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Understanding Mineral Stability and Crystallization Kinetics in Geology

This chapter explores the stability of minerals and the kinetics of crystallization processes. It discusses how reaction rates slow down with cooling, allowing minerals that typically form at high temperatures to be preserved at room temperature. The chapter also covers phase diagrams, which graphically represent the relationships between temperature, pressure, and composition in mineral systems. Key concepts include continuous and discontinuous solid solutions, exsolution, order-disorder phenomena, and immiscibility gaps, underscoring the complex behavior of mineral formation during cooling.

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Understanding Mineral Stability and Crystallization Kinetics in Geology

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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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