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Crystal Growth

Crystal Growth. GLY 4200 Fall, 2012. 1. Mineral Size. Mineral size - nm’s to tens of meters Mineral mass - nanograms to megagrams Stibnite crystals. Methods of Crystal Growth. From solution, usually aqueous From a melt By sublimation from a gas phase. Nucleation.

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Crystal Growth

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  1. Crystal Growth GLY 4200 Fall, 2012 1

  2. Mineral Size • Mineral size - nm’s to tens of meters • Mineral mass - nanograms to megagrams • Stibnite crystals

  3. Methods of Crystal Growth • From solution, usually aqueous • From a melt • By sublimation from a gas phase

  4. 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

  5. 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

  6. 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

  7. Growth From Melt

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

  13. 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

  14. 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

  15. Discontinuous Vectorial Properties Examples • Color banding in minerals • Dendritic growth • Rate of solution etching by a solvent • Cleavage • Hardness

  16. Color Bands • Tourmaline often shows color banding

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

  18. Continuous Vectorial Properties Examples • Index of refraction, related to the velocity of light • Seismic velocities in crystals • Electrical and thermal conductivity • Thermal expansivity

  19. 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

  20. Epitaxial Overgrowth Examples • The (010) plane of staurolite has a structure similar to kyanite • Kyanite’s (100) may epitaxially overgrow staurolite • Similarly, plagioclase sometimes overgrows microcline.

  21. Epitaxis Photo • Epitaxial overgrowth of quartz on epidote • Green Monster Mine,Prince of Wales Island,Alaska

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