Geoscience Reference
In-Depth Information
found to be new phases, and most of them carry technological significance owing
to their unique physical properties. Although a great amount of work has been done
on the synthesis of several hundreds of phases of rare earth silicates with and with-
out alkali metals, there are only a few publications dealing with their crystal growth
and seeded growth. Nevertheless, a vast literature data accumulated on these com-
pounds yield a great variety of interesting results with reference to the phase for-
mation, pressure
temperature conditions, starting materials, type of rare earth
element, alkali metal, the role of solvent, and so on. A large number of structure
types have been obtained, which clearly indicates the importance of these rare earth
silicates from the academic point of view. Besides, these rare earth silicates exhibit
very interesting physical properties, which have been studied by various authors
and they have correlated these properties with their molecular structures. Though it
is extremely difficult to discuss each and every aspect dealt with in the literature,
we have made a sincere attempt to provide an overall picture of these rare earth
silicates of hydrothermal origin with a special reference to their phase formation
and structure types.
7.4 Phase Formation of Rare Earth Silicates
(in Aqueous Solvents)
It is well known that the hydrothermal solutions contain significant amounts of
chlorides, carbonates, and other well-soluble components. The distribution and
mobility of the rare earth elements greatly depend upon their interaction with Cl, F,
SO
4
, and CO
3
species. The formation of silicates in nature takes place essentially
from fluoride solutions having alkaline character. Among them, the main role is
played by the hydroxides of Na and K. In laboratory synthesis of rare earth sili-
cates, essentially aqueous solvents of hydroxides and carbonates of Na and K, and,
similarly, chloride, fluorides, and their mixtures are commonly used as solvents.
The solubility increases above 60
C in NaOH solutions, especially above the melt-
ing point of NaOH (476
C), there exists a complete mixing. Similarly, the increase
in the concentration of NaOH with temperature further improves the solubility
[31,32]
. Such studies have been carried out on carbonate solutions in detail
[33,34]
.
Table 7.5
gives the solubility of Na
2
CO
3
in water.
The bonding of the rare earth elements in the form of soluble species could be
represented as follows
[35]
:
R
I
y
X
z
2
R
1
3
2½
R
en
ð
AX
Þ
m
ð
7
:
2
Þ
where
R
5
rare earth ion
X and (AX)
5
any acidic ions
I and II
5
electroneutral compounds or complex anions.
