Which one of the following minerals contains mostly silica?

1. Composition of the Earth's Crust (basic)

Welcome! Let’s start our journey by looking at the very ground we stand on. The Earth's crust is the outermost, solid, and brittle layer of our planet. Though it seems vast to us, it is incredibly thin in the context of the whole Earth, making up less than 1% of the Earth’s total mass and volume Physical Geography by PMF IAS, Earths Interior, p.52. Think of it like the thin skin of an apple—essential, but remarkably delicate compared to the fruit inside.

The crust isn't uniform; it’s divided into two distinct types: Continental and Oceanic. They differ significantly in thickness and density. While the oceanic crust is thin and dense, the continental crust is much thicker, especially beneath massive mountain ranges like the Himalayas, where it can reach depths of up to 70–100 km FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI, Interior of the Earth, p.22.

When we look at the chemical composition, the crust is dominated by light elements. Oxygen and Silicon are the most abundant, often combining with metals like Aluminium, Iron, Calcium, Sodium, and Potassium. These elements don't usually exist in isolation; they bond together to form minerals. Interestingly, Feldspar is the most common mineral, making up roughly half of the Earth's crust Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.175. Another vital mineral is Quartz, which is composed entirely of Silicon and Oxygen (SiO₂).

Finally, it’s important to distinguish between minerals and ores. While minerals are naturally occurring compounds, an ore is a mineral from which a metal can be extracted profitably Science Class X, Metals and Non-metals, p.49. For example, Bauxite is the primary ore we use to get Aluminium.

Sources: Physical Geography by PMF IAS, Earths Interior, p.52-53; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI, Interior of the Earth, p.22; Science Class X, Metals and Non-metals, p.49; Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.175-176

2. Introduction to Silicate Minerals (basic)

To understand the Earth's crust, we must first understand silicates. Think of silicates as the primary building blocks of our planet. Most minerals found in the Earth's crust are not pure elements (like gold or silver) but are compounds made of multiple elements Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.129. At the very heart of these compounds is the relationship between Silicon (Si) and Oxygen (O). Silicon is a fascinating element because it holds intermediate properties between metals and non-metals, allowing it to bond strongly with oxygen to form the backbone of the most common rock-forming minerals Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.123.

The simplest and most famous silicate is Quartz. Chemically, Quartz is Silicon Dioxide (SiO₂), often referred to as pure silica. While other minerals are like a "stew" containing various ingredients, pure Quartz is just the base—silicon and oxygen. This high silica content makes Quartz a hallmark of acidic rocks (also called felsic rocks), which are typically lighter in color and weight Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.176.

In contrast, most other silicate minerals are more complex. They take the basic Silicon-Oxygen structure and add other metallic elements like Aluminium, Potassium, Magnesium, or Iron. We can categorize them based on these "extra" ingredients:

• Feldspar & Mica: Include lighter elements like Aluminium and Potassium. These are major components of the continental crust, often called Sial (Silica + Aluminium) Physical Geography by PMF IAS, Earths Interior, p.53.
• Olivine & Pyroxene: These include heavier elements like Magnesium and Iron (Ferromagnesian minerals). These are more common in the oceanic crust, historically referred to as Sima (Silica + Magnesium).

Sources: Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.123, 129; Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.176; Physical Geography by PMF IAS, Earths Interior, p.53

3. The Dominant Rock-Forming Minerals (intermediate)

To understand the Earth's crust, we must look at the building blocks known as rock-forming minerals. While thousands of minerals exist, only a handful are responsible for the vast majority of the rocks we see. A rock is essentially an aggregate of one or more of these minerals held together by chemical bonds Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.169. The scientific study of these rocks is called petrology.

The undisputed heavyweight of the crust is Feldspar, which makes up about half of the Earth's crust. It is a complex silicate containing silicon and oxygen along with elements like sodium, potassium, calcium, and aluminium. Close behind is Quartz, which is unique because it is composed almost entirely of silicon dioxide (SiO₂). Unlike other minerals that include various metals, pure quartz is essentially pure silica Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.175. These two minerals are the primary components of "acidic rocks" like granite, which are lighter in color and less dense because they lack heavy minerals like iron and magnesium Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.170.

Beyond these, we find other silicate minerals like Mica (found in igneous and metamorphic rocks), Pyroxene, and Olivine. While most of these are non-metallic, the crust also contains metallic minerals like Bauxite (the ore for aluminium) and Cinnabar (the source of mercury) Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.175. Understanding these minerals is crucial because their chemical resistance determines how landforms weather over millions of years.

Sources: Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.169; Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.170; Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.175

4. Silica Content in Igneous Rocks (Acidic vs. Basic) (intermediate)

When we classify igneous rocks based on their chemical composition, the most important yardstick is the presence of silica (Silicon Dioxide, SiO₂). In geology, we don't use the term 'acidic' in the way a chemist does (pH levels); instead, it refers to the silicon radical content. Igneous rocks are broadly divided into Acidic and Basic (also known as Mafic) based on this silica percentage Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.169.

Acidic rocks are characterized by a high silica content, often reaching up to 80%. Because silica is relatively light, these rocks are dominated by minerals like quartz and feldspar, making them less dense and lighter in color. From a volcanic perspective, high silica acts like a 'thickener'; it makes the magma highly viscous (sticky). This means acidic magma does not flow far, cools quickly, and tends to build steep-sided stratovolcanoes Physical Geography by PMF IAS, Divergent Boundary, p.131. Granite is the most classic example of an acidic rock, forming the bulk of the continental crust (Sial).

In contrast, Basic rocks have a lower silica content and are enriched with heavier elements like Iron (Ferrous) and Magnesium (hence the term 'Mafic'). These rocks are denser and darker in color, such as Basalt. Because they lack high silica, their magma is less viscous and flows easily over long distances. This fluidity allows them to form vast, flat shield volcanoes or spread across the ocean floor Physical Geography by PMF IAS, Divergent Boundary, p.131.

Sources: Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.169-170; Physical Geography by PMF IAS, Divergent Boundary, p.131

5. Mineral Stability and Bowen's Reaction Series (exam-level)

When magma cools, minerals do not all crystallize at once. Instead, they follow a predictable sequence known as Bowen’s Reaction Series. This process explains why certain minerals are often found together and why others are never seen in the same rock. The series is divided into two branches: the discontinuous series (starting with Olivine and moving to Pyroxene) and the continuous series (plagioclase feldspars). As the temperature drops, the chemical composition of the remaining liquid magma changes, becoming increasingly enriched in silica (SiO₂).

The minerals at the top of the series, like Olivine, form at very high temperatures and are Mafic (rich in Magnesium and Iron). In contrast, minerals at the bottom, like Quartz, form at the lowest temperatures. Quartz is unique because it is essentially pure silica, whereas other minerals are complex silicates containing elements like Calcium, Aluminium, and Iron Physical Geography by PMF IAS, Chapter 13, p. 176. Because Quartz forms at temperatures closest to those found at the Earth's surface, it is chemically the most stable and resistant to weathering.

This sequence also dictates the physical nature of the resulting rocks. For instance, Plutonic rocks like granite cool slowly at depth, allowing large crystals of these minerals to form GC Leong, The Earth's Crust, p. 18. Furthermore, the silica content determines the viscosity of the magma: high-silica (felsic) magma is thick and viscous, often leading to explosive stratovolcanoes, while low-silica (mafic) magma is fluid and forms broad shield volcanoes Physical Geography by PMF IAS, Divergent Boundary, p. 131.

Sources: Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.176; Certificate Physical and Human Geography, GC Leong, The Earth's Crust, p.18; Physical Geography by PMF IAS, Divergent Boundary, p.131

6. Detailed Chemistry of Silicate Groups (exam-level)

To understand the chemistry of rocks, we must first recognize that the Earth's crust is essentially a giant chemical laboratory dominated by two elements: Oxygen (46.6%) and Silicon (27.7%) Physical Geography by PMF IAS, Earths Interior, p.53. Together, they form the silicon-oxygen tetrahedron (SiO₄⁴⁻), the fundamental building block of all silicate minerals. While the base unit is simple, the variety of silicate minerals arises from how these tetrahedra link together and which metallic "guest" ions (like Iron, Magnesium, or Calcium) join the structure.

Silicates are generally categorized by their chemical complexity. At one end of the spectrum is Quartz, which is the most "pure" silicate. Chemically, it is Silicon Dioxide (SiO₂), often reaching nearly 100% purity in its crystalline form. Unlike other silicates, Quartz does not rely on metallic cations to balance its charge; it is a framework of pure silica. This makes it highly resistant to chemical weathering and a primary constituent of "acidic" or silica-rich rocks Physical Geography by PMF IAS, Some Rock-Forming Minerals, p.176.

In contrast, other major silicate groups incorporate various elements that change their physical and chemical properties. For instance, Olivine and Pyroxene are known as ferromagnesian silicates because they are rich in Magnesium (Mg) and Iron (Fe). Olivine is a primary component of the Earth's mantle and basaltic rocks, typically appearing as greenish crystals GC Leong, The Earth's Crust, p.17. Mica represents a more complex group; it includes Potassium and Aluminium and is unique for its physical structure of thin, sheet-like layers or "plates" NCERT Contemporary India II, Non-Metallic Minerals, p.111. These sheets are so flexible and insulating that mica is indispensable in the electrical industry.

Sources: Physical Geography by PMF IAS, Earths Interior, p.53; Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.176; NCERT Contemporary India II, Non-Metallic Minerals, p.111; Certificate Physical and Human Geography, GC Leong, The Earth's Crust, p.17

7. Solving the Original PYQ (exam-level)

You have just explored the fundamental building blocks of the Earth's crust: rock-forming minerals. This question tests your ability to apply the concept of chemical composition to mineral identification. While you have learned that the majority of the crust is composed of silicate minerals, this question asks you to distinguish between complex silicates and pure silica. The key building block here is the silicon-oxygen tetrahedron; however, the degree to which other elements like Iron, Magnesium, or Potassium are integrated into that structure determines the mineral's classification.

To arrive at the correct answer, reason through the chemical simplicity of the options. Quartz is unique because it is a specific chemical compound consisting almost entirely of silicon dioxide (SiO2). In its pure form, it reaches nearly 100% silica content. As highlighted in Physical Geography by PMF IAS, this makes Quartz a primary constituent of acidic rocks. When a question asks which mineral contains "mostly" silica, your mind should immediately go to the most chemically straightforward oxide of silicon, which is (B) Quartz.

The other options—Mica, Olivine, and Pyroxene—represent a common UPSC trap. While they are indeed silicates, they are complex silicates. This means they "contain" silica, but they are also packed with other metallic elements. For example, Olivine and Pyroxene are rich in Magnesium and Iron, while Mica contains Aluminium and Potassium. The examiners are testing whether you can differentiate between a mineral that is silica and minerals that merely incorporate silica into a more complex metallic structure.