Unit 2: Rocks, Minerals and Weathering

1. Introduction to Earth Materials

Soil science begins with the understanding of the parent material from which soil is formed. The Earth's crust is the source of all soil components.

• Petrology: The science dealing with the description, composition, classification, and origin of rocks.
• Mineralogy: The science dealing with the study of minerals.

2. Minerals

A mineral is a naturally occurring, homogeneous, inorganic solid substance having a definite chemical composition and a distinct atomic structure.

2.1 Classification of Minerals

Minerals are classified based on origin, chemical composition, and specific gravity.

A. Based on Mode of Origin

1. Primary Minerals:
   • Formed from the crystallization of molten magma.
   • They remain unchanged in composition from the original rock.
   • Examples: Quartz, Feldspars (Orthoclase, Plagioclase), Micas (Muscovite, Biotite).
   • Significance: Major source of sand and silt fractions in soil.
2. Secondary Minerals:
   • Formed through the weathering and alteration of primary minerals.
   • Examples: Clay minerals (Kaolinite, Montmorillonite, Illite), Hematite, Limonite, Gypsum.
   • Significance: Dominate the clay fraction; crucial for soil fertility and cation exchange capacity.

B. Based on Quantity (in Rocks)

1. Essential Minerals: Minerals that make up the bulk of the rock and determine its classification (e.g., Feldspar in granite).
2. Accessory Minerals: Present in small quantities; they do not characterize the rock (e.g., Zircon, Apatite).

C. Chemical Classification (Silicates vs. Non-Silicates)

• Silicates: Compose 90% of the Earth's crust. Based on the silica tetrahedron ().
  • Tectosilicates: Quartz, Feldspars.
  • Phyllosilicates (Sheet silicates): Micas, Clay minerals.
• Non-Silicates:
  • Oxides: Hematite ().
  • Carbonates: Calcite (), Dolomite.
  • Sulphates: Gypsum ().
  • Phosphates: Apatite (source of Phosphorus).

2.2 Physical Properties of Minerals

To identify minerals without chemical analysis, physical properties are used:

1. Color: Visual appearance (e.g., Quartz is colorless/white; Hematite is red). Note: Least reliable due to impurities.
2. Streak: The color of the mineral in powdered form (rubbed on a porcelain plate). Pyrite (Fool's Gold) looks gold but has a black streak.
3. Lustre: The way light reflects off the surface (Metallic, Vitreous/Glassy, Pearly, Dull).
4. Cleavage: Tendency to break along smooth, flat planes of structural weakness (e.g., Mica has perfect basal cleavage).
5. Fracture: Breakage usually along irregular surfaces (e.g., Quartz shows conchoidal/curved fracture).
6. Hardness: Resistance to scratching (Measured on Mohs Scale).

Mohs Scale of Hardness:

1. Talc (Softest)
2. Gypsum
3. Calcite
4. Fluorite
5. Apatite
6. Orthoclase Feldspar
7. Quartz
8. Topaz
9. Corundum
10. Diamond (Hardest)

3. Rocks

A rock is a hard, consolidated aggregate of one or more minerals. Rocks are the parent materials for soils.

3.1 The Rock Cycle

The continuous process where rocks are created, changed from one form to another, destroyed, and formed again.

• Magma cools Igneous Rocks
• Weathering/Erosion Sediments Lithification Sedimentary Rocks
• Heat/Pressure Metamorphic Rocks
• Melting Magma

3.2 Classification of Rocks

A. Igneous Rocks ("Fire Rocks")

Formed by the cooling and solidification of molten magma (below surface) or lava (above surface).

• Intrusive (Plutonic): Cool slowly beneath the surface. Large crystal size.
  • Example: Granite.
• Extrusive (Volcanic): Cool rapidly on the surface. Small/fine crystal size.
  • Example: Basalt.

Chemical Classification (Based on Silica content):

1. Acid Rocks: Silica. (e.g., Granite, Rhyolite). Soil forming implication: Produces acidic, sandy soils.
2. Intermediate Rocks: Silica. (e.g., Andesite, Diorite).
3. Basic Rocks: Silica. (e.g., Basalt, Gabbro). Soil forming implication: Produces dark, clay-heavy, fertile soils.

B. Sedimentary Rocks

Formed by the accumulation, compaction, and cementation (lithification) of sediments derived from the breakdown of pre-existing rocks. They form in layers (stratification).

• Clastic (Mechanically formed): Formed from broken fragments.
  • Sandstone: Cemented sand grains.
  • Shale: Compacted clay/mud (most abundant sedimentary rock).
  • Conglomerate: Rounded pebbles cemented together.
• Chemical/Biochemical: Formed from precipitation of minerals from water or organism remains.
  • Limestone: Calcium carbonate ().
  • Dolomite: Calcium magnesium carbonate.

C. Metamorphic Rocks ("Changed Form")

Formed from pre-existing igneous or sedimentary rocks under the influence of intense heat and pressure (without melting).

• Foliated: Minerals align in bands/layers due to pressure.
  • Gneiss (from Granite).
  • Slate (from Shale).
  • Schist (from Shale/Basalt).
• Non-Foliated: No banding; massive structure.
  • Marble (from Limestone).
  • Quartzite (from Sandstone).

4. Weathering

Weathering is the disintegration and decomposition of rocks and minerals at or near the Earth's surface through physical, chemical, and biological agents. It is the first step in soil formation (pedogenesis).

Result: Rock Regolith (Unconsolidated material) Soil.

4.1 Physical (Mechanical) Weathering

Disintegration of rocks into smaller fragments (detritus) without any change in chemical composition. Dominant in cold and dry climates (deserts, arctic).

Mechanisms:

1. Thermal Expansion (Exfoliation):
   • In arid regions, rocks expand during the day (heat) and contract at night (cold).
   • Differential expansion causes outer layers to peel off like an onion.
2. Frost Wedging (Freeze-Thaw):
   • Water enters cracks in rocks. Upon freezing, water expands by ~9%.
   • This pressure widens cracks and shatters the rock.
3. Abrasion:
   • Grinding of rock surfaces by particles transported by wind, water, or glaciers.
4. Unloading (Pressure Release):
   • When overlying erosion removes weight, deep rocks expand upward, creating cracks (joints) parallel to the surface.

4.2 Chemical Weathering

Decomposition of rock minerals involving a change in chemical identity. Water and temperature are the key accelerators. Dominant in hot and humid (tropical) climates.

Major Processes:

1. Hydration:

   • Chemical attachment of water molecules to a mineral. Causes expansion and softening.
   • Reaction: (Anhydrite) + (Gypsum).
   • Result: Volume increase leads to physical stress.

2. Hydrolysis (Most important for Soil Formation):

   • Reaction of minerals with water (specifically and ions).
   • Water acts as a weak acid, displacing cations (K, Na, Ca) from the crystal lattice.
   • Significance: Primary method by which Feldspars convert to Clay minerals.
   • Reaction: Orthoclase + Water Acid Clay + KOH (Soluble).

3. Carbonation:

   • Reaction between Carbonic acid () and minerals.
   • dissolves in rainwater/soil water to form weak Carbonic acid.
   • Significance: Major process in decomposing Limestone ().
   • Reaction: (Calcium bicarbonate - highly soluble).

4. Oxidation:

   • Addition of oxygen to minerals. Occurs chiefly in iron-bearing minerals.
   • Visual indicator: Rocks turn rust-red or yellow.
   • Reaction: Ferrous Iron () Ferric Iron ().
   • (Hematite).

5. Solution:

   • Minerals dissolve directly in water (e.g., Halite/Salt, Gypsum).

4.3 Biological Weathering

Involves the physical and chemical actions of living organisms.

1. Physical-Biological:

   • Root Wedging: Plant roots penetrate rock crevices; as they thicken, they exert immense pressure, splitting rocks.
   • Burrowing Animals: Earthworms, termites, and rodents mix soil and bring fresh rock material to the surface for weathering.

2. Chemical-Biological:

   • Organic Acids: Lichens and mosses growing on rocks secrete organic acids (chelating agents) that dissolve minerals.
   • Microbial Action: Bacteria and fungi produce acids (citric, carbonic) during respiration and decomposition, accelerating chemical weathering.

4.4 Factors Affecting Weathering Stability

Not all minerals weather at the same rate. This is described by Goldich's Stability Series (inverse of Bowen's Reaction Series):

• Least Stable (Weathers Fast): Olivine, Ca-Plagioclase (formed at high temp/pressure, unstable at surface).
• Intermediate: Pyroxene, Amphibole, Biotite.
• Most Stable (Weathers Slow): Quartz (formed at lower temp, highly resistant).

General Resistance Order:

Summary of Weathering Products