Quick Revision Notes: Class 11 Geography

Chapter: Minerals and Rocks (खनिज एवं शैल)

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1. Introduction to Minerals (खनिज परिचय)

• Mineral (खनिज): A naturally occurring, inorganic, solid substance having a definite chemical composition and atomic structure.
• Key Characteristics:
  • Formed by inorganic processes.
  • Has a definite chemical composition (निश्चित रासायनिक संगठन).
  • Has an internal atomic structure (crystalline form).

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2. Physical Characteristics of Minerals (खनिज के भौतिक गुण)

Minerals are identified based on their physical properties:

• External Crystal Form (बाह्य क्रिस्टलीय रूप): Determined by internal arrangement of molecules (cubes, pyramids, hexagonal prisms, etc.).
• Cleavage (विदलन): Tendency to break along planes of weak atomic bonding (results in smooth, flat surfaces).
• Fracture (भंग): The way a mineral breaks when it does not cleave along flat planes (uneven, conchoidal, hackly).
• Luster (चमक): The appearance of a mineral's surface in reflected light (Metallic, Vitreous/Glassy, Pearly, Silky, Earthy).
• Color (रंग): Determined by internal molecular structure (some minerals show color due to impurities like quartz).
• Streak (धारिता/चूर्ण का रंग): The color of a mineral's powder when rubbed on an unglazed porcelain plate.
• Transparency (पारदर्शिता): Transparent (आर-पार), Translucent (धुंधला), or Opaque (अपरदर्शी).
• Structure (संरचना): Particular arrangement of individual grains or crystals (fibrous, bladed, granular, massive).
• Hardness (कठोरता): Resistance to being scratched. Measured on Mohs' Hardness Scale (मोह्स का पैमाना) from 1 to 10.
• Specific Gravity (विशिष्ट गुरुत्व): Ratio of the weight of a given volume of the mineral to the weight of an equal volume of water.

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3. Mohs' Scale of Hardness (मोह्स का कठोरता पैमाना)

Standard reference scale to measure mineral hardness (1 = Softest, 10 = Hardest):

1. Talc (टैल्क) - Softest
2. Gypsum (जिप्सम)
3. Calcite (कैल्साइट)
4. Fluorite (फ्लोराइट)
5. Apatite (एपैटाइट)
6. Feldspar (फेल्डस्पार)
7. Quartz (क्वार्ट्ज)
8. Topaz (टोपाज)
9. Corundum (कोरंडम)
10. Diamond (हीरा) - Hardest

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4. Major Groups of Minerals (खनिज के प्रमुख वर्ग)

• Feldspar (फेल्डस्पार): Half of the earth's crust is composed of feldspar. Color ranges from cream to pink. Used in ceramics and glass making.
• Quartz (क्वार्ट्ज): Consists of silica. Highly weather-resistant. Used in radio and radar.
• Pyroxene (पाइरोक्सीन): Consists of calcium, aluminum, magnesium, iron, and silica. Makes up 10% of the earth's crust. Found in meteorites.
• Amphibole (एम्फीबोल): Aluminum, calcium, silica, iron, magnesium. Green or black color. Makes up 7% of earth's crust. Used in asbestos industry.
• Mica (अभ्रक): Contains potassium, aluminum, magnesium, iron, silica. Forms thin sheets. Used in electrical instruments.
• Olivine (ऑलिवाइन): Magnesium, iron, silica. Used in jewellery; major component of basaltic rocks.
• Other Minerals: Carbonates (Calcite, Dolomite), Sulphides, Oxides, Native Elements (Copper, Gold, Silver).

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5. Classification of Rocks (शैलों का वर्गीकरण)

Rocks are aggregates of minerals. Based on their mode of formation, rocks are classified into three major groups:

A. Igneous Rocks (आग्नेय शैल - "Primary Rocks")

• Formed out of magma and lava from the interior of the earth.
• Sub-types based on cooling location:
  • Intrusive Igneous Rocks (अंतर्भेदी आग्नेय शैल): Magma cools slowly deep inside the crust (e.g., Granite, Gabbro, Pegmatite). Forms coarse grains.
  • Extrusive Igneous Rocks (बहिर्भेदी आग्नेय शैल): Lava cools rapidly on the earth's surface (e.g., Basalt, Deccan Traps). Forms fine-grained rocks.

B. Sedimentary Rocks (अवसादी शैल - "Stratified Rocks")

• Formed through the process of lithification (compaction and cementation of sediments).
• Sub-types based on formation:
  • Mechanically Formed (यांत्रिक रूप से निर्मित): Sandstone, Conglomerate, Limestone, Shale, Loess.
  • Organically Formed (जैविक रूप से निर्मित): Geyserite, Chalk, Limestone, Coal.
  • Chemically Formed (रासायनिक रूप से निर्मित): Chert, Halite, Potash, Gypsum.

C. Metamorphic Rocks (कायांतरित शैल)

• Formed under the influence of heat, pressure, and chemical solutions on pre-existing rocks.
• Types of Metamorphism:
  • Dynamic Metamorphism (गतिटिक कायांतरण): Caused by mechanical disruption and reorganization due to breaking and crushing without chemical changes.
  • Thermal Metamorphism (तापीय कायांतरण): Caused by high heat from magma/lava. Divided into Contact Metamorphism and Regional Metamorphism.
• Foliation / Lineation (पातीभवन / रेखन): Arrangement of minerals in parallel lines or layers during metamorphism.

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6. The Rock Cycle (शैल चक्र)

• Definition: The continuous cycle of transformation in which old rocks are converted into new ones.
• Key Steps:
  1. Magma cools to form Igneous Rocks.
  2. Igneous rocks weather and erode into sediments, forming Sedimentary Rocks.
  3. Igneous and Sedimentary rocks undergo extreme heat and pressure to form Metamorphic Rocks.
  4. Under excessive heat, all rocks melt down into Magma, restarting the cycle.