Question

In Chapter \(4,\) you learned that feldspars are very common minerals in igneous rocks. When you learn about the common minerals that compose sedimentary rocks in Chapter \(7,\) 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 easily to clay, making them rare in sedimentary rocks. Quartz is resistant to weathering, thus common in sedimentary rocks.

Step-by-step solution

Identification of Feldspar's Properties

Feldspars, found in igneous rocks, are susceptible to chemical weathering processes. Chemical weathering occurs when minerals react with water and atmospheric gases, forming new minerals and soluble ions. Feldspars are silicate minerals that can easily undergo hydrolysis reactions, which break down feldspar into clay minerals.

Chemical Weathering in Sedimentary Rocks

In sedimentary environments, minerals are subject to weathering and erosion. Chemical weathering causes feldspars to be relatively unstable and less likely to be preserved in sedimentary rocks as they break down into other minerals such as clay.

Expectation of Predominant Minerals in Sedimentary Rocks

Unlike feldspars, some minerals are more resistant to chemical weathering. Quartz is a mineral that is highly resistant to weathering. It does not break down easily during chemical weathering processes, making it a common mineral found in sedimentary rocks.

Key concepts

Igneous Rocks

Igneous rocks are formed from the cooling and solidification of magma or lava. They are primarily composed of a variety of minerals that crystallize under high-temperature conditions deep within the Earth's crust or on its surface during volcanic eruptions. These rocks are distinguished by their mineral content and texture, which are dictated by the environment in which they form.
Some common types of igneous rocks include:

• Granite
• Basalt
• Andesite
• Diorite

The minerals found within igneous rocks, like feldspars, are initially stable but can become unstable under conditions on the Earth's surface. This instability is particularly true with regard to chemical weathering. Chemical weathering involves the alteration of minerals when they interact with atmospheric moisture or other environmental compounds. Over time, this process transforms these minerals into new, more stable forms at the Earth's surface.

Sedimentary Rocks

Sedimentary rocks form from the accumulation and compaction of sediment materials, which can include fragments of other rocks, minerals, and organic matter. These materials are often deposited in layers through processes such as weathering and erosion. Such layered sediments are eventually cemented together over vast periods of time.
The distinctive characteristic of sedimentary rocks is their composition, generally a mix of mineral fragments that survive transportation and settling in a stable environment. Because sedimentary rocks are formed at or near the Earth’s surface, the minerals within them have already been subjected to weathering.
Some key features of sedimentary rocks include:

• Presence of fossils
• Layered appearance
• Usually consisting of clays, carbonates, and other stable minerals

Chemical weathering plays a vital role in transforming less stable minerals, like feldspars, into more stable ones like clays and quartz, which are then featured prominently in sedimentary rocks.

Feldspars

Feldspars are a group of minerals that are highly significant in geology due to their abundance in the Earth's crust. They are primarily found in igneous rocks and are known for their diverse range of colors and hardness levels. Feldspars make up nearly two-thirds of the Earth's crust and are composed of aluminosilicate minerals containing calcium, sodium, or potassium.
These minerals are particularly sensitive to chemical weathering processes, especially hydrolysis, a chemical reaction with water. When feldspars are exposed to the elements, they break down into smaller particles and convert into clay minerals like kaolinite. This transformation is why feldspars are rare in sedimentary rocks compared to their abundance in igneous rocks.
Key reasons for feldspar's breakdown include:

• Sensitivity to acidic conditions
• Relatively unstable structure compared to quartz
• Easily altered by water reactions

As a result, the absence of feldspars is notable in sedimentary rocks, where more chemically resilient minerals dominate.

Quartz

Quartz, one of the most abundant and durable minerals on Earth, plays a significant role in the context of chemical weathering. It is primarily known for its resistance to weathering processes, which makes it a common component of both igneous and sedimentary rocks.
In comparison to feldspars, quartz remains relatively unaffected by chemical weathering due to its strong silicon-oxygen bonds, which do not easily break down by natural acidic elements. This resistance explains quartz’s prominence in sedimentary settings, where more reactive minerals have already been degraded or washed away.
Some characteristics of quartz include:

• High hardness and chemical stability
• Common in both igneous and sedimentary rocks
• Lacks cleavage, contributing to its durability

Given these properties, quartz is often found in layers of sedimentary rocks such as sandstones, where it has resisted the change while other minerals have transitioned into different forms or been lost entirely.