Geoscience Reference
In-Depth Information
Figure 7.43 Characteristic photographs of
α
-eucryptite and
β
-spodumene crystals
[200]
.
-spodumene. The partial replacement of Al
1
3
by Cr
1
3
reduces the lower limit of pressures for the formation of spodumene; spod-
umene containing traces of Cr
3
1
grows on the seed at 3000 atm. A complete
replacement of Al
2
O
3
by Fe
2
O
3
or Cr
2
O
3
leads to the synthesis of iron and chro-
mium spodumene in the low-pressure range: approximately 250 and 750 atm,
respectively.
Figure 7.43
shows the characteristic photographs of
decomposes into
α
-eucryptite and
β
α
-eucryptite and
β
-spodumene crystals
[200]
.
Ghobarkar
[201]
has studied the hydrothermal synthesis of the petalite mineral
phases and their morphological changes with temperature in great detail. Here,
petalite minerals were synthesized hydrothermally at different temperatures from a
glass with the composition Li
2
CO
3
:Al
2
O
3
:SiO
2
5
1:1:8. It was found that the devel-
opment of the mineral phases and the volume and habit of crystals depended on the
rise of temperature from 410
C to 490
C,
-petalite crystallized in the monoclinic
system with a constant change in the habit. Alpha-petalite splits into quartz and an
orthorhombic
α
β
1
-petalite from 490
C onward. The tetragonal phase of petalite
forms from 560
C to 750
C with changes in the habit.
Thus, the hydrothermal synthesis of lithium silicates is a challenging field in
hydrothermal research area. Recently, researchers are more inclined toward the
synthesis of the lithium silicate family of materials using hydrothermal method
owing to their gemological interest.
7.11 Hydrothermal Growth of Germanates
Germanates form an important group of inorganic compounds having a close prox-
imity to the silicates in many respects. The ionic radii of both Si
4
1
(0.39
˚
) and
Ge
4
1
(0.44
˚
) are closer to one another. Researchers are synthesizing the solid
solutions of silicates and germinate in order to enhance the specific properties for
various applications, e.g., Si
1
2
x
Ge
x
O
2
, to enhance the piezoelectric properties. The
natural germanates are very few and, hence, their synthetic analogs permit to
understand their structures, crystal chemistry, physicochemical conditions, etc.
In nature, germanium usually occurs in the form of isomorphous admixtures in
