Geoscience Reference
In-Depth Information
geological conditions. Zincosilicates phase formation in the system Na
2
O
a
ZnO
a
SiO
2
a
181]
. The fol-
lowing zincosilicates have been identified in this system: Zn
2
[SiO
4
]-willemite,
Na
2
Zn[Si
2
O
6
], NaZn[SiO
4
], Na
2
Zn[Si
3
O
8
], Na
2
Zn
3
(SiO
4
)
2
, and Na
2
Zn
2
[Si
2
O
7
].
Figure 7.33
shows the NC diagram of fields of crystallization of various zincosili-
cate phases.
Figure 7.34
shows the schematic diagram of various zincosilicate
phase formations. Litvin
[175]
proposed the following univariant equilibrium:
H
2
O has been studied in great detail by several workers
[178
Zn
2
SiO
4
1
2Na
2
O
3SiO
2
2
2Na
2
Zn Si
2
O
6
1
3Zn
2
SiO
4
1
2Na
2
O
SiO
2
2
2Na
2
Zn
3
Si
2
O
8
1
Zn
2
SiO
4
1
2Na
2
O
5SiO
2
2
2Na
2
ZnSi
3
O
8
1
Zn
2
SiO
4
1
Na
2
O
SiO
2
2
Na
2
Zn
2
Si
2
O
7
1
2Na
3
Zn
3
Si
2
O
8
1
Na
2
O
1
2SiO
2
2
3NaZn
2
Si
2
0
7
2SiO
2
2
Na
2
Zn
2
Si
2
O
7
1
Na
2
O
1
2Na
2
ZnSi
2
O
6
Na
2
ZnSi
2
O
6
2
Na
2
ZnSiO
4
1
SiO
2
Na
2
ZnSi
3
O
8
2
Na
2
ZnSi
2
O
6
1
SiO
2
Na
2
Zn
3
Si
2
O
8
1
2Na
2
O
SiO
2
2
3Na
2
ZnSiO
4
1
3Na
2
ZnSiO
4
2
Na
2
Zn
3
Si
2
O
8
1
SiO
2
1
2Na
2
O
Zn
2
SiO
4
(willemite): Willemite crystallizes in pure water, in NaOH solution, and
in mixed solvents of NaOH
Na
2
CO
3
. The optimum conditions for spontaneous
recrystallization of willemite are NaOH
1
450
C;
8
C,
4
6wt%;T
375
Δ
T
5
5
5
5
P
600 atm. Single crystals of willemite grow in the form of a hexagonal
prism. The fast growing face is the rhombohedral face (1120), the growth rate
of which is 0.8
400
5
1.0 mm/day. When the concentration of NaOH increases beyond
10 wt%, the solubility of willemite increases and the formation of other phases
begins. Also, when the NaOH concentration is 10 wt%, the boundaries of monomi-
neralic synthesis move up to the ratio ZnO/SiO
2
5
1:2.5. Further increase in the
concentration of the solvent increases the field of quartz. Willemite doped with
active ions like Mn
2
1
exhibits excellent luminescence properties when exposed to
UV radiation, and it is an excellent phosphor.
The field of crystallization of Na
2
ZnSi
3
O
8
falls in the silica-rich region
(
Figure 7.35
). Na
2
ZnSi
3
O
8
crystallizes usually together with willemite, with an
increase in the NaOH concentration, especially silica. Na
2
ZnSi
3
O
8
starts melting
congruently and recrystallizes in the upper part of the autoclave. The presence of
Na
2
CO
3
helps crystals and increases the crystal size. The crystals of Na
2
ZnSi
3
O
8
