Geology Reference
In-Depth Information
redissolved by the liquid. In practice, therefore, melts
cool below the liquidus, becoming supersaturated
with the crystal components, before the first visible
crystals appear, a phenomenon called supercooling .
The subsequent growth of a crystal can be seen in
terms of adding new material (depicted as rectangular
blocks) in layers on an existing crystal surface, as
depicted in Figure  8.7a. The blocks here can be inter-
preted as ions, molecules or entire unit cells. Exposed
corners and edges are features of very high surface
energy owing to the number of unsatisfied bonds.
Adding a block at A is therefore highly unfavourable in
energy terms, and would occur only in a strongly super-
saturated solution. Sites B and C are less hostile, but
continued exploitation of such step sites will complete
the layer and eliminate the step. In subsequent crystal-
lization on this face, use of sites like A is unavoidable.
Calculations suggest that a high threshold of supersatu-
ration or supercooling has to be surmounted before
crystallization at sites like A can proceed, and crystalli-
zation ought therefore to be an extremely slow process,
yet in practice this seems not to be the case.
∆G L
UNSTABLE
0
STABLE
Very small nucleus:
+ 6 r 2 σ s term
predominates
Larger nucleus:
- r 3 L v term
predominates
Size of crystal nucleus ( r )
Figure 8.6 Energetics of crystal nucleation. Δ G L is the free
energy of the nucleus relative to the equivalent amount of melt.
(a)
(b)
C
Re-entrant
corner in step
A
Open
surface
B
Step
(c)
Figure 8.7 (a) Sites of accretion on a crystal face. (b) Screw dislocation, (c) Subsequent positions of the step as crystallization
proceeds. (Source: Read 1953. Reproduced with permission of McGraw-Hill.)
 
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