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Chapter 17

Chapter 17. Stability of minerals. Introduction. Kinetics (the rate of reactions): Reaction rates slow down on cooling Minerals that are normally stable at high T, can be preserved at room T Mineral formation involves:

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Chapter 17

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  1. Chapter 17 Stability of minerals

  2. Introduction • Kinetics (the rate of reactions): • Reaction rates slow down on cooling • Minerals that are normally stable at high T, can be preserved at room T • Mineral formation involves: • An assemblage (system) of phases and chemical components (elements, oxides) • A phase is a homogenous, physically distinct part of the system: solid, liquid or gas • Each crystallized mineral is a separate phase, the homogenous melt is a phase and the homogenous gas is another phase • Components: chemical constituents – usually compounds

  3. Phase diagrams • A function of two variables • T vs total pressure P • T vs partial pressure p/ fugacity f / activity a • T vs composition of system X • Eh vs pH

  4. Phase diagrams Fig 17.2 Fig 17.5 Fig 17.6

  5. Diagrams for crystallization from a melt The binary system diopside-anorthosite • Two components • One free parameter (T) • Solid lines: equilibrium between two phases - mark onset of crystallization (liquidus) • Eutectic point (E): 3 coexisting phases – lowest T for coexisting of solid and liquid phases • Horizontal line (solidus): Temperature limit - only solid phases below solidus

  6. Chapter 18 Solid solutions

  7. Crystallization of solid solutions from melt • Continuous isomorphism: Complete solid solution at all temperatures Olivine system Plagioclase system Fe2SiO4 – Mg2SiO4 NaAlSi3O8 – CaAl2Si2O8

  8. Exsolution • Discontinuous isomorphism: For some systems solid solution occur only at certain conditions • Alkali feldspars only continuous solid solution, resulting in homogeneous crystals, at high temperatures • Upon cooling crystal become heterogenous in one of two ways: • Ordering: • Unlike atoms attract each other upon cooling • results in regular alternating arrangement of unlike atoms in the crystal • Exsolution: • Like atoms attract each other upon cooling • results in separation of a homogeneous crystal into local domains of differing composition • Both processes are subsolidus transformations – occur after original homogeneous crystal has solidified

  9. Exsolution in feldspars • Immiscibility gap

  10. Exsolution • Examples of exsolution: Table 18.1 Albite K-feldspar (Na-K) AlbiteAnorthite (Na-Ca) AugitePigeonite (Ca-Mg/Fe) Hematite Ilmenite (Fe-Ti) Bornite Chalcopyrite (Fe-Cu) Sphalerite Chalcopyrite (Zn-Cu)

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