Crystal Growth GLY 4200 Fall, 2016 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, 2016 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 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

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

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

Growth From Melt Video: CrystalGrowth.wmv

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

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

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

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

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

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

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

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

Color Bands Tourmaline often shows color banding

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

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

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

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.

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