Which of the following is the correct ascending order of the given minerals in terms of their presence in the Earth's crust ? 1. Amphibolite 2. Mica 3. Pyroxene Select the answer using the code given below :

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

To understand the ground we walk on, we must look at its chemical 'ingredients.' The Earth's crust is the thin, outermost skin of our planet, representing less than 1% of the Earth's total mass Physical Geography by PMF IAS, Earths Interior, p.52. While it may be thin, it is chemically diverse. When we break down the crust into its basic chemical elements, we find that just eight elements account for over 98% of its total weight. Oxygen is the most abundant, followed closely by Silicon. Together, these two elements form the backbone of most rocks, often referred to as the 'Sial' (Silica and Alumina) layer in continental areas Certificate Physical and Human Geography - GC Leong, The Earth's Crust, p.17.

It is crucial to distinguish between the composition of the Crust and the Whole Earth. While the Crust is dominated by lighter elements like Oxygen and Silicon, the Earth as a whole is much heavier because its core is packed with Iron. This difference is a result of planetary differentiation, where heavier materials sank to the center during the Earth's formation, leaving lighter materials to float on top and cool into the crust.

Rank	Most Abundant Elements (Crust)	Percentage (by weight)
1	Oxygen (O)	46.6%
2	Silicon (Si)	27.7%
3	Aluminium (Al)	8.1%
4	Iron (Fe)	5.0%

These elements rarely stay alone; they bond together to form minerals. For instance, half of the Earth's crust is composed of just one mineral group: Feldspar Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.175. Other significant rock-forming minerals include Quartz (made of Silicon and Oxygen), Pyroxene (common in volcanic rocks), and Mica. Understanding this chemical 'soup' is our first step in mastering how different rocks are formed and why they behave the way they do.

Sources: Physical Geography by PMF IAS, Earths Interior, p.52-53; Certificate Physical and Human Geography - GC Leong, The Earth's Crust, p.17; Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.175

2. Introduction to Rock-Forming Minerals (Silicates) (basic)

To understand the building blocks of our planet, we must look at minerals—naturally occurring inorganic substances with a definite chemical composition and physical properties. While the Earth's crust contains a vast variety of elements, it is primarily composed of eight elements, with Oxygen and Silicon being the most dominant. Consequently, the most common rock-forming minerals are Silicates, which are formed by the combination of Silicon and Oxygen with other metallic elements (Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.49).

Among the silicates, three groups are particularly significant for their role in defining crustal composition: Pyroxene, Amphibole, and Mica. These minerals don't just exist in isolation; they are the primary ingredients of the rocks we see around us, whether they are igneous, sedimentary, or metamorphic (Certificate Physical and Human Geography, GC Leong, The Earth's Crust, p.17). Understanding their relative abundance helps us understand why certain rocks, like basalt or granite, look and behave the way they do.

Mineral Group	Approx. % in Crust	Key Characteristics
Pyroxene	~11%	Commonly found in basic igneous rocks (like basalt) and meteorites. Usually dark green or black.
Amphibole	< 11%	Often forms as an alteration of pyroxene. It is a major constituent of the metamorphic rock amphibolite.
Mica	~4%	Known for its ability to split into thin, flexible sheets. Contains elements like Potassium and Aluminum (INDIA PEOPLE AND ECONOMY, NCERT Class XII, Mineral and Energy Resources, p.57).

In the overall volume of the Earth's crust, Pyroxene is significantly more abundant than Mica. Amphiboles occupy a middle ground—while they are essential rock-formers, they are generally less prevalent than pyroxenes in the total crustal volume. This hierarchy (Mica < Amphibole < Pyroxene) is a fundamental fact geologists use to classify the density and origin of different crustal layers.

Sources: Science , class X (NCERT 2025 ed.), Metals and Non-metals, p.49; Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.), The Earth's Crust, p.17; INDIA PEOPLE AND ECONOMY, TEXTBOOK IN GEOGRAPHY FOR CLASS XII (NCERT 2025 ed.), Mineral and Energy Resources, p.57

3. Properties of Major Silicate Groups: Feldspars and Quartz (intermediate)

When we look at the Earth's crust, we are essentially looking at a massive collection of silicate minerals. These are minerals built around a silicon-oxygen framework. Among them, two groups stand out as the absolute heavyweights: Feldspars and Quartz. Together, they form the backbone of the continental crust, often referred to as the Sial layer because of the dominance of Silica and Aluminium Physical Geography by PMF IAS, Earths Interior, p.52. Understanding these is vital because they dictate how rocks weather, the type of soil they form, and even their industrial utility.

Feldspar is the single most abundant mineral group, making up nearly 50% of the Earth's crust Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.175. It is a versatile group containing silicon, oxygen, and aluminium, but its specific variety depends on whether it hosts Potassium (Orthoclase), Sodium, or Calcium (Plagioclase). Feldspars are generally light-colored and are the primary reason why rocks like granite look pinkish or off-white. In the industrial world, their ability to melt at lower temperatures makes them indispensable for making ceramics and glass.

Quartz, on the other hand, is the purest form of silica (SiO₂). Unlike feldspar, it contains no other metals in its primary structure. It is famous for its hexagonal crystalline structure and its remarkable durability. One of its most distinctive physical properties is that it has no cleavage—meaning when it breaks, it doesn't split along smooth planes but rather shatters with irregular, glass-like edges Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.175. Because it is highly resistant to chemical weathering, quartz often remains as sand long after other minerals in a rock have decomposed.

Feature	Feldspar	Quartz
Abundance	~50% of the crust	Found in sand and granite
Composition	Si, O, Al + (K, Na, or Ca)	Pure Silica (SiO₂)
Structure	Varies by type	Hexagonal; Uncleaved
Common Use	Ceramics and Glass	Radio and Radar (due to piezoelectricity)

Together, high concentrations of these two minerals define Acidic Rocks. These rocks are characterized by being lighter in color and less dense than their "basic" counterparts because they lack heavy minerals like iron and magnesium Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.170. This explains why the continental crust (rich in granite) "floats" higher than the denser oceanic crust.

Sources: Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.169-175; Physical Geography by PMF IAS, Earths Interior, p.52; Certificate Physical and Human Geography, GC Leong, The Earth's Crust, p.17

4. Igneous Rocks: Mafic and Felsic Mineral Composition (intermediate)

When we look at Igneous rocks, their personality—color, weight, and even how they flow when molten—is determined by their mineral DNA. Geologists broadly classify these rocks into two families based on their chemical composition: Felsic and Mafic. Understanding this distinction is like knowing the difference between oil and water; it tells you everything about the rock's origin and behavior.

Felsic rocks (a name derived from Feldspar and Silica) are rich in silica (SiO₂) and aluminum. These rocks, like Granite, are typically light-colored and have lower densities. Key minerals in this group include Quartz and Mica. While Mica is a well-known mineral, it actually forms a relatively small portion of the Earth's crust—approximately 4% Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.176. On the other hand, Mafic rocks (Magnesium and Ferrous/Iron) are low in silica but rich in magnesium and iron. These rocks, such as Basalt and Gabbro, are dark, heavy, and dense Certificate Physical and Human Geography, GC Leong, The Earth's Crust, p.18.

In the transition from Felsic to Mafic, we encounter specific silicate minerals that define the Earth's crustal volume. Pyroxene is a heavyweight in this regard; it is a major group of silicate minerals making up about 11% of the Earth's crust and is a primary constituent of mafic rocks. Amphiboles, which are often found as alterations of pyroxenes, are also significant but generally sit between Mica and Pyroxene in terms of total crustal abundance. Therefore, if you were to rank these by their presence in the crust, the sequence rises from Mica (4%) to Amphibole, and finally to Pyroxene (11%) Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.176.

Feature	Felsic Rocks	Mafic Rocks
Key Elements	Silica (SiO₂), Aluminum	Magnesium, Iron (Fe)
Color	Light (Grey, Pink, White)	Dark (Black, Dark Green)
Example	Granite	Basalt, Gabbro
Mineral Examples	Quartz, Feldspar, Mica	Pyroxene, Olivine

Sources: Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.170, 176; Certificate Physical and Human Geography, GC Leong, The Earth's Crust, p.18

5. Metamorphic Processes and Amphibole Formation (intermediate)

Metamorphism is the process by which pre-existing rocks (protoliths) undergo a transformation in mineralogy, texture, or chemical composition without melting into magma. This change is driven by intense heat, high pressure, and the presence of chemically active fluids. When a rock is subjected to both directed pressure and heat, it undergoes dynamo-thermal metamorphism, leading to a complete recrystallization of the rock and the creation of entirely new structures Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.173. For instance, under these conditions, granite might transform into gneiss, while clay or shale turns into schist.

Central to this transformation are the silicate minerals, specifically Pyroxene and Amphibole. Pyroxene is a major group of rock-forming minerals, accounting for approximately 11% of the Earth's crust, and is commonly found in basic igneous rocks like basalt and even in meteorites Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.176. However, when these igneous rocks are buried deep and subjected to metamorphic conditions, the pyroxene often alters into Amphibole. This is why we find amphiboles as the primary constituent of the metamorphic rock amphibolite. While pyroxene is more abundant in the total crustal volume, amphibole represents a critical phase in the life cycle of minerals as they adapt to the Earth's internal pressures.

To understand the relative presence of these minerals in the Earth's crust, we can look at their percentages. Mica, used extensively in electrical instruments, accounts for roughly 4% of the crust Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.176. If we arrange them by abundance, we see a clear hierarchy:

Mineral	Estimated Crustal Abundance	Common Context
Pyroxene	~11%	Igneous rocks and meteorites
Amphibole	Significant (but less than Pyroxene)	Metamorphic rock (Amphibolite)
Mica	~4%	Igneous and Metamorphic rocks

Sources: Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.173; Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.176

6. Relative Abundance Ranking of Crustal Minerals (exam-level)

To understand the Earth's crust, we must look beyond individual elements like Oxygen and Silicon and focus on how they combine to form rock-forming minerals. While thousands of minerals exist, only a handful make up the vast majority of the crust. The hierarchy of these minerals is determined by the chemical environment of the crust — the continental crust is more felsic (rich in silica and aluminum), while the oceanic crust is mafic (rich in magnesium and iron) Physical Geography by PMF IAS, Earths Interior, p.53.

The most dominant group is Feldspar (making up about half the crust), followed by Quartz. However, when we look at the next tier of abundance, we find the Pyroxene group, which accounts for approximately 11% of the crustal volume. These are typically dark-colored minerals common in basaltic rocks and even meteorites Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.176. Following them are the Amphiboles; while significant, they are generally less abundant than pyroxenes and are often found in metamorphic rocks like amphibolite.

Further down the ladder is Mica, which constitutes about 4% of the Earth's crust. Mica is easily recognized by its ability to split into extremely thin, flexible sheets and is prized in the electrical industry for its insulating properties Geography of India by Majid Husain, Resources, p.22. Understanding this relative ranking — from the ubiquitous Feldspars down to the rarer Micas — allows geologists to predict the behavior and origins of different rock types.

Mineral Group	Approx. Abundance	Primary Elements
Pyroxene	~11%	Ca, Al, Mg, Fe, Silica
Amphibole	~5-7%	Complex Silicates (Al, Mg, Fe, etc.)
Mica	~4%	K, Al, Mg, Fe, Silica

Sources: Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.176; Physical Geography by PMF IAS, Earths Interior, p.53; Geography of India by Majid Husain, Resources, p.22

7. Solving the Original PYQ (exam-level)

Now that you have mastered the basics of Geomorphology, you can see how the fundamental percentages of rock-forming minerals dictate the structure of the Earth's crust. This question tests your ability to recall the hierarchical abundance of the silicate group beyond the well-known leaders like Feldspar and Quartz. In your recent lessons, we identified that while the crust is dominated by oxygen and silicon, these elements manifest in specific mineral proportions. Recognizing the relative volume of Pyroxene, Amphibole, and Mica is essential for understanding the composition of both continental and oceanic lithosphere.

To arrive at the correct answer, we must apply the specific percentages learned in Physical Geography by PMF IAS. Mica, despite its visibility in many rocks, accounts for only about 4% of the crustal volume. Amphiboles (the mineral group found in the rock Amphibolite) have a slightly higher presence, typically around 5-7%. Pyroxene is much more significant, comprising roughly 11% of the crust as a major component of basic igneous rocks. Therefore, the ascending order (from smallest to largest) is Mica (2), followed by Amphibole (1), and then Pyroxene (3). This logic leads us directly to (C) 2, 1, 3.

UPSC frequently employs the "order trap" to catch students who understand the minerals but rush the final step. A common mistake is selecting Option (A), where Mica is correctly identified as the least abundant, but the positions of Pyroxene and Amphibole are swapped. Always verify the specific sequence requested; if you accidentally arranged them in descending order (highest to lowest), you would have chosen a completely different sequence. Another trap is the term Amphibolite; do not be distracted by the rock name—the question is testing your knowledge of the Amphibole mineral group's presence in the overall crustal makeup.