Question

When you studied Chapter \(3,\) you learned that feldspars are very common minerals in igneous rocks. When you learn about the common minerals that compose sedimentary rocks in Chapter 6 , you will find that feldspars are relatively rare. Applying what you have learned about chemical weathering, explain why this is true. Based on this explanation, what mineral might you expect to be common in sedimentary rocks that is not found in igneous rocks?

Short answer

Feldspars weather to clay minerals and silica, making quartz more common in sedimentary rocks.

Step-by-step solution

Understanding Chemical Weathering

Chemical weathering refers to the breakdown and alteration of minerals in rocks through chemical reactions with water, air, and other substances. Feldspars, which are a group of minerals found in many igneous rocks, are particularly susceptible to chemical weathering due to their chemical composition. This process often transforms feldspars into clay minerals and silica.

Feldspars to Clay Conversion

When exposed to weathering, feldspars undergo a chemical transformation. Hydrolysis, a key process in chemical weathering, involves feldspars reacting with water, leading to the formation of secondary minerals like kaolinite (a type of clay) and dissolved silica. This renders feldspars less abundant in sedimentary rocks since they have been converted into clay over time.

Mineral Composition in Sedimentary Rocks

Sedimentary rocks are often formed from the deposition and compaction of weathered material. Since feldspars transmute into clay minerals and silica due to weathering, these clays accumulate in sedimentary environments. As a consequence, minerals that are more resistant to chemical weathering, such as quartz, persist and become more common in sedimentary rocks.

Common Minerals in Sedimentary Rocks

Quartz is known for its durability against chemical weathering and is thus more resistant compared to feldspars. This resistance allows quartz to accumulate as a prevalent mineral in sedimentary rocks, contrasting with its relative abundance in igneous rocks, where feldspars are more common.

Final Explanation

In summary, the susceptibility of feldspars to chemical weathering renders them scarce in sedimentary rocks. Instead, minerals resistant to weathering processes, like quartz, are more likely to be found in sedimentary environments. Thus, quartz becomes a common mineral in sedimentary rocks, filling the gap left by the weathered feldspars.

Key concepts

Feldspars

Feldspars are one of the most ubiquitous groups of minerals on Earth. They make up a significant portion of igneous rocks, which are rocks formed from the cooling and solidification of molten lava or magma. However, feldspars are chemically reactive and prone to weathering. This means they don't stick around as long in their original form as some other minerals might.

During the process of chemical weathering, feldspars react with elements like water and carbon dioxide from the atmosphere. This causes them to break down into other minerals and dissolved ions. As a result, they are less commonly found in sedimentary rocks, which form over long periods from the accumulation of such weathered materials.

The transformation of feldspars into different materials is a key part of how rocks change over geologic time spans.

Sedimentary Rocks

Sedimentary rocks are fascinating collections of particles from pre-existing rocks or organic materials that are compacted and cemented over time. They often tell a story of Earth's past environments.

As rocks on the Earth's surface break down through weathering processes, the resulting sediments are transported by wind, water, or ice, before settling in layers in places like riverbeds, oceans, and deserts. Once these sediments accumulate, they slowly compact and cement together, forming sedimentary rocks.

• Feldspars, which begin in igneous rocks, are rarely found in sedimentary rocks as they tend to weather into clay minerals or other substances before reaching sedimentary environments.
• Instead, more durable minerals like quartz persist through these environments, remaining intact enough to become major constituents of sedimentary rocks.

Understanding sedimentary rocks can provide valuable insights into past climates and environments.

Quartz

Quartz stands out as a resilient mineral that tends to be long-lived in rock cycles and is quite common in sedimentary rocks. This durability is largely due to its impressive resistance to chemical weathering processes that typically break down other minerals.

Unlike feldspars, quartz is chemically stable, weathering slower than many other minerals. This allows it to remain largely unchanged in environments where chemical weathering is prevalent. As rocks break down and sediments are transported, quartz often remains intact.

Because of this, when sedimentary rocks form from accumulated sediments, quartz often emerges as a significant component. Its presence is a testament to its enduring nature against the natural elements.

• Quartz's stability and resistance to weathering make it not only a staple in sedimentary rocks but also valuable in various industrial applications.

Hydrolysis

Hydrolysis is an important chemical weathering process that specifically affects minerals like feldspars. In this chemical reaction, water interacts with the minerals to change their composition, ultimately forming new substances.

When feldspars undergo hydrolysis, they react with water often coupled with carbon dioxide to create clay minerals and dissolved ions such as silica. This process is fundamental to the breakdown of rocks and is a primary reason why feldspars disappear over time in sedimentary environments.

The transformation of feldspars during hydrolysis is an excellent example of how original rock materials get recycled into new forms. This constant process contributes to the evolution of Earth's surface and the formation of various geological features we see today.

• Hydrolysis plays a key role in creating soil, as clay minerals generated are important components of many soils.

Clay Minerals

Clay minerals are a group of minerals that are typically formed from the weathering of rocks like feldspars. These minerals are small in size but play a significant role in the ecosystem.

Through the process of chemical weathering, particularly hydrolysis, feldspars transform into clay minerals like kaolinite. These minerals are often found in soils and sedimentary rocks, contributing to their texture and structure.

Clay minerals are crucial in agriculture, retaining moisture and nutrients that plants need to grow. They also influence the physical properties of the soil, making them essential for robust soil health.

• In sedimentary rocks, clay minerals help cement the sediments together, forming what we call shale or mudstone.
• These minerals, although simple, make up a lively component of Earth's surface processes.