Crystallization - Crystallization, Reactions, Defects and...

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Crystallization, Reactions, Defects and Mineraloids Stability of minerals Stability: stable, metastable, and unstable Stability and energy Gibbs Free Energy “G” of a mineral: the lower the energy, the more stable it is. Factors affecting G: Enthalpy, entropy, P, T and X. Mineral reactions: Lower energy assemblage is more favored. Phase diagrams and their uses for depicting stability. Activation energy: energy needed to overcome kinetic barriers to some reaction. Crystallization Crystals form from (i) melts (upon cooling), (ii) solutions (through evaporation of solvent or supersaturation + nucleation), solids (through recrystallization or neocrystallization), or (iv) vapors (sublimation or decreasing P). In the case of crystallization from solutions (which can be undersaturated, saturated, or supersaturated), the solubility product of a compound has to be exceeded (i.e. saturated or supersaturated). Formation of crystals starts through the process of nucleation (i.e. the creation of a center around which ions or atoms aggregate) and proceeds through crystal growth. A large # of nuclei results in a large surface area to volume. In such cases, the energy of the system is high, and there is a large # of unsatisfied chemical bonds. The largest # of unsatisfied chemical bonds (hence largest amount of energy of attachment) exists at corners, followed by edges, and is lowest along crystal faces (Fig. 1). The greater attraction of atoms to corners may result in a dendritic state of
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This note was uploaded on 02/27/2012 for the course GLY 314 taught by Professor Staff during the Spring '09 term at Marshall.

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Crystallization - Crystallization, Reactions, Defects and...

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