Identify the Mineral on the basis of the following characteristics relating to it : I. It is usually a greenish crystal. II. It is often found in basaltic rocks. II. Magnesium, iron and silica are its major elements. Select the answer using the code given below :

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

Welcome to your first step in mastering the Earth's building blocks! To understand rocks and minerals, we must first look at the Earth's Crust— the thin, outermost shell where we live and where all our mineral wealth is found. Think of the crust as the 'skin' of an apple; it is incredibly thin, making up less than 1% of the Earth's total mass and volume Physical Geography by PMF IAS, Chapter 13, p.52. Despite its thinness, it is remarkably diverse, varying in thickness and composition depending on whether you are standing on a continent or sailing over an ocean.

The crust is divided into two main types: Continental and Oceanic. The continental crust is much thicker, especially under massive mountain ranges like the Himalayas, where it can reach depths of 70 to 100 km. In contrast, the oceanic crust is much thinner (averaging 5–30 km) but denser Physical Geography by PMF IAS, Chapter 13, p.52. This density difference is why continents 'float' higher on the mantle than the ocean floors do.

Feature	Oceanic Crust	Continental Crust
Average Thickness	5 – 30 km	50 – 70 km (up to 100 km in Himalayas)
Main Elements	Silica and Magnesium (SiMa)	Silica and Aluminium (SiAl)
Density	Higher (~3.0 g/cm³)	Lower (~2.7 g/cm³)

Chemically, the crust is dominated by just a handful of elements. While the entire Earth is mostly made of Iron, the Crust specifically is dominated by Oxygen and Silicon. These elements combine with others to form minerals—naturally occurring substances with a definite chemical composition INDIA PEOPLE AND ECONOMY, Chapter 7, p.53. When these minerals are concentrated enough to be mined profitably, we call them ores Science Class X, Chapter 3, p.49.

Sources: Physical Geography by PMF IAS, Chapter 13: Types of Rocks & Rock Cycle, p.52-53; INDIA PEOPLE AND ECONOMY (NCERT), Chapter 7: Mineral and Energy Resources, p.53; Science Class X (NCERT), Chapter 3: Metals and Non-metals, p.49

2. Physical Properties Used to Identify Minerals (basic)

When you look at a landscape, the rocks you see are made up of building blocks called minerals. Identifying these minerals is like being a detective; you look for specific physical clues that each mineral leaves behind. Because minerals are inorganic substances with a definite chemical composition, their internal structure manifests in outward physical traits that we can observe without needing a laboratory INDIA PEOPLE AND ECONOMY, Mineral and Energy Resources, p.54.

One of the first things a geologist notices is Color. While some minerals can vary in color due to impurities, others have a very distinctive "diagnostic" color. For instance, Haematite is famous for its reddish or coral-red appearance Geography of India, Resources, p.7. However, color can be tricky! A more reliable property is Streak, which is the color of the mineral in its powdered form. Even if a mineral's surface is tarnished, its streak remains consistent. Another vital property is Luster—the way light reflects off the surface. This helps us categorize minerals into Metallic (shiny like metal) or Non-metallic (pearly, glassy, or dull) groups INDIA PEOPLE AND ECONOMY, Mineral and Energy Resources, p.53.

We also look at how a mineral holds together. Hardness measures how easily a mineral can be scratched, usually ranked on the Mohs Scale (where Talc is the softest and Diamond is the hardest). Most minerals, including precious ones like diamonds, are formed under extreme pressure deep within the Earth's mantle and are brought to the surface by volcanic activity Physical Geography by PMF IAS, Earths Interior, p.57. When these minerals break, they do so in specific ways: Cleavage refers to breaking along flat, parallel planes, while Fracture is an irregular, jagged break. By combining these observations—color, luster, hardness, and breakage—we can uniquely identify almost any mineral found in the crust.

To help you remember how these properties compare, look at this table:

Property	Description	Example
Hardness	Resistance to scratching	Diamond (Hardest)
Streak	Color of the powdered mineral	Red streak for Haematite
Luster	Appearance in reflected light	Metallic vs. Glassy
Cleavage	Tendency to break along smooth planes	Mica (breaks into thin sheets)

Sources: INDIA PEOPLE AND ECONOMY, Mineral and Energy Resources, p.53-54; Geography of India, Resources, p.7; Physical Geography by PMF IAS, Earths Interior, p.57

3. Classification of Igneous Rocks: Mafic vs. Felsic (intermediate)

When we look at igneous rocks, we don't just categorize them by where they cooled (intrusive vs. extrusive); we also look at what they are made of. This chemical classification primarily hinges on the amount of silica (SiO₂) present in the magma. We generally divide these into two major families: Felsic (acidic) and Mafic (basic) rocks. Understanding this distinction is crucial because it determines everything from the color of the rock to the explosiveness of a volcano.

Felsic rocks get their name from a combination of Feldspar and Silica. These rocks are rich in lighter elements like aluminum, potassium, and sodium, and they contain a high percentage of silica (usually over 65-70%). Because of this chemistry, felsic rocks like Granite are typically light-colored and have a lower density. In the context of our planet's structure, the continental crust is largely composed of these lighter felsic rocks Physical Geography by PMF IAS, Earths Interior, p.53. A key physical property to remember is that high silica content makes magma very "thick" or viscous. This is why felsic (or andesitic) magma often leads to the formation of steep, conical stratovolcanoes because the lava doesn't flow very far before solidifying Physical Geography by PMF IAS, Divergent Boundary, p.131.

On the opposite end of the spectrum are Mafic rocks, a name derived from Magnesium and Ferrum (iron). These rocks are silica-poor (around 45-55%) but rich in heavy minerals like Olivine and pyroxene Physical Geography by PMF IAS, Some Rock-Forming Minerals, p.176. Because of the iron and magnesium, mafic rocks like Basalt and Gabbro are dark-colored and much denser than felsic rocks. This density explains why the oceanic crust, which is mostly basaltic, sits lower than the continental crust Physical Geography by PMF IAS, Earths Interior, p.53. Mafic magma has low viscosity, meaning it is "runny." This allows it to flow over vast distances, creating wide, low-profile shield volcanoes or contributing to seafloor spreading Physical Geography by PMF IAS, Divergent Boundary, p.131.

Feature	Felsic (Acidic)	Mafic (Basic)
Silica Content	High (>65%)	Low (45-55%)
Key Elements	Aluminum, Potassium, Sodium	Magnesium, Iron (Ferrum)
Color & Density	Light color; Lower density	Dark color; Higher density
Viscosity (Flow)	High (Thick/Sticky)	Low (Runny/Fluid)
Common Example	Granite	Basalt

Sources: Physical Geography by PMF IAS, Earths Interior, p.53; Physical Geography by PMF IAS, Some Rock-Forming Minerals, p.176; Physical Geography by PMF IAS, Divergent Boundary, p.131

4. Major Rock-Forming Silicate Groups (intermediate)

To understand rocks, we must first understand the silicates—the most important group of rock-forming minerals. Nearly 90% of the Earth's crust is made of silicate minerals. Their basic building block is the Silicon-Oxygen tetrahedron (SiO₄), where one silicon atom is bonded to four oxygen atoms. How these tetrahedra link together determines the mineral's properties, such as its hardness and how it breaks. Among the silicates, Feldspar is the undisputed heavyweight, making up about half of the Earth's crust. It is light-colored and composed of silicon, oxygen, sodium, potassium, calcium, and aluminium Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.175. Closely following is Quartz, which consists purely of silicon and oxygen. Quartz is highly resistant to weathering and is a primary component of acidic rocks like granite Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.170. Another vital group includes the Ferromagnesian silicates (rich in Iron and Magnesium). These are typically darker and denser. A prime example is Olivine, which is characterized by its distinctive olive-green color and is found in mafic rocks like basalt. While Pyroxene also contains magnesium and iron, it is distinguished by the addition of calcium and aluminium and often appears black or green in igenous rocks and meteorites.

Mineral Group	Main Elements	Distinguishing Features
Feldspar	Si, O, Al, Na, K, Ca	Most abundant; used in ceramics.
Quartz	Si, O	Hexagonal crystals; no cleavage; very hard.
Olivine	Mg, Fe, Si, O	Greenish color; common in basalt.
Pyroxene	Mg, Fe, Si, Ca, Al	Commonly black; found in meteorites.

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

5. Metamorphic Rocks and Non-Mineral Options (intermediate)

The word metamorphic literally translates to a 'change of form.' Unlike igneous rocks (which freeze from a liquid) or sedimentary rocks (which settle from fragments), metamorphic rocks are the result of recrystallization and reorganization of minerals within existing rocks. This transformation occurs while the rock remains in a solid state, driven by intense changes in Pressure, Volume, and Temperature (PVT) Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.173. These changes usually happen when rocks are forced deep into the Earth's crust by tectonic movements or when they come into contact with searing hot magma rising through the crust.

During metamorphism, minerals may arrange themselves in distinct patterns. Foliation or lineation occurs when mineral grains align in layers or lines due to uneven pressure. In some cases, minerals of different groups are arranged into alternating thin and thick layers, appearing as light and dark bands; this is often referred to as gneissoid structure. It is vital to distinguish between a mineral (a chemically uniform substance like Olivine or Quartz) and a metamorphic rock (an aggregate of minerals like Amphibolite or Schist). For example, while Olivine is a specific magnesium-iron-silicate mineral found in basaltic rocks, Amphibolite is a complex rock formed through the metamorphism of mafic igneous rocks Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.169.

To master this topic, you should recognize the 'parent' rock and its metamorphic 'daughter' product. These transformations completely alter the original character and appearance of the rock:

Original (Parent) Rock	Metamorphic Product
Limestone	Marble
Sandstone	Quartzite
Shale / Clay	Slate or Schist
Granite	Gneiss
Coal	Graphite

GC Leong, The Earth's Crust, p.19

Sources: Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.173; Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.169; Certificate Physical and Human Geography, GC Leong, The Earth's Crust, p.19

6. Distinguishing Olivine and Pyroxene (exam-level)

When we look at the building blocks of the Earth's interior, two mineral groups stand out: Olivine and Pyroxene. Both are classified as ferromagnesian silicates, meaning they are rich in iron (Fe) and magnesium (Mg). However, for a UPSC aspirant, the devil is in the details—specifically their chemical complexity and where they are typically found.

Olivine is the simpler of the two. It is primarily composed of magnesium, iron, and silica Physical Geography by PMF IAS, Some Rock-Forming Minerals, p.176. Its most striking feature is its olive-green to deep bottle-green color, which makes it popular in jewelry (as the gemstone peridot). Geologically, it is the MVP of the Earth's interior; the mantle is composed mainly of very dense rocks rich in olivine Certificate Physical and Human Geography, The Earth's Crust, p.17. You will frequently find it in basaltic rocks (oceanic crust) and ultramafic rocks like peridotite.

Pyroxene, on the other hand, is chemically more "crowded." While it also contains magnesium, iron, and silica, it introduces calcium and aluminum into its structure Physical Geography by PMF IAS, Some Rock-Forming Minerals, p.176. Physically, while it can be green, it is often black or very dark in appearance. A unique diagnostic feature for exam purposes is its frequent occurrence in meteorites, though it is also a common component of the Earth's crust.

Feature	Olivine	Pyroxene
Key Elements	Mg, Fe, Silica	Mg, Fe, Silica + Ca, Al
Common Color	Distinctive Olive Green	Green or Black
Primary Habitat	Upper Mantle, Basalt	Crust, Meteorites

Sources: Physical Geography by PMF IAS, Some Rock-Forming Minerals, p.176; Certificate Physical and Human Geography, The Earth's Crust, p.17

7. Solving the Original PYQ (exam-level)

Now that you have mastered the building blocks of rock-forming minerals and their chemical signatures, this question asks you to synthesize those concepts. It requires you to bridge the gap between a mineral's physical properties (color), its geological context (occurrence in basalt), and its chemical composition. In Physical Geography by PMF IAS, we learn that minerals are categorized by the elements they contain; here, the presence of Magnesium, Iron, and Silica serves as the primary chemical fingerprint for identifying mafic minerals found deep within the crust and mantle.

To arrive at the correct answer, look for the "smoking gun" in the description: the greenish crystal. While many minerals can exhibit various colors, the distinct olive-green to deep bottle-green hue is a definitive trait of (D) Olivine. Think of the name itself as a mnemonic for its color. When you combine this visual cue with its chemical makeup and its frequent occurrence in basaltic rocks (which are rich in iron and magnesium), the reasoning becomes clear. Olivine is a primary component of the Earth's upper mantle, making it a staple in the mafic and ultramafic rocks you studied in the igneous rock module.

UPSC often includes "distractor" options that share similar traits to test your precision. Pyroxene is a common trap because it also contains Magnesium, Iron, and Silica; however, it typically includes Calcium and Aluminum and usually appears black rather than a vibrant green crystal. Feldspar is the most abundant mineral in the crust, but it is an Aluminum silicate often containing Potassium or Sodium, rarely appearing green. Finally, Amphibolite is a classic trap because it is actually a metamorphic rock, not a single mineral. By distinguishing between chemical formulas and rock types, you can confidently isolate Olivine as the only option that fits all three criteria.