1. Crystal Growth GLY 4200 Fall, 2012 1
Giant Gypsum Crystals in the Cueva de los Cristales, Naica Mine complex, south-east of Chiuahua City, Mexico – reported in Geology, March?, 2007
Some xtals exceed 11 meters
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: Rate of change is important
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
Video: CrystalGrowth.wmv

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