Geoscience Reference
In-Depth Information
Figure 5.8 Alpha-quartz
crystals
[38]
.
5.2.4 Solubility
Solubility is one of the most important aspects in hydrothermal crystal growth. The
early literature surveyed clearly shows that the very slow growth rate achieved until
the works of Nacken (Germany 1950)
[21]
and Wooster and Wooster (1946)
[40]
were published was mainly attributed to the lack of knowledge of the solubility of
quartz. The same applies to other compounds also. The first systematic study of the
solubility of quartz was carried out by Spezia (1905)
[39]
. Since then, a lot of prog-
ress has been achieved in the understanding of the solubility in general for various
inorganic compounds. During 1960s, new methods of investigating the solubility
and the new experimental setup for the determination of solubility under hydrother-
mal conditions were proposed
[41,42]
.
The solubility of quartz in pure water was found to be too low for crystal growth
(0.1
0.3 wt%), but the solubility could be markedly increased by the addition of
OH
2
,Cl
2
,F
2
,Br
2
,I
2
, and acid media which act as mineralizers. For example,
the reactions:
SiO
3
2
2
1
SiO
2
1
2OH
2
!
H
2
O
Si
3
O
7
2
2
1
2OH
2
1
3SiO
2
!
H
2
O
show the formation of various complexes or species during the hydrothermal crys-
tallization of quartz. Hosaka and Taki
[43]
have used Raman spectra to identify
and quantify such species.
In pure aqueous solutions (even at 400
C and 25,000 psi), the solubility of
quartz is too low to allow growth to take place in any reasonable time. Alkaline
additions, such as NaOH, Na
2
CO
3
, KOH, and K
2
CO
3
are all effective as minerali-
zers in this pressure and temperature range. A small increase in molarity results in
only a slight increase in the growth rate, whereas a large increase begins to produce
an additional phase along with quartz. The minimum molarity for a good growth
