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
Search WWH ::




Custom Search