Nucleation and crystallization

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Nucleation and crystallization
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Forming crystals

• From solutions, melts, vapors
• From disordered and random to ordered
• Halite crystals demo

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Halite

• Solution with Na and Cl-
• Finally super saturated, solid precipitates ion
  by ion

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Halite

• Evaporation rate impacts crystal size
• Can force precipitation with evaporation, drop in
  T, drop in P
• Same from melts

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Stability of minerals

• Stable
• Unstable
• metastable

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Gibbs free energy (G)

• Comparison of G tells you which mineral is more
  stable
• Stable minerals have lowest ?G at specific T and P

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Nucleation

• Appropriate atoms must find each other,
  chemically bond, form a nucleus of a crystal
• Then nucleus progressively grows adding more
  atoms to surface

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Nucleation

• Atoms constantly bumping into each other forming
  embryos
• Some embryos will by chance have correct
  composition and structure

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Nucleation

• Embryos try to reach size-critical radius
• Whether a mineral will grow depends on Gibbs Free
  energy

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Nucleation

• Embryos dont form at exact equilibrium T
• To nudge growth, you need to undercool (like
  activation energy)
• To no undercooling
• T1 low undercooling
• T3 large undercooling

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Nucleation

• Small undercooling, slow cooling (magma in
  crust), less nuclei but larger crystals
• Large undercooling, fast cooling (volcanic
  rocks), more nuclei but small crystal size

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Nucleation

• Rate of nucleation and growth as function of T
  below melting T

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Crystal growth

• If nucleus is to survive, grows fast enough to
  reduce surface energy and therefore solubility
• Single large crystal more stable than many small
  ones

• Perfect shaped crystal more stable than irregular
  one

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Crystal growth

• Once nuclei stable, need to add atoms
• Atoms stable with lowest surface area
• Where is energetically best place to put new
  atoms?

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Rate of growth

• Calcite in lab 14.8 ?m/day - 6 cm in 11.26 years
• Plag in magma chamber can be 10,000 years for 1
  cm
• Calcareous concretions in se rocks 5 cm in 1.7
  Ma

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Rate of growth

• 1700 lbs!!

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Rate of growth

• What is the face that had fastest growth?

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Rates of growth

• Hypothetical crystal all faces grow at same rate
• Realistic crystal and 110 grows faster

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Rates of growth

• Halite growth
• 111 face is layers of Na and Cl
• Strong attraction of next layer because of
  charges
• Fast growth
• 001 face equal Na and Cl, neutral charge, slow
  growth

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Growth rate and crystal shape

• Shape of zircon as function of growth rate and
  melt chemistry

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Growth rate and crystal shape

• Shape of plag as function of growth rate and melt
  chemistry

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Growth rate and crystal shape

• What tectonic environment would you find these
  each in assuming plag forming in a basalt during
  cooling

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Other issues impacting crystal size and shape

• Room to grow

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Other issues impacting crystal size and shape

• fluids