2 4 crystal growth and phase diagrams

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Mineral Stability  What controls when and where a particular mineral forms?    Commonly referred to as “Rock cycle” Rock cycle: Mineralogical changes that occur because of variations in geologic environment Knowing answer provides information about earth history or processes Mineral formation  Why would you want to know earth history or processes:    Find: ore deposits, oil and gas, building materials Understand engineering hazards, water cycle Understand how humans effect the earth: climate… The Rock Cycle  A system for organizing mineralogical changes Fig 5-1 Bowen’s reaction series Fe, Mg - silicates Changing composition Ca, Na - silicates Ca, Na, Fe, Mg - silicates K-spar Qtz requirements for mineral stability  Constituents   Available reactants/elements (X) Correct environmental conditions (energy)   Pressure (P) Temperature (T) Mineral Stability   More stable position is one of lower energy Minerals may not be stable – e.g metastable minerals   Mineral contains more energy than expected from their environment Energy required to overcome metastability – activation energy Activation Energy: - energy to shake book off shelf - Energy required to change mineral phases Fig 5-2 How can stability be estimated?  Algebraically:    Physical chemistry/Thermodynamics Estimates of ∆G – Gibbs free energy Graphically – “phase diagrams”:   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  H2O  Al2SiO5 Phase – physically separable part of a system; e.g  for H20: ice, water, water vapor  for Al2SiO5: Sillimanite, Kyanite, Andalusite One and two component phase diagrams  Several types of 2-component 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 Two component phase diagrams   What happens if there are two components in a system? Example: Plagioclase feldspars – two components with complete solid solution (at high T, otherwise “exsolution”)  Albite– NaAlSi3O8  Anorthite – CaAl2Si2O8  Any composition in between the two end member compositions     How does solid (and melt) composition vary during crystallization? How does composition vary as solids melt melt to form magma? OR… If you know the composition of a plagioclase feldspar, can you determine T and P of crystallization? Two component phase diagram with complete solid solution = Na, Ca, Al, SiO2 = (Na,Ca)xAlySizO8 100% Albite – NaAlSi3O8 Mole % Anorthite 100% Anorthite – CaAl2Si2O8 Equilibrium Crystallization Start An77 An68 End 100% Albite – NaAlSi3O8 Mole % Anorthite An55 100% Anorthite – CaAl2Si2O8 (1) The crystals are always in equilibrium with the melt (2) Minerals have homogeneous compositions throughout Fig 5-14a Lever Rule %B = qr/qs %A = rs/qs Fraction of two components relate to the relative lengths of tie lines Fig 5.5 Non-equilibrium crystallization  Results in “zoning”    Individual mineral grains may vary in composition from center to edge Easily observed petrographically Very common in plagioclase feldspars Zoned Plagioclase crystal Oscillatory zoning Fig 12-12 Other types of zoning include: (1)Normal zoning (Carich centers) (2)Reverse zoning (Narich centers)  Zoning reflects change in P and T when mineral crystallizes   Crystallizing mineral in disequilibrium with composition of melt Can be explained by non-equilibrium crystallization using phase diagram Non-Equilibrium Crystallization Start Normal Zoning An77 An77 An77 An68 An55 Mole % Anorthite Minerals show zoning – heterogeneous compositions Fig 5-14b Controls on zoned crystals    Diffusion rate through solid crystal Time allowed for diffusion to occur Diffusion is rapid in olivine – few zoned crystals   Mostly equilibrium Diffusion slow in plagioclase  Commonly zoned Two component phase diagram - No solid solution Ca, Mg, Al, SiO2 = At me diopside, anorthite, and melt present Fig 5.4 At me, diopside begins xtll, anorthite continues xtll NO HEAT LOST – remains 1237º C – until all solid Composition is 75% An, 25% Di When first reach 1237º C, system is 48% anorthite, 52% melt Rates of growth    Slowest growing faces are often most prominent Fast growth causes faces to disappear This is why minerals have common forms 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     High surface charge on face Comes from unsatisfied bonds from element Strong attraction from this face to oppositely charge ion Result is {111} face grows faster than {001} face  Thicker layer for a given amount of time Start with octahedral faces End with cube faces Boundaries are “time lines” Fig 5-7 [...]...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  Al2SiO5 Phase diagram ∆G = f(P,T) Phase with lowest ∆G is stable Lines mark boundaries of regions with the lowest ∆G Very useful to remember for metamorphic reactions Fig 5.3 H2O phase diagram... feldspars Zoned Plagioclase crystal Oscillatory zoning Fig 12- 12 Other types of zoning include: (1)Normal zoning (Carich centers) (2) Reverse zoning (Narich centers)  Zoning reflects change in P and T when mineral crystallizes   Crystallizing mineral in disequilibrium with composition of melt Can be explained by non-equilibrium crystallization using phase diagram Non-Equilibrium Crystallization Start Normal... Anorthite – CaAl2Si2O8 Equilibrium Crystallization Start An77 An68 End 100% Albite – NaAlSi3O8 Mole % Anorthite An55 100% Anorthite – CaAl2Si2O8 (1) The crystals are always in equilibrium with the melt (2) Minerals have homogeneous compositions throughout Fig 5-14a Lever Rule %B = qr/qs %A = rs/qs Fraction of two components relate to the relative lengths of tie lines Fig 5.5 Non-equilibrium crystallization... to remember for metamorphic reactions Fig 5.3 H2O phase diagram Only component is H2O More complete H2O diagram There are 15 polymorphs of ice Ice IX stability: T < 140 K 2 kbar < P < 4 kbar Commonly shown P & T conditions tetragonal Ice 9: Kurt Vonnegut, Cat’s Cradle, melting T = 45 .8ºC at P = 1 Atm Two component phase diagrams   What happens if there are two components in a system? Example: Plagioclase... Anorthite – CaAl2Si2O8  Any composition in between the two end member compositions     How does solid (and melt) composition vary during crystallization? How does composition vary as solids melt melt to form magma? OR… If you know the composition of a plagioclase feldspar, can you determine T and P of crystallization? Two component phase diagram with complete solid solution = Na, Ca, Al, SiO2 = (Na,Ca)xAlySizO8... heterogeneous compositions Fig 5-14b Controls on zoned crystals    Diffusion rate through solid crystal Time allowed for diffusion to occur Diffusion is rapid in olivine – few zoned crystals   Mostly equilibrium Diffusion slow in plagioclase  Commonly zoned Two component phase diagram - No solid solution Ca, Mg, Al, SiO2 = At me diopside, anorthite, and melt present Fig 5 .4 At me, diopside begins xtll,... anorthite continues xtll NO HEAT LOST – remains 123 7º C – until all solid Composition is 75% An, 25 % Di When first reach 123 7º C, system is 48 % anorthite, 52% melt Rates of growth    Slowest growing faces are often most prominent Fast growth causes faces to disappear This is why minerals have common forms Halite  {001} faces parallel to layers of bonded Na and Cl   Face is charge neutral Weak attraction... minerals have common forms 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     High surface charge on face Comes from unsatisfied bonds from element Strong attraction from this face to oppositely charge ion Result is {111} face grows faster than {001} face  Thicker
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