Geoscience Reference
In-Depth Information
The study of the phase diagrams of crystallization is the first step in the search
of new compounds, the conditions of their synthesis, and the prediction of their
physical properties. The next step in the hydrothermal synthesis is the selection of
compounds with promising properties and the development of methods providing
the synthesis of crystals with sizes appropriate for their practical use in science and
technology. The study of phase interactions in multicomponent systems yields a
wealth of information for hydrothermal physical and crystal chemistry and simula-
tion of natural processes. In fact, the number of single crystals grown on seeds
under hydrothermal conditions is much smaller than that of spontaneously grown
single crystals. Today we have the techniques for crystal growth on seeds for the
following materials:
Oxides
Quartz SiO
2
, GeO
2
,Si
1
2
x
Ge
x
O
2
, zincite ZnO, cuprite Cu
2
O, sapphire
A1
2
O
3
, ruby A1
2
O
3
:Cr
3
1
, TeO
2
, ABO
4
(A
2
Sb, B
2
Sb, Nb), ZnO
Sulfides
ZnS, HgS
Complex oxides
Sillenite Bi
12
TO
20
(T
5
Si, Ge, etc.), garnets Ln
3
Fe
5
O
12
, ferrites
LnFeO
3
Silicates
Nepheline (Na, K) AlSiO
4
, cancrinite Na
8
Al
6
Si
6
O
24
(OH,C1)
2
NH
2
O,
sodalite Na
8
Al
6
Si
6
O
24
(OH)
2
NH
2
O; beryl (emerald) Al
2
Be
3
Si
6
O
18
:
Cr
3
, eulytine Bi
4
(SiO
4
)
3
Phosphates and
family
A1PO
4
, GaPO
4
, KTiOPO
4
, KTiOAsO
4
, GaAsO
4
Carbonates
CaCO
3
, CoCO
3
, MnCO
3
Germanates
KAlGeO
4
,Na
8
Al
6
Ge
6
O
24
(OH)
2
nH
2
O (two modifications), Bi
4
Ge
3
O
12
Nitrides
GaN, AlN
Others
KNbO
3
(KN), KTaO
3
(KT), KTa
1
2
x
Nb
x
O
3
(x
5
0.44, KTN)
The number of spontaneously nucleated crystals/compounds is very high and it
is practically impossible to list the individual phases and their preparation condi-
tions. Therefore, we have made an effort to group them into broader divisions and
discuss only some selected phases.
7.3 Rare Earth Silicates
Among silicates obtained under hydrothermal conditions, the rare earth silicates are
the most extensively studied ones owing to their academic as well as technological
importance. The rare earth elements have a significant influence on the crystalliza-
tion of various rocks and minerals and in the magmatic processes on the whole.
Their distribution in various rocks and minerals clearly demonstrates the physico-
chemical influence they exert upon the crystallization of the magma. Their distribu-
tion and association further depend upon the alkalinity of the melt. Therefore, an
extensive work has been carried out on various alkali rare earth metal silicate sys-
tems in order to understand the crystallization of a large number of rare earth sili-
cates with and without alkali metals. A majority of these rare earth silicates were
