Geoscience Reference
In-Depth Information
Figure 7.41 NC diagram of
crystallization of the system
Na
2
O
a
MnO
a
SiO
2
a
H
2
O
[195]
.
μ
SiO
2
Na
2
MnSi
3
O
15
Na
2
Mn
3
Si
4
O
12
Na
2
MnSiO
4
Mn
2
SiO
4
μ
Na
2
O
Some of them have been obtained as single crystals using spontaneously grown
crystals as seeds
[197]
.
7.10 Hydrothermal Growth of Lithium Silicates
Lithium aluminum silicate systems are being studied extensively because of their
ultra-low thermal expansion and their geological importance. Almost all the natural
lithium aluminosilicates have been synthesized, together with a number of artificial
lithium compounds with no natural analogs. However,
the only exception is
α
-spodumene, which has never been synthesized with an adequate degree of repro-
ducibility,
in spite of
the high pressure
temperature conditions of synthesis
600
C, P
(T
4000 atm)
[198,199]
. In the Li
2
O
a
Al
2
O
3
a
SiO
2
a
H
2
O system, in
5
5
700
C and at pressures of up to 9000 atm,
temperature range 400
α
-eucryptite,
β
-spodumene, petalite, lithium silicate, and
α
-spodumene have been synthesized
over a wide range of pH
[200]
. Instead of
α
-spodumene, others—like eucryptite,
β
-spodumene, petalite, and a number of artificial lithium compounds—have been
obtained.
The results of the foregoing investigations lead to the conclusions that (a) spodu-
mene is formed at pressures of over 4000 atm as indicated, in particular, by the fact
that all the phases synthesized have lower densities than that of
α
-spodumene, and
(b) isomorphic impurities reduce the minimum pressure required for the formation
of
α
-spodumene.
