Rocks & Minerals — Set 4

Correct Answer: B. Aluminum

• **Aluminum** = the most abundant metal in Earth's crust at approximately 8% by weight, primarily found in bauxite ore and silicate minerals. • **Bauxite ore** — aluminum is rarely found in pure form; it must be extracted from bauxite ore (aluminum hydroxides) through the energy-intensive Bayer process. • 💡 Wrong-option analysis: Iron: second most abundant metal (~5% of crust); Silicon: most abundant metalloid (~27% of crust) but not a metal; Copper: trace metal, very rare in the crust (~0.006%).

Correct Answer: C. Oxygen

• **Oxygen** = the most abundant element in Earth's crust at ~46-47% by weight, mostly combined with silicon to form silicate minerals that make up most of the crust. • **46-47% by weight** — oxygen dominates the crust as it bonds with silicon, aluminum, iron, and others to form oxides and silicates. • 💡 Wrong-option analysis: Aluminum: third most abundant element (~8%) and most abundant metal; Iron: fourth most abundant (~5%); Silicon: second most abundant (~28%).

Correct Answer: D. Igneous

• **Igneous Rocks = Primary Rocks** = called 'primary' because they were the first rocks to form on Earth, directly from cooling magma; all other rocks ultimately derive from them. • **First to form** — the early Earth was molten; as it cooled, igneous rocks formed first; all sedimentary and metamorphic rocks trace back to original igneous material. • 💡 Wrong-option analysis: Sedimentary: secondary rocks formed from weathered and deposited material; Fossils: organic remains preserved in rocks, not a rock type; Metamorphic: tertiary rocks transformed from other rock types.

Correct Answer: A. Red Sandstone

• **Red Sandstone** = the primary construction material of the Red Fort in Delhi, a sedimentary rock widely used in Mughal architecture for its durability and carvability. • **Mughal architecture** — red sandstone was the preferred material for Mughal monuments (Agra Fort, Fatehpur Sikri); marble was reserved for finer elements. • 💡 Wrong-option analysis: Basalt: dark volcanic rock, not typically used for Mughal monument construction; Marble: white metamorphic rock used for Taj Mahal, not Red Fort; Granite: hard intrusive rock used in South Indian temples.

Correct Answer: B. White Marble

• **White Marble (Makrana Marble)** = the metamorphic rock used to construct the Taj Mahal, sourced from Makrana quarries in Rajasthan, prized for its pure white color and polishability. • **Makrana, Rajasthan** — the specific quarry source of the white marble used in the Taj Mahal; the same marble is still quarried today. • 💡 Wrong-option analysis: Granite: hard, coarse-grained igneous rock, not used for Taj Mahal's main structure; Quartzite: metamorphic rock from sandstone, very hard but not used for Taj; Sandstone: used for Red Fort, not Taj Mahal.

Correct Answer: A. Basalt

• **Black Soil from Basalt** = weathering of basaltic lava rocks (Deccan Trap) produces black/regur soil rich in iron, magnesium, and alumina, ideal for cotton cultivation. • **Deccan Trap** — the volcanic basalt plateau covering large areas of Maharashtra, Karnataka, and MP; its weathered black cotton soil (regur) is India's most fertile cotton soil. • 💡 Wrong-option analysis: Limestone: weathers into red/brown soil rich in calcium; Granite: weathers into sandy, nutrient-poor soil; Sandstone: weathers into sandy, coarse soil with low fertility.

Correct Answer: D. Carbon

• **Carbon allotropes** = Diamond and Graphite are both pure forms of carbon, differing only in how carbon atoms are bonded — Diamond is cubic, Graphite is layered hexagonal. • **Hardest vs softest** — Diamond (Mohs 10, hardest) has strong 3D covalent bonds; Graphite (Mohs 1-2, very soft) has weak van der Waals forces between layers. • 💡 Wrong-option analysis: Silicon: a metalloid forming silicate minerals, not the element of diamond/graphite; Sulfur: yellow nonmetal with its own allotropes; Iron: metal, forms iron ore minerals like magnetite and hematite.

Correct Answer: B. 1

• **Talc = Mohs 1 (softest)** = talc is assigned the value 1 on the Mohs scale, the minimum, meaning it can be scratched by any other mineral including a fingernail. • **Qualitative scale** — the Mohs scale (1-10) ranks scratch resistance; Talc(1) is softest, Diamond(10) hardest; the scale is ordinal, not linear. • 💡 Wrong-option analysis: 10: assigned to Diamond (hardest); 7: assigned to Quartz (scratches glass); 5: assigned to Apatite (hardness of tooth enamel).

Correct Answer: C. 10

• **Diamond = Mohs 10 (hardest)** = diamond is assigned the maximum value of 10 on the Mohs hardness scale, the hardest known natural material. • **Industrial applications** — diamond's extreme hardness makes it essential for industrial cutting tools, drill bits, and grinding wheels, beyond its use in jewelry. • 💡 Wrong-option analysis: 1: assigned to Talc (softest mineral); 9: assigned to Corundum (includes ruby and sapphire); 5: assigned to Apatite (hardness of human tooth enamel).

Correct Answer: D. Magma

• **Magma** = the term for liquid molten rock stored within the Earth's crust and mantle, a high-temperature mixture of rock material, volatiles, and crystals. • **Becomes lava at surface** — magma is called 'lava' only after it erupts onto the Earth's surface; underground it remains magma regardless of temperature. • 💡 Wrong-option analysis: Lava: same material as magma but after surface eruption; Cinder: small fragments of solidified lava ejected from a volcano; Crater: the bowl-shaped depression at a volcano's summit.