Geoscience Reference
In-Depth Information
could be one of the factors favorable for the preparation of hydrated zirconium sili-
cates, possessing the ability for ion exchange. Then, the second controlling factor,
giving a certain diversity of compounds, is the type of the initial silicon-containing
reagent. The comparison shows that in a wide range of Si:Zr ratios (2
5:1) used in
the systems Zr(OC
3
H
7
)
4
Na
2
Si
3
O
7
NaOH
a
H
2
O and Zr(OC
3
H
7
)
4
Na
2
SiO
3
NaOH
a
H
2
O,
the main product
is Na
4
Zr
2
Si
5
O
16
H
2
O. No formation of
2H
2
O or the novel 12.8
˚
phase was found. The second feature is the
formation of Na
4
ZrSi
3
O
10
(as admixture or main phase) even at a Si:Zr ratio of 1:1,
which was not observed when silicic acid as the silicon-containing reagent was used.
The hydrothermally grown crystals of the sodium zirconium silicates have a
broad size range (from 0.2 to 25 mm) and an irregular form. The lack of a clearly
defined morphology correlates with a low crystallinity of the compound.
A majority of the works on the synthesis of alkali-zirconium silicates suggest
that all of them are layered compounds. Thermal and chemical stability, different
Zr/Si molar ratio, and intercalation ability suggest that these compounds could be
promising for pillaring with inorganic and organic species for synthesis of new
functionally designed and tailored catalysts, and support for catalyst and adsor-
bents. High affinity for some transition and toxic heavy metals makes sodium zir-
conium silicates attractive for high-level purification of certain technological
solutions and wastes containing these elements.
Na
2
ZrSi
3
O
9
7.9 Growth of Selected Silicates
It would be extremely difficult to describe the growth of all the silicates
under hydrothermal conditions as there have been a great number of publications
in this field since the 1850s. Therefore, we only discuss the synthesis of some
selected silicates with an emphasis on the current trends in the growth of silicate
crystals.
The first work on silicates on the whole by hydrothermal method was carried
out by W¨hler
[105]
. Followed by this, several other silicate minerals were synthe-
sized during the nineteenth century. Allen et al.
[106]
carried out a systematic
investigation of the hydrothermal synthesis of several pyroxene and amphibole
group of minerals. For example, these authors quote an experiment of the synthesis
of amphibole at low temperatures by Chroustschoff, who obtained hornblende in
which a mixture of silicic acid, alumina, lime, magnesia, ferrous and ferric oxides,
and the alkalies, partly in solution partly precipitated were heated in sealed glass
vessels at a temperature of 550
C for 3 months. The results of these experiments
were used to explain the geological relations. Later, Morey and Niggli
[107]
pub-
lished an exhaustive review on the hydrothermal formation of silicates, giving a
complete list of silicates obtained till that time, and all the important publications
were included.
Figure 7.24
shows the equilibrium diagram of the system diopside
anorthite at pressure 1 bar, and it was studied by Bowen (1915)
[108]
. Soon after
