Geoscience Reference
In-Depth Information
The system is a four-component one with temperature and pressure as constants.
According to the Gibb's phase rule at the invariant equilibrium, only four phases
can exist (NdPO
4
, NdP
3
O
9
, NdP
5
O
14
, and MNdP
4
O
12
). The variation in the concen-
tration of phosphorus and alkaline metal oxides determines the fields of crystalliza-
tion of different phases in the composition diagram. According to Le Chatalae's
principle, the increase in the potential of M
2
O(P
2
O
5
) provides the reaction taking
place with the absorption of this component. The following reactions can explain
the various nonvariant points in the ternary phosphate systems:
NdPO
4
1
P
2
O
5
1
G
1
"
Nd
ð
PO
3
Þ
3
1
G
2
2NdPO
4
1
M
2
O
P
2
O
5
1
G
3
2
"
MNdP
4
O
12
1
G
4
1
Nd
ð
PO
3
Þ
3
1
P
2
O
5
1
G
5
"
NdP
5
O
14
1
G
6
ð
7
:
15
Þ
2Nd
ð
PO
3
Þ
3
1
M
2
O
P
2
O
5
1
G
7
"
2MNdP
4
O
12
1
G
8
1
2NdP
5
O
14
1
M
2
O
G
9
"
2MNdP
4
O
12
1
P
2
O
5
1
G
10
1
where G is a liquid phase having transitional composition. The presence of a divar-
iant field of stability of MNdP
4
O
12
(
Figure 7.55
) indicates the possibility of grow-
ing these crystals from solutions with variable concentrations of phosphorus and
alkaline metals. The relevant empirical AB-diagrams of fields of crystallization,
where A
Nd
2
O
3
, are shown in
Figure 7.56
.The
dependence of PT conditions and the equilibrium state of the system has been stud-
ied (
Figures 7.57 and 7.58
). It is observed that with changes in temperatures, there
will be different polymorphic modifications of tetraphosphates. In hydrothermal
systems, the pressure measured corresponds to the pressure of aqueous solutions
and its influence on the crystallization process is quite important. At a constant
temperature, it increases with reference to the molar fraction of the water. In the
pressure range of 1
M
2
O/Nd
2
O
3
,B
P
2
O
5
/M
2
O
5
5
1
600 atms, the boundaries of phase formation do not change. In
this case, the changes in phase formation take place only during changes in the
concentration of water in the ampoule. With an increase in the molar fraction of
water, the following phase transformation takes place:
Figure 7.55 Composition diagram for the
system Me
2
O
a
Nd
2
O
3
a
P
2
O
5
a
H
2
O
[248]
.
M
2
O
P
2
O
5
