Geoscience Reference
In-Depth Information
Figure 9.11 Dependence of solubility of
corundum
[70]
.
6
5
4.
420
460
500
540
580
T (°C)
Figure 9.12 Dependence of the growth rate of
different faces and the temperature
[70]
.
1
2
0.4
3
0.2
0
8
16
24
Δ
T°C
crystals with (211), (311), and (111) faces were obtained. Thomas
[71]
has studied
the influence of KOH/CO
2
ratio in alkaline-bicarbonate solutions on the growth
rate of corundum crystals under hydrothermal conditions. It has been shown that
the concentration of CO
2
in the system has a main influence on the growth rate of
corundum crystals; as for the KOH content in the bicarbonate solutions, it influ-
ences growth velocity with only a small concentration of the alkaline component.
A mechanism for aluminum transfer with the growth of corundum crystals in the
carbonate and bicarbonate solutions has been discussed. The corundum of hydro-
thermal origin is used both as gemstone and technological material.
The hydrothermal technique is useful for obtaining large crystals of sapphire and
ruby. The corundum modifications of Al
2
O
3
, which are relatively strain free, can be
prepared at moderate temperature conditions and without the excessive gradients
characteristic of flame fusion and melt techniques. These are the gemstone varieties
of corundum and named according to their colors: sapphire blue and ruby red.
Probably, sapphire is the second material after quartz to be grown in any size by
hydrothermal methods. Similarly, large ruby crystals can be grown using carbonate
solutions. The solubility of sapphire and ruby is the same as that of corundum and it
increases with temperature. Oxides of chromium and iron, when taken in the nutrient
along with Na
2
CO
3
solution, more strongly influence the crystallization than when
