Crystal Growth

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Crystal Growth

• GLY 4200
• Fall, 2012

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Mineral Size

• Mineral size - nms to tens of meters
• Mineral mass - nanograms to megagrams
• Stibnite crystals

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Methods of Crystal Growth

• From solution, usually aqueous
• From a melt
• By sublimation from a gas phase

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Nucleation

• Usually form from the initial crystallization
  products of solutions or melts
• Various ions must combine to form an initial
  regular structure pattern of a crystal
• Usually requires supersaturation

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Supersaturation

• Achieved by
• Increasing concentration
• Changing temperature
• Changing pressure
• Rate of change is important
• Slow cooling leads to a few nuclei and large
  crystals
• Rapid cooling leads to many nuclei, small
  crystals

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Melts

• Growth is similar to aqueous dehydration
• High temperatures mean large thermal vibrations,
  which quickly break atomic clusters apart,
  destroying nuclei
• Low temperatures allow the attractive forces to
  overcome thermal vibration holding clusters
  together

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Growth From Melt
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Vapor

• Cooling allows dissociated atoms or molecules to
  join
• Examples
• Formation of snowflakes
• Growth of ice on a window
• Formation of sulfur crystals around fumaroles

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Destruction of Nuclei

• Nuclei have very large surface area/volume
• Unsatisfied bonding on outer surfaces leads to
  dissolution
• Crystallization only takes place when some nuclei
  survive long enough for growth to occur

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Critical Size

• If nuclei grow rapidly, their surface area/volume
  declines, and they may reach and exceed a
  critical size
• Above the critical size, the nuclei are
  relatively stable, and growth can begin

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Law of Bravais

• The most likely crystal face to grow are those
  planes having the highest density of lattice
  points

• However, these faces have lowest surface energy
• This makes them stable, but slow growing
• Anions or cations in solution are not attracted
  to these faces

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Rate of Growth

• Faces composed of all anions or all cations are
  very high energy
• They attract ions of the opposite sign, and grow
  rapidly
• Eventually they grow themselves out of existence,
  leaving the slower growing faces

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Vectorial Properties

• Some properties of crystals depend on the
  direction in which they are measured
• These are called vectorial properties
• Examples Hardness, electrical and thermal
  conductivity, speed of light, speed of seismic
  waves, thermal expansion, solution rate, and
  diffraction of X-rays

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Variation of Vectorial Properties

• Many vectorial properties vary discontinuously as
  direction is changed
• Values of these properties pertain to a given
  crystallographic direction
• Values of the property in crystallographic
  directions intermediate to two given directions
  do not very smoothly as the direction is changed

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Discontinuous Vectorial Properties Examples

• Color banding in minerals
• Dendritic growth
• Rate of solution etching by a solvent
• Cleavage
• Hardness

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Color Bands

• Tourmaline often shows color banding

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Dendritic Mineral Habit

• Dendritic formation of bright native silver
  crystals.
• State of Maine Mine, Tombstone District, Cochise
  Co., Arizona, USA

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Continuous Vectorial Properties Examples

• Index of refraction, related to the velocity of
  light
• Seismic velocities in crystals
• Electrical and thermal conductivity
• Thermal expansivity

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Crystal Intergrowths

• During crystal growth, one crystalline substance
  may grow on a crystalline substance of different
  composition and structure
• Such growths are known as epitaxial growths

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Epitaxial Overgrowth Examples

• The (010) plane of staurolite has a structure
  similar to kyanite
• Kyanites (100) may epitaxially overgrow
  staurolite
• Similarly, plagioclase sometimes overgrows
  microcline.

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Epitaxis Photo

• Epitaxial overgrowth of quartz on epidote
• Green Monster Mine,Prince of Wales
  Island,Alaska