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Crystallisation from melts. Why do crystals form? What controls their shape and size? Three aspects to the growth of a crystal are Nucleation: formation of a stable nucleus Diffusion of material to the nucleus Growth of the crystal by adding atoms
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Crystallisation from melts • Why do crystals form? • What controls their shape and size? • Three aspects to the growth of a crystal are • Nucleation: formation of a stable nucleus • Diffusion of material to the nucleus • Growth of the crystal by adding atoms The slowest of these three aspects controls the shape and size.
Different types of crystals • If diffusion is slow, the crystal grows spikes to get at the new material needed & forms a dendrite or a skeletal crystal • If nucleation is the restriction (a) spherulites can form (many radiating crystals grow from one nucleus or (b) a few large crystals • If growth is the restriction (slow crystallisation) then well formed crystals develop.
SIMPLE TWINS • Sanidine and orthoclase phenocrysts seem to invariably have a simple twin. Why might this be so? • Option A: It is groups of atoms in twin orientation that grow large enough to form a stable nucleus. • Option B: Trace elements that are concentrated in the first stage of growth (Ba,Sr) allow a twin to form. • Option C: ???????
Growth rate control • A)Interface kinetics-the movement of material in/out across crystal/melt interface and attachment onto the crystal • B)Transport of nutrients in and impurities away from boundary • C)Transport of heat of crystallisation away from the interface. • All linked to undercooling. (A) dominant at low degrees, (B)and/or (C) dominant at high.
GRAINSIZE • Largely controlled by the nucleation rate (influenced by the growth rate). Few nuclei leads to large crystals, many leads to small. • Few nuclei if degree of undercooling is small.(i.e. slow cooling of plutonic rocks). Few nuclei and high growth rate. Polyhedral (many faces) crystals common. Orthoclase phenocrysts in granites are prime examples. • Pegmatites are extreme examples but due to the hydrous character of the melt which increases diffusion. • Aplites (leuco-micro-granites) form during water loss events (pressure quench) which bring on strong undercooling by increasing the freezing temp.
Fine Grained Rocks. • Strong undercooling (loss of heat to the cold surfaces for lavas and dykes) promotes strong nucleation and slow diffusion and growth. Polyhedral crystals are commonly formed. • Phenocrysts generally form as a result of two-stage cooling (slow plutonic stage followed by fast volcanic stage). A general increase in the undercooling as the magma intrudes up into cooler rocks produces a
Nucleation and growth relative to undercooling • For nucleation to occur some degree of undercooling is needed. The rate of nucleation and growth both rise rapidly with increasing undercooling and then fall off. The maximum for growth falls off before nucleation.
