Crystallisation%20from%20melts

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Crystallisation from melts

• Why do crystals form?
• What controls their shape and size?
• Three aspects to the growth of a crystal are
• Nucleation formation of a stable nucleus
• Diffusion of material to the nucleus
• Growth of the crystal by adding atoms
• The slowest of these three aspects controls the
  shape and size.

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Different types of crystals

• If diffusion is slow, the crystal grows spikes
  to get at the new material needed forms a
  dendrite or a skeletal crystal
• If nucleation is the restriction (a) spherulites
  can form (many radiating crystals grow from one
  nucleus or (b) a few large crystals
• If growth is the restriction (slow
  crystallisation) then well formed crystals
  develop.

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SIMPLE TWINS

• Sanidine and orthoclase phenocrysts seem to
  invariably have a simple twin. Why might this be
  so?
• Option A It is groups of atoms in twin
  orientation that grow large enough to form a
  stable nucleus.
• Option B Trace elements that are concentrated in
  the first stage of growth (Ba,Sr) allow a twin to
  form.
• Option C ???????

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Growth rate control

• A)Interface kinetics-the movement of material
  in/out across crystal/melt interface and
  attachment onto the crystal
• B)Transport of nutrients in and impurities away
  from boundary
• C)Transport of heat of crystallisation away from
  the interface.
• All linked to undercooling. (A) dominant at low
  degrees, (B)and/or (C) dominant at high.

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GRAINSIZE

• Largely controlled by the nucleation rate
  (influenced by the growth rate). Few nuclei leads
  to large crystals, many leads to small.
• Few nuclei if degree of undercooling is
  small.(i.e. slow cooling of plutonic rocks). Few
  nuclei and high growth rate. Polyhedral (many
  faces) crystals common. Orthoclase phenocrysts in
  granites are prime examples.
• Pegmatites are extreme examples but due to the
  hydrous character of the melt which increases
  diffusion.
• Aplites (leuco-micro-granites) form during water
  loss events (pressure quench) which bring on
  strong undercooling by increasing the freezing
  temp.

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Fine Grained Rocks.

• Strong undercooling (loss of heat to the cold
  surfaces for lavas and dykes) promotes strong
  nucleation and slow diffusion and growth.
  Polyhedral crystals are commonly formed.
• Phenocrysts generally form as a result of
  two-stage cooling (slow plutonic stage followed
  by fast volcanic stage). A general increase in
  the undercooling as the magma intrudes up into
  cooler rocks produces a

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Nucleation and growth relative to undercooling

• For nucleation to occur some degree of
  undercooling is needed. The rate of nucleation
  and growth both rise rapidly with increasing
  undercooling and then fall off. The maximum for
  growth falls off before nucleation.

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DISLOCATIONS
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