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Chapter 2: Problem 8

What are silicate rocks? Give four examples of such rocks, and explain how they differ from one another in terms of their chemical composition.

Short Answer

Expert verified
Silicate rocks are primarily composed of silicate minerals, which mainly contain silicon and oxygen, along with other elements. Four examples of silicate rocks are granite, basalt, gabbro, and gneiss. Granite is a felsic, intrusive igneous rock with a high silica content (around 70%) and mainly composed of quartz and alkali feldspar. Basalt is a mafic, extrusive igneous rock with a lower silica content (45-52%) and mainly composed of plagioclase feldspar and pyroxene. Gabbro is an intrusive igneous rock that is also mafic, with a silica content of around 48-52%, and contains primarily plagioclase feldspar, pyroxene, and minor amounts of olivine and amphibole. Gneiss is a metamorphic rock with a medium to high silica content and is composed of quartz, feldspar, mica, and other minerals. The chemical compositions of these rocks differ in terms of mineral composition and relative abundances, resulting in different silica content and classification.

Step by step solution

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1. Definition of Silicate Rocks

Silicate rocks are a type of rock that is primarily made up of silicate minerals. These minerals are comprised mainly of silicon and oxygen, usually with various other elements, such as aluminium, potassium, magnesium, sodium, and calcium. Silicate rocks are the most abundant rocks on Earth and can be found in both the Earth's crust and mantle.
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2. Examples of Silicate Rocks

Here are four examples of silicate rocks: 1. Granite 2. Basalt 3. Gabbro 4. Gneiss
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3. Chemical Composition of Granite

Granite is an intrusive igneous rock that is primarily composed of quartz and alkali feldspar minerals (for example, potassium feldspar). It also contains minor amounts of plagioclase feldspar, mica, and amphibole minerals. Due to the presence of a significant amount of quartz, granite has a high silica content of around 70%, making it a felsic rock.
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4. Chemical Composition of Basalt

Basalt is an extrusive igneous rock that is mainly composed of plagioclase feldspar and pyroxene minerals. It also contains minor amounts of olivine and amphibole. Basalt has a lower silica content of around 45-52% and is rich in iron and magnesium, which classifies it as a mafic rock.
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5. Chemical Composition of Gabbro

Gabbro is an intrusive igneous rock that has a similar mineral composition to basalt, containing primarily plagioclase feldspar, pyroxene, and minor amounts of olivine and amphibole. However, because it is an intrusive rock, its mineral crystals are larger and more visible. Gabbro has a silica content of around 48-52% and is also classified as a mafic rock, similar to basalt.
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6. Chemical Composition of Gneiss

Gneiss is a metamorphic rock that is formed from the alteration of previously existing rocks, mostly granitic and sedimentary rocks, under high pressure and temperature conditions. It is composed of quartz, feldspar, mica, and other minerals such as amphiboles. The chemical composition of gneiss varies widely depending on the original rock it formed from. However, it typically has a medium to high silica content, classifying it as an intermediate to felsic rock. In summary, silicate rocks are mainly composed of silicate minerals and can be categorized into various types based on their mineral composition and formation processes. Though all these examples are silicate rocks, their chemical compositions differ in terms of the specific minerals present and their relative abundances, resulting in differences in their silica content and classification as felsic, intermediate, or mafic rocks.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Igneous Rocks
Igneous rocks form from the cooling and solidification of molten magma or lava. This process can occur beneath the Earth's surface, resulting in intrusive igneous rocks, such as granite and gabbro, or on the surface following a volcanic eruption, which creates extrusive igneous rocks like basalt.

These rocks are categorized based on their mineral composition and the speed at which they cool, leading to variations in crystal size and texture. Rapid cooling often leads to the fine-grained texture of extrusive rocks, while slower cooling allows for larger crystals in intrusive rocks. The mineral composition, specifically the silica content, further aids in classifying them as either felsic (high silica, lighter color) or mafic (lower silica, darker color).
Metamorphic Rocks
Metamorphic rocks, such as gneiss, are born from the transformation of existing rocks under the influence of high pressure and temperature without the rock reaching a molten state.

These conditions induce physical and chemical changes, resulting in new mineralogies and textures. The degree of metamorphism can vary, leading to a range of rocks from slightly modified to highly altered forms. The original rock, known as the protolith, plays a critical role in determining the characteristics of the metamorphic rock, influencing aspects like grain size, banding formation, and mineral content. While igneous rocks start from a molten state, metamorphic rocks always originate from a solid state, highlighting a key difference in their geological creation stories.
Mineral Composition
Mineral composition is fundamental in defining the nature of a rock. Silicate rocks consist of a diverse group of minerals, primarily made up of various combinations of silicon, oxygen, and other elements such as aluminum, potassium, and magnesium.

For example, quartz, a common component of granite, is pure silicon dioxide (SiO2), while other minerals like feldspar and micas exhibit a more complex structure with additional elements. The proportions of these minerals determine not only the rock's color and density but also its overall properties, like hardness and resistance to weathering.
Silica Content
Silica content, expressed as a percentage of silicon dioxide (SiO2) in the rock, is a crucial parameter in classifying silicate rocks. High silica content (over 65%) typically designates a rock as felsic, which tend to be light in color; quartz in granite is an example.

In contrast, mafic rocks like basalt and gabbro have significantly lower silica content (approximately 45-52%) and are often darker due to the presence of minerals such as pyroxene and olivine. The silica content not only influences the rock's classification but also its melting point, viscosity of the magma, and behavior during metamorphism.
Rock Classification
Rock classification involves multiple factors including origin (igneous, sedimentary, or metamorphic), mineral composition, texture, and chemical composition, such as silica content. This systematic categorization helps geologists to understand Earth processes and history.

For instance, the Streckeisen diagram is commonly used for igneous rocks, classifying them into four main groups based on their quartz, feldspar, and ferromagnesian (iron and magnesium-containing minerals) content – felsic, intermediate, mafic, and ultramafic. Metamorphic rocks are classified based on texture, such as foliated or non-foliated, and the degree of metamorphism. Understanding these classifications provides insight into tectonic movements, environmental conditions, and valuable resource locations.

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Most popular questions from this chapter

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