Geoscience Reference
In-Depth Information
Silicates of rare earth elements owing to their ionic radii belong to a large group
of silicates with larger cations. During the 1950s, the academican N.V. Belov
framed the structural principles for these silicates. Further, the structure formation
of silicon
oxygen radicals shall be treated essentially, as a result of cationic motif.
Their differentiation as chains, ribbons, cages, and rings are relatively more popu-
lar, but a significant variation depending upon their geometric relationship of cat-
ionic polyhedra and Si
O radicals can also be observed
[51]
.
Natural rare earth silicates are considered as very complex ones containing a
wide range of cations like: Ca, Fe, Be, Al, Ti, Sr, Mn, Ba, and Na. Their crystal
chemical elucidation is highly related to the naturally occurring rare earth elements.
Thus, we can have two broad groups: alkali rare earth silicates and rare earth sili-
cates without alkalis covering both natural and synthetic rare earth silicates.
Figure 7.7
shows the tetrahedral arrangement in silicates with [Si
6
O
14
] radical. The
typical compounds are Na
4
[Si
6
O
14
] and NaNd[Si
6
O
14
]
[49]
.
Today, over 15 structure types of mixed rare earth silicates without alkalis and
21 structure types of rare earth silicates with alkali cations have been reported.
Their crystal chemical characteristics are presented in
Tables 7.6
a
7.9 [35,52]
.
In these compounds, the following anionic groups have been identified: ortho-,
diortho-, mixed ortho-, triortho-, and tetraorthogroups, discrete 4-membered rings,
discrete 12-membered rings, spiral chains, layers from 6-, 8-, and 12-membered
rings, triple column and Si
O framework. Various types of anionic radicals, found
in rare earth silicates are, given in
Table 7.10 [52]
.
a
b
=6.5 Å
(a)
a
= 20.6 Å
Y
= 15.39 (27.69) Å
(b)
Z
= 19.67 Å
Figure 7.7 Tetrahedral arrangement in silicates with [Si
6
O
14
] radical
[7]
.
