Geoscience Reference
In-Depth Information
(a)
N
= 2:1-1:2
Na
2
R[SiO
4
](OH)
NaR[SiO
4
]
(rhomb.1)
La
Pr
Nd
Sm
Eu
NaR[SiO
4
]
(tetra.)
Gd
Tb
Dy
Ho
Er
Tu
Yb
Na
3
R[Si
2
O
7
]
NaR[SiO
4
]
(
rhomb II
)
Lu
Y
NaOH
5%
20%
60% 90%
100%
N
= 1:4
(b)
La
NaR[Si
6
O
14
] + Na
2
R[SiO
4
](OH)
Pr
Nd
Sm
Eu
Gd
NaR[SiO
4
]
(tetra.)
Tb
Dy
Ho
Er
Tu
Yb
Na
15
R
3
[Si
12
O
36
]
Lu
Y
NaOH
20%
60% 90%
100%
Figure 7.6 Crystallization fields for the system Na
2
O
a
R
2
O
3
a
SiO
2
a
H
2
O, T
5
450
650
C,
P
5
400
2500 atm (i—surplus R
2
O
3
; ii—surplus SiO
2
)
[35]
.
crystallization of phases with low “connectivities” between [SiO
4
] tetrahedra in the
structure. A decrease in the ratio of interstitial oxygen atoms and hydroxyl groups
broadly corresponds to a progressive change from a 3d structure, to one that is lay-
ered, to one based on chains or rings and eventually, to one that contains discrete
radicals, typically [SiO
4
]
2
4
groups. The changes in the structure of the crystalliza-
tion phase occur via changes in the structure of the silicate species in the solution.
The latter, in turn, responds to changes in the experimental conditions in accor-
dance with a generalized reaction:
Si
x
O
2
ð
2x
2
y
Þ
Si
x
O
2
ð
2x
2
y
2
z
Þ
2zOH
2
2
H
2
O
ð
7
:
3
Þ
1
1
y
y
