Crystal Growth GLY 4200 Fall 2012 1 Mineral

Crystal Growth GLY 4200 Fall, 2012 1

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

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

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 4

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 5

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 6

Growth From Melt 7

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 8

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 9

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 10

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 11

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 12

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 13

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 14

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

Color Bands • Tourmaline often shows color banding 16

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

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

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 19

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. 20

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