Class 11 Geography Notes Chapter 5 (Minerals and rocks) – Fundamental of Physical Geography Book

Right, let's focus now on Chapter 5, 'Minerals and Rocks' from your 'Fundamental of Physical Geography' textbook. This is a crucial chapter, forming the bedrock (pun intended!) of understanding landforms and earth processes, making it very important for your government exam preparations.

Chapter 5: Minerals and Rocks - Detailed Notes

1. Introduction: Earth's Composition

• The Earth's crust is the outermost solid shell, composed primarily of rocks.
• Rocks are aggregates of one or more minerals.
• About 98% of the total crust is composed of eight major elements:
  1. Oxygen (O) - ~46.6%
  2. Silicon (Si) - ~27.7%
  3. Aluminium (Al) - ~8.1%
  4. Iron (Fe) - ~5.0%
  5. Calcium (Ca) - ~3.6%
  6. Sodium (Na) - ~2.8%
  7. Potassium (K) - ~2.6%
  8. Magnesium (Mg) - ~2.1%
• The remaining 2% consists of elements like titanium, hydrogen, phosphorus, manganese, sulphur, carbon, nickel, etc.
• These elements usually combine to form substances called minerals.

2. Minerals

• Definition: A mineral is a naturally occurring, inorganic solid substance having an orderly atomic structure (crystalline structure) and a definite chemical composition (which may vary within given limits).
• Key Characteristics:
  • Naturally Occurring: Formed by natural geological processes.
  • Inorganic: Not formed from living organisms (Coal is an exception, often discussed with minerals but is organic).
  • Solid: Exists in a solid state at Earth's surface temperatures.
  • Orderly Crystalline Structure: Atoms are arranged in a specific, repeating pattern.
  • Definite Chemical Composition: Can be expressed by a chemical formula (e.g., SiO₂ for Quartz, NaCl for Halite). Some minerals allow substitution of similar ions, leading to composition variation within limits (e.g., Olivine - (Mg, Fe)₂SiO₄).
• Minerals are the fundamental building blocks of rocks.
• Over 2000 minerals identified, but only a few are abundant ("rock-forming minerals").

3. Physical Characteristics of Minerals (Used for Identification)

• Crystal Form: The external shape reflecting the internal atomic arrangement. Examples: Cubes (Halite), Hexagonal Prisms (Quartz).
• Luster: Appearance of a mineral surface in reflected light.
  • Metallic: Looks like metal (e.g., Pyrite, Galena).
  • Non-metallic: Includes various types like:
    • Vitreous/Glassy: (e.g., Quartz)
    • Pearly: (e.g., Talc, some Mica)
    • Silky: (e.g., Asbestos)
    • Resinous: (e.g., Sphalerite)
    • Earthy/Dull: (e.g., Bauxite)
• Color: Often the most obvious property, but unreliable for identification as impurities can cause variations (e.g., Quartz can be clear, white, pink, purple, etc.). Some minerals have characteristic colours (e.g., Malachite - green, Azurite - blue).
• Streak: The color of the mineral's powder when rubbed against an unglazed porcelain plate (streak plate). More reliable than mineral color (e.g., Hematite may be black or reddish, but its streak is always reddish-brown).
• Transparency: How light passes through a mineral.
  • Transparent: Light passes through, and objects are visible (e.g., clear Quartz).
  • Translucent: Light passes through, but objects are not clearly visible (e.g., Chalcedony).
  • Opaque: Light does not pass through (e.g., Galena).
• Structure: The particular arrangement or shape of individual crystals or aggregates. Examples: Fibrous, granular, tabular, columnar.
• Hardness: Resistance to scratching or abrasion. Measured using the Mohs Scale of Hardness (relative scale 1-10).
  1. Talc (Softest)
  2. Gypsum
  3. Calcite
  4. Fluorite
  5. Apatite
  6. Orthoclase Feldspar
  7. Quartz
  8. Topaz
  9. Corundum
  10. Diamond (Hardest)
  • Common tests: Fingernail (~2.5), Copper penny (~3.5), Glass plate/Steel knife (~5.5).
• Cleavage: Tendency of a mineral to break along planes of weak bonding, producing smooth, flat surfaces. Described by the number of planes and angles between them (e.g., Mica has one perfect cleavage plane; Halite has three planes at 90°).
• Fracture: How a mineral breaks when it does not split along cleavage planes. Surfaces are irregular. Examples: Conchoidal (curved, like broken glass - Quartz), Fibrous, Uneven.
• Specific Gravity: Ratio of the mineral's weight to the weight of an equal volume of water (density). Most common minerals have specific gravity between 2.5 and 3.0. Metallic minerals are generally denser.

4. Major Rock-Forming Minerals

• The vast majority of the Earth's crust is composed of Silicate Minerals (Silicon + Oxygen + other elements).
• Feldspar:
  • Most abundant group (over 50% of crust). Silicon and oxygen are common; Sodium, Potassium, Calcium, Aluminium are usually included.
  • Two main types: Alkali Feldspar (Potassium/Sodium rich, e.g., Orthoclase) and Plagioclase Feldspar (Sodium/Calcium rich).
  • Used in ceramics and glassmaking. Color varies (light cream to salmon pink for Orthoclase; white to grey for Plagioclase). Hardness ~6.
• Quartz:
  • Second most abundant mineral in the continental crust. Major component of sand and granite.
  • Composition: SiO₂. Very hard (7 on Mohs scale).
  • Typically colorless or white but varies due to impurities. Lacks cleavage, exhibits conchoidal fracture.
  • Used in glassmaking, electronics (radio, radar).
• Pyroxene:
  • Complex silicates containing Calcium, Aluminium, Magnesium, Iron, Silicon, Oxygen.
  • Common member: Augite. Usually greenish or black. Common in igneous and metamorphic rocks (especially basalt, gabbro). Hardness 5-6. Cleavage at nearly 90°.
• Amphibole:
  • Complex silicates similar to pyroxenes but with Hydroxyl (OH) ions. Major elements: Aluminium, Calcium, Silicon, Iron, Magnesium.
  • Common member: Hornblende. Usually dark green to black. Found in igneous and metamorphic rocks. Hardness 5-6. Cleavage at ~60° and 120°. Used in asbestos (fibrous variety).
• Mica:
  • Characterized by perfect basal cleavage (splits into thin sheets). Contain Potassium, Aluminium, Magnesium, Iron, Silicon, Oxygen.
  • Common types: Muscovite (light-colored/clear), Biotite (dark/black). Found in igneous and metamorphic rocks. Used as insulators in electrical equipment. Hardness 2-3.
• Olivine:
  • Magnesium Iron Silicates - (Mg, Fe)₂SiO₄.
  • Usually greenish, glassy luster, conchoidal fracture. Common in high-temperature igneous rocks (basalt, gabbro, peridotite). Often weathers easily at the surface. Used in refractory bricks, gemstones (Peridot). Hardness 6.5-7.
• Other Minerals: Non-silicates like Carbonates (Calcite - CaCO₃), Oxides (Hematite - Fe₂O₃), Sulphides (Pyrite - FeS₂), Sulphates (Gypsum - CaSO₄·2H₂O), Halides (Halite - NaCl), Native Elements (Gold - Au, Copper - Cu) are also important but less abundant as rock-formers.

5. Rocks

• Definition: A rock is a naturally formed, consolidated aggregate composed of one or more minerals. Some rocks (like Obsidian - volcanic glass) lack crystalline structure, and some (like Coal) are organic.
• The scientific study of rocks is called Petrology.

6. Classification of Rocks (Based on Mode of Formation)

• A. Igneous Rocks (Primary Rocks):

  • Formation: Formed from the cooling and solidification (crystallization) of molten rock material called magma (below the surface) or lava (on the surface).
  • Characteristics: Usually hard, crystalline, lack strata (layers), lack fossils.
  • Classification based on Location of Cooling:
    • Intrusive (Plutonic) Igneous Rocks: Magma cools slowly deep beneath the Earth's surface. Results in large, well-formed crystals (coarse-grained texture). Examples: Granite, Gabbro, Diorite, Peridotite. Often exposed later by erosion.
    • Extrusive (Volcanic) Igneous Rocks: Lava cools rapidly on the Earth's surface. Results in small crystals (fine-grained texture) or no crystals (glassy texture). Examples: Basalt, Rhyolite, Andesite, Obsidian (glassy), Pumice (frothy, vesicular).
  • Classification based on Texture: Texture refers to the size, shape, and arrangement of mineral grains. Primarily determined by the rate of cooling.
    • Phaneritic: Coarse-grained (crystals visible to naked eye) - Slow cooling (Intrusive).
    • Aphanitic: Fine-grained (crystals too small to see) - Rapid cooling (Extrusive).
    • Glassy: No crystals - Very rapid cooling (Extrusive).
    • Porphyritic: Mix of large and small crystals - Indicates two stages of cooling.
    • Vesicular: Contains holes (vesicles) from trapped gas bubbles - Rapid cooling of gassy lava (Extrusive).
  • Classification based on Chemical Composition (Silica content): Felsic (high silica, light color - Granite, Rhyolite), Intermediate (Andesite, Diorite), Mafic (low silica, high Mg/Fe, dark color - Basalt, Gabbro), Ultramafic (very low silica - Peridotite).

• B. Sedimentary Rocks (Secondary Rocks):

  • Formation: Formed from the accumulation, compaction, and cementation of sediments. Sediments are fragments derived from the weathering and erosion of pre-existing rocks (igneous, metamorphic, or older sedimentary), organic matter, or chemical precipitation. The process of turning sediments into rock is called Lithification.
    • Compaction: Weight of overlying sediments squeezes water out and reduces pore space.
    • Cementation: Dissolved minerals (like calcite, silica, iron oxide) precipitate in pore spaces, binding grains together.
  • Characteristics: Often occur in layers or strata (stratified), may contain fossils, generally softer than igneous rocks, often porous.
  • Classification based on Mode of Formation:
    • Mechanically Formed (Clastic/Detrital): Formed from fragments (clasts) of pre-existing rocks. Classified by grain size.
      • Conglomerate/Breccia: Large, rounded (conglomerate) or angular (breccia) gravel-sized clasts.
      • Sandstone: Sand-sized grains (often quartz).
      • Siltstone: Silt-sized grains.
      • Shale/Mudstone: Clay and silt-sized grains, often laminated (shale).
    • Organically Formed (Biogenic): Formed from the accumulation of organic matter (plant or animal remains).
      • Limestone: Primarily composed of Calcite (CaCO₃) from shells, coral, skeletal debris (e.g., Chalk, Coquina).
      • Coal: Compressed and altered plant remains.
    • Chemically Formed (Non-Clastic): Formed by precipitation of minerals from water (often seawater) due to evaporation or chemical reactions.
      • Limestone (some types): Precipitated calcite.
      • Rock Salt (Halite): Evaporation of saline water.
      • Gypsum: Evaporation of sulphate-rich water.
      • Chert/Flint: Microcrystalline quartz (SiO₂).

• C. Metamorphic Rocks:

  • Formation: Formed when pre-existing rocks (igneous, sedimentary, or other metamorphic rocks) are changed (metamorphosed) by heat, pressure, and/or chemical reactions deep within the Earth, without melting completely. The original rock is called the parent rock or protolith.
  • Agents of Metamorphism:
    • Heat: From magma intrusions or deep burial. Increases reaction rates, can cause recrystallization.
    • Pressure:
      • Confining Pressure: Equal pressure from all directions due to burial. Compacts rock, may cause phase changes.
      • Differential Stress/Directed Pressure: Unequal pressure, often during mountain building. Causes deformation and alignment of minerals (foliation).
    • Chemically Active Fluids: Hot water containing dissolved ions can react with minerals and facilitate recrystallization.
  • Types of Metamorphism:
    • Contact Metamorphism (Thermal): Occurs when rocks are "baked" by contact with magma/lava. High temperature, low pressure. Affects a localized area (aureole). Often produces non-foliated rocks.
    • Regional Metamorphism: Occurs over large areas, associated with mountain building. Involves high temperature and high directed pressure. Often produces foliated rocks.
    • Dynamic Metamorphism: Associated with fault zones where rocks are ground and sheared under high pressure, with less emphasis on temperature change.
  • Characteristics: Often harder and denser than parent rocks. May exhibit foliation (parallel alignment of platy or elongated mineral grains, giving a layered or banded appearance) or be non-foliated (minerals are not aligned).
  • Classification based on Texture:
    • Foliated Rocks: Minerals aligned in parallel layers or bands due to directed pressure. Examples (increasing metamorphic grade):
      • Slate: Fine-grained, splits easily into flat sheets (from Shale).
      • Phyllite: Slightly coarser than slate, glossy sheen (from Slate).
      • Schist: Medium-to-coarse grained, platy minerals visible (mica), scaly/platy appearance (from Phyllite/Granite/Basalt).
      • Gneiss: Coarse-grained, banded appearance with alternating layers of light and dark minerals (from Schist/Granite/Diorite).
    • Non-Foliated Rocks: No layered appearance, minerals are randomly oriented. Often form from parent rocks with equi-dimensional grains or under contact metamorphism. Examples:
      • Marble: Metamorphosed Limestone or Dolostone (composed of Calcite).
      • Quartzite: Metamorphosed Sandstone (composed of Quartz), very hard.
      • Hornfels: Fine-grained, baked by contact metamorphism (various parent rocks).

7. The Rock Cycle

• A fundamental concept illustrating the processes that create, change, and destroy rocks.
• Rocks are constantly being transformed from one type to another in a continuous cycle.
• Key Processes: Cooling & Solidification, Weathering & Erosion, Transportation, Deposition, Lithification (Compaction & Cementation), Metamorphism (Heat & Pressure), Melting.
• Pathways:
  • Magma cools -> Igneous Rock
  • Igneous Rock undergoes weathering -> Sediments -> Lithification -> Sedimentary Rock
  • Sedimentary Rock undergoes heat/pressure -> Metamorphic Rock
  • Metamorphic Rock melts -> Magma
  • Shortcuts exist: Igneous Rock -> Metamorphism -> Metamorphic Rock; Metamorphic Rock -> Weathering -> Sediments -> Sedimentary Rock; Sedimentary Rock -> Weathering -> Sediments -> Sedimentary Rock.
• The rock cycle demonstrates the interrelationship between internal (tectonic, volcanic) and external (weathering, erosion) earth processes.

8. Importance of Minerals and Rocks

• Economic: Source of metals (Iron, Copper, Gold), building materials (Granite, Marble, Sandstone), industrial minerals (Gypsum, Halite), gemstones (Diamond, Ruby), fuel (Coal).
• Soil Formation: Weathering of rocks is the first step in creating soil.
• Landforms: Different rock types erode at different rates, shaping landscapes.
• Scientific Study: Provide clues about Earth's history, climate, and processes.

Multiple Choice Questions (MCQs)

1. Which of the following is the most abundant mineral group in the Earth's crust?
   (a) Carbonates
   (b) Oxides
   (c) Silicates
   (d) Sulphides

2. The tendency of a mineral to break along planes of weak bonding is called:
   (a) Fracture
   (b) Hardness
   (c) Streak
   (d) Cleavage

3. Granite is an example of which type of rock?
   (a) Extrusive Igneous
   (b) Intrusive Igneous
   (c) Clastic Sedimentary
   (d) Foliated Metamorphic

4. Which process is primarily responsible for the formation of sedimentary rocks?
   (a) Melting and cooling
   (b) Heat and pressure
   (c) Lithification
   (d) Volcanic eruption

5. Marble is a metamorphic rock formed from which parent rock?
   (a) Sandstone
   (b) Shale
   (c) Granite
   (d) Limestone

6. Which of the following is NOT a defining characteristic of a mineral?
   (a) Naturally occurring
   (b) Organic origin
   (c) Definite chemical composition
   (d) Orderly crystalline structure

7. Mohs scale is used to determine which property of minerals?
   (a) Luster
   (b) Specific Gravity
   (c) Hardness
   (d) Streak

8. Fossils are most commonly found in which type of rock?
   (a) Igneous rocks
   (b) Metamorphic rocks
   (c) Sedimentary rocks
   (d) Volcanic rocks

9. Which of these rocks exhibits foliation?
   (a) Marble
   (b) Quartzite
   (c) Slate
   (d) Basalt

10. The rock cycle illustrates that:
    (a) All rocks eventually turn into igneous rocks.
    (b) Rocks are static and unchanging over geologic time.
    (c) Any rock type can be transformed into any other rock type through geological processes.
    (d) Sedimentary rocks are formed only from weathered igneous rocks.

Answer Key:

1. (c) Silicates
2. (d) Cleavage
3. (b) Intrusive Igneous
4. (c) Lithification
5. (d) Limestone
6. (b) Organic origin (Minerals are typically inorganic)
7. (c) Hardness
8. (c) Sedimentary rocks
9. (c) Slate
10. (c) Any rock type can be transformed into any other rock type through geological processes.

Make sure you understand the definitions, processes, and examples thoroughly. Pay special attention to the rock cycle and the characteristics used to differentiate the three main rock types. Good luck with your preparation!