2.4 Crystal growth and phase diagrams

2.4 Crystal growth and phase diagrams - Mineral Stability...

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Unformatted text preview: Mineral Stability What controls when and where a particular mineral forms? Knowing answer provides information about earth history or processes Mineral formation Rock cycle essentially mineralogical changes that occur because of variations in geologic environment Why? Geologic history Processes: ore deposits, oil and gas, building materials, engineering hazards, water cycle, climate... "The Rock Cycle" Bowen's Reaction Series Fe, Mg silicates Ca, Na silicates Fig. 51 Ca, Na, Fe, Mg silicates Kspar Qtz 3 Requirements for minerals Constituents Correct environmental conditions Available reactants/elements (X) Pressure (P) Temperature (T) Mineral Stability More stable position is one of lower energy Change may not be stable e.g. metastable minerals Energy required to overcome metastability activation energy Mineral contains more energy than expected by environment Activation Energy energy to shake book off shelf Fig 52 How can stability be estimated? Algebraically: Graphically "phase diagrams": Physical chemistry/Thermodynamics Estimates of G Gibbs free energy Essentially figures of solutions to G problems Many types, common ones: One component P & T variable, X fixed (i.e. the component Two (or more) components T & X variable, P fixed Components and Phases Component Chemical entity; e.g., H2O; Al2SiO5 Phase physically separable part of a system; e.g., ice, water, water vapor; Sillimanite, Kyanite, Andalusite One and two component phase diagrams One component diagrams Fields where only one phase (mineral) is stable Lines where two phases are stable simultaneously Points where three phases are stable One component diagrams If P and/or T changes One phase converts to another Examples: H2O component; ice, water, and vapor are phases Al SiO component; Kyanite, Andalusite, 2 5 Sillimanite are phases Water phase diagram Only component is H2O Complete H2O diagram At least 7 polymorphs of ice Commonly shown P & T conditions Al2SiO5 Phase diagram Lowest G is the most stable phase Lines mark boundaries of regions with the lowest G Very useful to remember for metamorphic reactions Zoned Crystals Individual mineral grains often vary in composition from center to edge Easily observed petrographically Zoning reflects conditions under which crystal grew Disequilibrium with composition of melt for parts of crystals Example of zoning Plagioclase feldspar Two end member composition albite and anorthite Complete solid solution (at high T) Two component phase diagram illustrates how temperature controls composition of mineral Zoned Plagioclase crystal Oscillatory zoning Fig. 1212 Other types of zoning include: (1)Normal zoning (Ca rich centers) (2)Reverse zoning (Na rich centers) Origin of zoning can be explained with a two component phase diagram Diagram shows complete solid solution crystals and melt may have any composition 100% Albite Mole % Anorthite 100% Anorthite How does the composition of the crystals relate to the composition of the melt? Fig. 510a Equilibrium Crystallization Start 77 68 End 55 Mole % Anorthite Minerals show no zoning homogeneous compositions Fig. 510b NonEquilibrium Crystallization Start 77 77 68 77 55 Mole % Anorthite Minerals show zoning heterogeneous compositions Fig. 510c Controls on zoned crystals Diffusion rate through solid crystal Time allowed for diffusion to occur Diffusion is rapid in olivine few zoned crystals Diffusion slow in plagioclase Mostly equilibrium Commonly zoned Mineral crystallization Two types of crystallization 1. 2. Homogeneous: crystallization from fluid Heterogeneous: crystallization on a surface Hematite (Fe2O3, cubic packing) growing on Magnetite (Fe3O4, hexagonal packing) Fig. 55 Freezing of magma Precipitation from solution Rates of growth Slowest growing faces are often most prominent Fast growth causes faces to disappear Halite {001} faces parallel to layers of bonded Na and Cl Face is charge neutral Weak attraction from this face to either ion {111} faces parallel layers of pure Na and Cl Result is {111} faster than {001} High surface charge on face Comes from unsatisfied bonds from element Strong attraction from this face to oppositely charge ion Thicker layer for a given amount of time Start with octahedral faces End with cube faces Boundaries are "time lines" Fig 57 ...
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