Lecture Notes: Understanding Silicates and Aluminosilicates

Introduction

• Topic: Part 3 of 4.3 on Soils.
• Focus: Understanding silicon dioxide, silicates, and aluminosilicates.
• Importance: Key components of rocks and soils; constitute 92-93% of Earth's crust.

Composition of Earth's Crust

• Elements: Silicon and oxygen are the most abundant, followed by aluminum.
• Compounds: Silicates and aluminosilicates are prevalent due to this composition.

Structure of Silicates and Aluminosilicates

• Silicates:
  • Based on SiO4 tetrahedra (1 silicon covalently bonded to 4 oxygen atoms).
  • Formation of stable chains and rings through bonding.
• Aluminosilicates:
  • Silicon atoms replaced by aluminum, increasing the net negative charge.
  • Aluminum is considered a metalloid due to covalent bonding.

Types of Silicate Structures

• Ortho silicate: Single SiO4 unit.
• Di silicate: Two SiO4 units joined by one oxygen.
• Hexa silicate: Cyclic structure.
• Other Structures: Single chains, double chains, sheet structures, and continuous networks (e.g., in silicon dioxide).

Determining Charge of Silicate and Aluminosilicate Ions

• Method 1: Charge Balancing

  • Sum of ion charges equals zero.
  • Example 1: Lawsonite
    • Contains Ca, Al, Si ions, hydroxide, and water.
    • Charge balance: 2 (Ca) + 3 (Al) + unknown (Si) - 2 (OH) = 0.
    • Result: Silicate ion charge = -6.
  • Example 2: Sodalite
    • Contains Na, Cl, and aluminosilicate ions.
    • Charge balance: 8 (Na) + 6 (Aluminosilicate) - 2 (Cl) = 0.
    • Result: Aluminosilicate ion charge = -1.

• Method 2: Oxidation Numbers

  • Use oxidation states: Al = +3, Si = +4, O = -2.
  • Example: Beryl
    • Formula: Si6O18
    • Oxidation calculation: 6(Si) + 18(O) = -12
    • Result: Formula is Si6O18 with charge -12.
  • Example: Anorthite
    • Formula: Al2Si3O8
    • Suggest charge: -2 (based on Ca balance).
    • Oxidation calculation confirms: Charge = -2.

Conclusion

• More practice examples will be covered in class.
• This concludes part 3 of 4.3. Next up is the final video in the series.

Note: Ensure to understand how to write formulas and balance charges using both methods as they are fundamental to comprehending silicates and aluminosilicates.