Transcript File 2 - gemoc

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.
• 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: ???????
SIMPLE TWINS
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 twostage 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.
DISLOCATIONS