Geoscience Reference
In-Depth Information
Figure 7.32 Hexagonal prism shaped crystals grown from 0.1 N NaOH solution
[161]
.
There are several other beryllium-bearing silicates—like chkalovite
Na
2
BeSi
2
O
6
,
iron beryllosilicate
BeSi
2
O
8
,
barylite-BaBe
2
Si
2
O
7
and so on—have been synthesized as single crystals using the
hydrothermal method
[162
Fe
3
BeSi
3
O
9
(F,OH)
2
,
aluminoberyllosilicate
Na
3
Al
165]
. Sobolev et al.
[166]
prepared chkalovite and its
germainum analog by hydrothermal
techniques from the system BeO
a
MeO
2
a
NaOH
Si, Ge). Beus and Dikov
[167]
have studied its field of
stability in hydrothermal solutions containing NaOH, Na
2
CO
3
,NaF,andHF,and
prepared from beryllium hydroxide and nitrate with amorphous SiO
2
in hydrothermal
NaOH and NaCl
a
H
2
O(whereMe
5
600
C,
solutions. The experimental
temperature was 400
Δ
60
C, P
700 atm and the experimental duration was 5 days. It is pos-
sible that in natural hydrothermal process, where chkalovite is very rarely encoun-
tered, undergoes further changes owing to its chemical instability, forming other
more stable beryllium compounds. Most of the beryllium-bearing compounds exhibit
piezoelectric properties, especially the Na
1
-andBu
1
2
-bearing beryllium silicates.
For example, barylite crystals cannot be synthesized by the ordinary solid-state reac-
tion. Maeda et al.
[165]
obtained this crystal under the following conditions:
T
5
40
5B
P: 500 bar
T: 385
C
Δ
T:15
C
Solvent: 3 M KOH
Baffle opening: 15%
Seed: BaBe
2
Si
2
O
7
ceramics.
Likewise, there are several such reports on the hydrothermal synthesis of
beryllium-bearing silicates and most of
them exhibit
interesting electronic
properties.
7.9.3 Tourmaline
Tourmaline is a complex silicate of boron and aluminum with either magnesium,
iron, or the alkali metals prominent. The general formula can be written as
