Geoscience Reference
In-Depth Information
Table 7.12 Results of Electron Microprobe Analyses at Three Different Locations Within a
Single Crystal of the Present Sodium Neodymium Silicate
[76]
Position
Na mol%
Nd mol%
Si mol%
Total wt% oxides
1
10.81
3.49
22.40
94.55
2
10.79
3.50
22.50
94.77
3
10.74
3.46
22.24
94.19
Average
10.78(4)
3.48 (2)
22.4 (1)
94.5 (3)
Expected
10.5
3.5
21.0
94.7
The expected values are those calculated for the composition Na
3
NdSi
6
O
15
2H
2
O, deduced from the chemical analysis
and the structure determination.
in NaOH concentration, silicates of type Na
15
R
3
[Si
12
O
36
] are obtained with Si
a
O
radicals made up of 12-membered rings, [Si
12
O
36
](O/Si
5
3).
In the region with high silica concentration, R
2
O
3
is insufficient for the forma-
tion of complexes with 3d cationic motif. When R
2
O
3
:SiO
2
5
1:4 and concentration
of NaOH is 5%, NaR[Si
6
O
14
] type was obtained. In this, the cationic motif is repre-
sented by rare earth chains. During the increase in NaOH concentration, rare earth
chain silicates cease to form with the appearance of compounds with discrete rare
earth cations. In Na
15
R
5
[Si
12
O
36
], rare earth ion lies as isolated octahedra, whereas
in Na
3
R[Si
6
O
15
]
2H
2
O, the rare earth ion lies as isolated seven-sided polyhedra.
More recently, Haile et al.
[75]
carried out hydrothermal investigations in the
high silica region of the Na
2
O
a
Y
2
O
3
a
SiO
2
system and obtained a new phase—
8.385
˚
; Z
trisodium yttrium, Ibmm, with a
4, and n
atoms in the asymmetric unit. The structure is unique in the sense that silica tetra-
hedra form isolated [Si
6
O
15
] rings, rather than layers as might be expected from the
Si:O
10.468, b
15.246, c
5
5
5
5
4. No isomorphs to Na
3
YSi
6
O
15
have been reported.
Figure 7.17a
shows the
structure of Na
3
YSi
6
O
15
as viewed along c-axis. The SiO
4
and YO
6
units are repre-
sented as polyhedra.
Similarly, Haile
5
et
al.
[76]
reported another phase
rich in sodium,
Na
3
NdSi
6
O
15
SiO
2
under
hydrothermal conditions. Although this compound has very similar lattice constants
and diffraction symbols to data reported for NaNdSi
6
O
13
(OH)
2
2H
2
O, a layered silicate in the system Na
2
O
a
Nd
2
O
3
a
nH
2
O (special
group Cmm2) by Karpov et al
[70]
, the microprobe analysis given by Haile et al.
(
Table 7.12
) shows the composition substantially richer in Na
1
. The asymmetric
unit of the structure contains 22 atomic sites: 1Nd, 4Na (three of which are only
partially occupied).
Figure 7.17b
shows the structure of Na
3
NdSi
6
O
15
2H
2
O
viewed along a. The SiO
4
and NdO
7
units are represented as polyhedra.
Figure
7.17c
shows an isolated NdO
7
polyhedron viewed along a, and the figure also gives
oxygen
oxygen distances (
˚
).
Tuttle autoclaves have been used in the above works. The solvents used are either
pure water or a dilute solution of a sodium base (0.1 M NaOH, Na
2
CO
3
,orNaHF
2
),
experimental temperature was 500
10
5
Pa.
In systems containing Fe
3
1
and weak NaOH solutions, aegirine, NaFeSi
2
O
6
type silicates were obtained, whereas in the strongly concentrated NaOH solutions,
600
C, and pressure was 8.25
3
