Themes of the Day:

• Mineral Structures - Isostructuralism & Polymorphism
• Mineral Classification

Mineral Structures - Isostructuralism & Polymorphism

• Illustrating Mineral Structures - e.g., Olivine
• Isostructural Minerals - e.g., Halite and Galena, Calcite group
• Polymorphism - same formula, different structures
  Examples:
  • SiO₂
  • FeS₂
  • C - graphite, diamond
  • Al₂SiO₅kyanite, andalusite, sillimanite
  • KAlSi₃O₈
  Types:
  • Reconstructive polymorphism - e.g., graphite, diamond
  • Dispalcive polymorphism - e.g., alpha-quartz, beta-quartz
  • Order-disorder polymorphism - e.g., K-feldspars
  • polytypism - e.g., orthorhombic vs. monoclinic sheet silicates

Mineral Classification

• Primarily grouped by anion or anionic complex, secondarily by structure - See Table 4.5
• Compositional Variation - Solid Solution - requires similarly sized and charged ions
  Varieties:
  • Simple Substitution - e.g., Fe²⁺ = Mg²⁺ in olivine
  • Coupled Substitution - e.g., Ca²⁺ + Al³⁺ = Na⁺ + Si⁴⁺ in plagioclase feldspar
  • Omission Substitution - e.g., 3Fe²⁺ = 2Fe³⁺ + vacancy in pyrrhotite
  • Interstitial Substitution - e.g., K⁺ in beryl
• Mineral Formulas - e.g., Diopside
• Graphic Representation
  • Binary systems
  • Ternary systems