Geology: Primary and Secondary Minerals

Primary and Secondary Minerals

• Primary minerals form during magma cooling. Examples include feldspar, mica, and hornblende.
• Secondary minerals form from the chemical alteration of primary minerals. Examples include silicate minerals and non-silicate minerals.

Silicate Minerals

• Silicate minerals contain silicon as a component. Clay minerals are an example.
• The basic units are Si tetrahedron and Al octahedron.

Non-Silicate Minerals

• Non-silicate minerals do not contain silicon.
• Examples include native elements (gold, silver, diamond, graphite), halides (halite, fluorite), oxides (hematite), sulfides (pyrites), sulfates (gypsum), and carbonates (calcite).

1:1 Clay Minerals

• Structure: One tetrahedral sheet bonded to one octahedral sheet.
• Characteristics:
  • Low swelling and shrinking (non-expandable).
  • Low surface area (10-20 m²/g).
  • Low cation exchange capacity (CEC) (5-10 cmol/kg).
  • Kaolinite and Halloysite are examples.

2:1 Clay Minerals

• Structure: Two tetrahedral sheets bonded to one octahedral sheet.
• Characteristics:
  • High swelling and shrinkage (expandable).
  • High surface area (600-800 m²/g).
  • High cation exchange capacity (CEC) (80-120 cmol/kg).
  • Examples include montmorillonite (smectite), illite, and vermiculite.

Properties of Clay Minerals

• The high surface area and structure of clay minerals significantly affect soil properties.
• They absorb water due to micropores, leading to high water-holding capacity.
• Ions are adsorbed due to surface charges.
• Clay minerals swell when wet and become sticky, and shrink and become hard when dry.
• Isomorphous substitution is a key factor in the formation of clay charges.
• Clay particles contain numerous layers with negative charges on their surfaces and edges. Plant nutrients like calcium, magnesium, potassium and ammonia are held on the clay layers by these negative charges.