks due to their reaction with acids. These include minerals such as calcite and dolomite. Calcite, which consists of calcium carbonate, is perhaps the most well-known carbonate mineral. Many sedimentary rocks, like limestone, contain substantial amounts of calcite.

What sets carbonate minerals apart from quartz is their chemical reaction to acids. Even a weak acidic solution, such as vinegar, will cause calcite to effervesce, or fizz. This fizzing is due to the release of carbon dioxide gas as the acid reacts with the carbonate compound. Consequently, a rock showing effervescence when in contact with acid likely contains carbonate minerals.

• Common Carbonate Minerals: Calcite, Dolomite
• Reaction: Fizzes with acid
• Composition: Often calcium carbonate (\( \text{CaCO}_3 \)) in calcite

Acid Test

The acid test is a simple yet effective method for identifying carbonate minerals within a rock. When a drop of weak acid, such as hydrochloric acid, is applied to a rock surface, it can reveal hidden clues about the rock's composition. If the rock fizzles and bubbles, that’s a sure sign of a reaction.

This reaction happens because the acid reacts with carbonate minerals to release carbon dioxide gas. For quartz-rich rocks, like some types of sandstone, the effervescence indicates carbonate minerals' presence. These minerals might not be visible but are part of the rock, likely as a minor binding agent.

• Acid Used: Typically dilute hydrochloric acid
• Reaction: Fizzing indicates carbonate presence
• Carbon Dioxide: Released gas causing bubbles

Cementing Material

In sedimentary rocks, grains aren't just thrown together; they need a binding agent to stay intact. This cementing material often includes minerals that provide cohesion. Quartz grains can be trapped in a matrix that binds them together, commonly comprising carbonate minerals.

This binding process occurs as minerals precipitate from water and fill spaces between the grains. In many cases, this cement could be calcite, causing the rock to effervesce when subjected to an acid test. The presence of this cementing material explains why a quartz-heavy rock might still react to acid, as the filler is made up of carbonate minerals.

• Role: Binds sedimentary particles
• Common Material: Calcite, a carbonate mineral
• Consequence: Rocks fizz when in contact with an acid