Geoscience Reference
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studied by several workers. Some major conclusions have been drawn based on the
studies on the growth of various crystals.
Supersaturation in hydrothermal experiments is given by the magnitude of the
temperature gradient (
T) between the dissolution and growth zones, so that the
relationship between the rate of growth and the magnitude of
Δ
T is usually investi-
gated. If the solubility changes with temperature according to a law which is close
to linear with
Δ
T, the growth rate is also practically linear. For all crystals, which
have been studied under hydrothermal conditions, the rates of growth of the faces
increase linearly with supersaturation. The high activation energies of the growth
of the faces, combined with other factors—the anisotropy of the rates of growth of
the faces, the marked dependence of the rate of crystallization on the composition
of the solution, etc.—provide strong evidence in support of the suggestion that in
hydrothermal crystallization under conditions of excess mass transfer, a primary
role is played by surface processes taking place directly at the crystal
Δ
solution
interface. It should be remembered that the activation energy of diffusion in solu-
tions usually does not exceed 4
5 kcal/mol
[75]
, and the activation energies of dis-
solution rarely exceed 10 kcal/mol. These values are much lower than the
activation energies of growth, indicating that diffusion in the solution and dissolu-
tion of the charge do not limit the rate of crystallization with increase in the con-
centration of the solution. The rate of the crystallization can increase in two ways.
For crystals that do not contain components of the solvent, the rate increases
sharply at low concentrations and remains practically unchanged at high concentra-
tions. Similarly, in some cases, an increase in the rate of growth of the faces with
increase in pressure has been observed. Pressure apparently does not have any sig-
nificant direct effect on the rate of growth of crystals, but it may have an influence
through other parameters: mass transfer and solubility.
Kuznetsov
[67]
reported the effects of temperature, seed orientation, filling, and
temperature gradient,
T, on the gr
o
wth kinetic
s
of corundum crystals. The activa-
tion energies for growth on the
Δ
faces, together with some pre-
liminary evidence on the rate-limiting step in the hydrothermal crystallization of
Al
2
O
3
are gi
v
en..
Figur
e
s 4.11 and 4.12
show the growth rates as functions of fill-
ings—x:
ð
1120
Þ
and
ð
1011
Þ
ð
1011
Þ
;
o:
ð
1120
Þ;
and relation of log R to 1/T for the filling factors given
Figure 4.11 Growth rates as a function of
filling—x: (1011); o: (1120)
[67]
.
R
1.4
550
°
×
1.2
×
×
0.8
520
°
×
550
°
0.4
×
×
×
0
0.45
0.50
0.55
0.60
0.65
0.70 K
