.
Week 6: Feb 20 and 22
  • Compositional variation in minerals (cont'd ...)
    • Exsolution - ("unmixing") the process whereby an initially homogenous solid solution separates into 2 (or more) distinct crystalline phases. This may occur with a drop in T from higher T where solid solution is favored to lower T where it is not.
      • exsolution requires ions to move and segregate into the two different crystalline domains
      • exsolution textures in minerals may take the form of lamellae, "blebs", stringers, rods, etc.
      • example: at high T alkali feldspar--(Na,K)AlSi3O8-- shows complete solid solution between albite--NaAlSi3O8 and K-feldspar--KAlSi3O8 wherein there are no restrictions on the amount of Na and K in the mineral. At lower T, however, albite-rich lamellae begin to segregate from K-f rich host along the {100} plane
      • (subsolidus) phase diagram for alkali feldspar showing variation in composition of exsolving domains (see Fig. 5.23 of Nesse)
      • (subsolidus) phase diagram for plagioclase (NaAlSi3O8-CaAl2Si2O8) showing the peristerite and Boggild-Huttenlocher miscibility gaps.
    • Zoning - The (usually) concentric variation of composition in a crystal having solid solution. The core usually has a composition that was in equilibrium with the liquid (or some other matrix as in a metamorphic rock) at higher T, and the rim has a lower T composition. The reverse may also occur.
      • example: zoning in plagioclase
      • plagioclase phase diagram (see fig. 5.10 of Nesse)
  • Growth of Minerals
    • stable, unstable, and metastable states
    • Gibbs Free Energy and mineral stability - the mineral assemblage with the lowest Gibbs free energy is the stable assemblage
    • Gibbs free energy is a function of pressure and temperature; since P and T affect the Gibbs free energy of each mineral differently, some minerals will have a lower free energy than others and will end up being more stable.
    • Homogenous vs. heterogenous nucleation
    • Role of defects in aiding crystal growth

Mineralogy homepage | Saint Louis University | Earth & Atmospheric Sciences