Hot Under the Collar (part II) Crystallisation Fluorite Igneous petrology is all about crystals and how they get there.

Melting & Crystallisation: Different Sides of the Same Coin MeltingCrystallisation

Crystallisation and Melting at Equilibrium? H - total energy, S - energy wasted in disorder, G -free energy available for work

Equilibrium?

Melting, Crystallisation & Equilibrium  T is the supercooling - the difference in T between the melting temperature and the T at which crystallisation occurs

How Crystallisation Occurs Foundations (a crystal nucleus) Man carrying bricks (with bum hanging out) - diffusion Addition of bricks one at a time and in the right order - surface reactions

How Crystallisation Occurs Crystal nucleus is a small region of crystal structure in a melt. Those smaller than the critical radius are known as embryos.

Group of Hippies Mellow Hippie Irate Hippie Evil Developers

More Mellow (Stable) Group of Hippies

How Crystallisation Occurs Surface energy increases with surface area 4  R 2 Internal energy increase with volume (4/3)  R 3

Nucleation Rate

Crystal Growth Man carrying bricks (with bum hanging out) - diffusion Addition of bricks one at a time and in the right order - surface reactions

Diffusion of Hippies

Diffusion of Atoms

Diffusion Controlled Growth The building blocks of the crystal (the atoms and molecules) have to diffuse towards the crystal. The unwanted atoms and molecules have to diffuse away. If the diffusion rate is slow it controls the rate of growth

Surface Reaction Mechanisms Step Growth Step growth is the addition of atoms or molecules one layer at a time. The layer grows over the surface as spreading islands. Molecules are added at the edge of the islands and must be in the right orientation. Step growth occurs when the atomic building blocks of the crystal are present in high abundance (saturated).

Surface Reaction Mechanisms Spiral Growth Spiral growth occurs by the addition of atoms around a screw dislocation on the surface of a crystal. Molecules and atoms are more easily absorbed (held on to the surface) here. Spiral growth is dominant when the atomic building blocks of the crystal are present in low abundance (undersaturated).

Surface Reaction Mechanisms Nuclei Absorption Entire crystal nuclei can be incorporated directly on to a crystal. This occurs when the atomic building blocks of the crystal are present in very high abundance (supersaturated).

Crystal Growth Rates On cooling crystal growth rates are largest at higher temperatures before the nuclei have formed. How does a magma crystallise?