Chapter 17 Stability of minerals. Introduction Kinetics (the rate of reactions): Kinetics (the rate of reactions): –Reaction rates slow down on cooling.

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Presentation transcript:

Chapter 17 Stability of minerals

Introduction Kinetics (the rate of reactions): 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 Minerals that are normally stable at high T, can be preserved at room T Mineral formation involves: 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 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 Components: chemical constituents – usually compounds

Phase diagrams A function of two variables 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

Phase diagrams Fig 17.6 Fig 17.5 Fig 17.2

Diagrams for crystallization from a melt The binary system diopside-anorthosite Two components Two components One free parameter (T) One free parameter (T) Solid lines: equilibrium between two phases - mark onset of crystallization (liquidus) 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 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 Horizontal line (solidus): Temperature limit - only solid phases below solidus

Chapter 18 Solid solutions

Crystallization of solid solutions from melt Continuous isomorphism: Complete solid solution at all temperatures Continuous isomorphism: Complete solid solution at all temperatures Olivine systemPlagioclase system Fe 2 SiO 4 – Mg 2 SiO 4 NaAlSi 3 O 8 – CaAl 2 Si 2 O 8

Exsolution Discontinuous isomorphism: For some systems solid solution occur only at certain conditions 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: Upon cooling crystal become heterogenous in one of two ways: –Ordering: Unlike atoms attract each other upon cooling 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 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 Both processes are subsolidus transformations – occur after original homogeneous crystal has solidified

Exsolution in feldspars Immiscibility gap Immiscibility gap

Exsolution Examples of exsolution: Table 18.1 Examples of exsolution: Table 18.1 AlbiteK-feldspar(Na-K) AlbiteAnorthite(Na-Ca) AugitePigeonite(Ca-Mg/Fe) HematiteIlmenite(Fe-Ti) BorniteChalcopyrite(Fe-Cu) SphaleriteChalcopyrite(Zn-Cu)