Geoscience Reference
In-Depth Information
Orthotetrahedra (orthosilicates) have a free oxygen not shared with neighboring
[Si
O] tetrahedra but are bonded only with the rare earth cations. R
2
[SiO
4
]O has
two structure types: Eu
1
2
2
[SiO
4
] and Gd
2
[SiO
4
]O; NaR[SiO
4
] has three structure
types, and similarly in the apatite-type R
4.66
[SiO
4
]
3
O and KNd
9
[SiO
4
]
6
O.
Diorthogroup (diorthosilicates) contains compounds of R
2
[Si
2
O
7
](fivestruc-
ture types)—Y
2
[Si
2
O
7
], Sc
2
[Si
2
O
7
], Na
3
R[Si
2
O
7
], K
3
R[Si
2
O
7
], and Na
6
YSc
[Si
2
O
7
].
Triorthosilicates are always characteristic of Y
3
[Si
3
O
10
] and K
3
Y[Si
3
O
8
](OH)
2
(
Figure 7.8
)
[53,54]
. Likely triorthogroups in the form is sufficiently rare Si
a
O
radical and twice observed in Gd-silicates: Na
4
Gd
2
[Si
3
O
10
]
[55]
and Na
2
Gd
3
[Si
3
O
10
]
[56]
.
Tetraorthogroups [Si
4
O
13
] as seen in Na
4
Sc
2
[Si
4
O
13
] is a linear radical which
extends to a series of island radicals SiO
4
a
a
Si
2
O
7
a
Si
3
O
10
a
Si
4
O
13
.
Table 7.6 Crystal Chemical Characteristics of Rare Earth Silicates
[35]
Crystal Chemical Group
R-element
Symmetry
Na
X
R
IX
[SiO
4
]
La
Nd
Orthorhombic (i)
Na
VII
R
VIII
[SiO
4
]
Pr
Ho
Tetragonal
Na
VI
R
VI
[SiO
4
]
Ho
Lu
Orthorhombic (ii)
Na
a
13
R
3
VI
[Si
12
O
36
]
Ho
Lu,
Y Monoclinic
Na
a
R
VI
[Si
2
O
7
]
Er
Lu,
Y orthorhombic
Na
a
R
VI
[Si
6
O
14
]
La
Eu
Orthorhombic
Na
2
a
R
VIII
[SiO
4
](OH)
La, Pr
Orthorhombic
Na
a
R
VII
[Si
6
O
13
](OH)
2
La
Lu
Orthorhombic
a
Coordination number of Na in the work is not given.
Table 7.7 Crystal Chemical Characteristics of Silicates
[35,52]
Compound
O/Si SiO
2
/
R
2
O
3
Si/Na
1
R
Structural
Si
a
O
Motif R
a
O
Co.
No. R
a
Co. No.
of Na
a
R
4.67
[SiO
4
]
3
O
4
0.78
0.64
[SiO
4
]
3d bond
9.7
NaR[SiO
4
]
4
2
0.5
[SiO
4
]
[R
2
O
8
]
NN
9
10
NaR[SiO
4
]
4
2
0.5
[SiO
4
]
4-Membered
ring
8
7
NaR[SiO
4
]
4
2
0.5
[SiO
4
]
[R
2
O
8
]
NN
6
6
Na
2
R[SiO
4
](OH)
4
2
0.33
[SiO
4
]
3d-bond
8
5; 7
Na
3
R[Si
2
O
7
]
3.5
4
0.5
[Si
2
O
7
]
[RO
6
]
0
6
4; 5
Na
15
R
3
[Si
12
O
36
]
3
8
0.66
[Si
12
O
36
]
[RO
6
]
0
6
4; 6
Na
3
R[Si
6
O
15
]
2H
2
O
2.5
12
1.5
[Si
6
O
15
]
7
NaR[Si
6
O
14
]
2.33
12
3
[Si
6
O
14
]
[R
3
O
12
]
N
8
4; 8
a
Co. No.
Coordination number.
5