Geoscience Reference
In-Depth Information
5
Hydrothermal Growth of Some
Selected Crystals
The hydrothermal technique has produced a wide variety of minerals and crystals,
both in nature and in laboratory in all sizes from nanosize to bulk size. In fact, it is
the only technique to synthesize some of the inorganic compounds like quartz and
berlinite. However, the method has some limitations as far as the size is concerned.
Except quartz, the method has not yielded large single crystals. Although berlinite,
gallium berlinite, and lithium tetraborate show many superior piezoelectric proper-
ties compared to
-quartz
crystals. Further, it is difficult to discuss here the growth of all the compounds of
hydrothermal origin. Hence, the scope of this chapter has been restricted only to
the growth of some bulk crystals of technological importance.
α
-quartz, it is not possible to obtain them as large as
α
5.1 Quartz
Quartz is one of the most abundant minerals and occurs as both essential and acces-
sory constituents of rocks. References to quartz are known from 1505 onward. It
may have been derived from the Saxon word querkluftertz, or cross-vein-ore, which
could easily have become condensed to querertz and then to quartz
[1]
.
Quartz, SiO
2
, exists in both crystalline and amorphous forms in nature.
The crystalline form of quartz has over 22 polymorphic modifications. The three
principal crystalline forms of SiO
2
(quartz, tridymite, and cristobalite) each possess
a well-defined field of stability under equilibrium conditions. The transformations
from one to another are, however, somewhat sluggish, so that the higher tempera-
ture forms, cristobalite and tridymite, can exist metastably below their inversion
temperatures. Each crystalline form of quartz, metastable tridymite and
metastable cristobalite, has furthermore low- and high-temperature modifications
designated as
-quartz, which is
stable below 573
C at atmospheric pressure, is the most popular and technologi-
cally important. The structure of
α
- and
β
-, respectively
[2
4]
. Among them,
α
-quartz
has trigonal symmetry, belongs to the enantiomorphous crystal class-32, and its
space group is P3
1
21 or P3
2
21 according to its right- or left-handedness. Its struc-
ture was among the first to be investigated by X-ray techniques as early as 1914 by
Bragg
[6]
. The structure of quartz is made up of SiO
4
tetrahedra, which are linked
by sharing each of their corners with another tetrahedron. In the three-dimensional
framework thus formed, every Si
4
1
has four oxygens and every oxygen has two
α
-quartz is shown in
Figure 5.1 [5,6]
The
α
