Geoscience Reference
In-Depth Information
Na
15
Fe
5
Si
12
O
36
forms. The heavier rare earth elements (Ho to Lu) follow Fe
1
3
,
whereas the lighter lanthanides form original structures, whose cationic nature has
not been observed in natural analogs.
7.6 Degree of Silification
One of the characteristic features of silicates is the molar ratio of rare earth oxides
and silicon. Variation in the value of molar ratio varies within a wide range from
0.78 for Na-free silicate (apatite
britolite structures) up to 12 for compounds with
highly condensed [Si
O] radicals. The degree of silification (S) can be represented
as the ratio of Si-atoms to the sum of the cations:
a
KSi
S
ð
7
:
6
Þ
5
ð
nM
11
1
mMe
11
1
?Þ
The degree of silification decreases with an increase in NaOH concentration.
This reflects the interrelation of cationic and anionic motif (crystal chemical fac-
tor). It also reflects the phase formation, chemical environment, and conditions of
synthesis (genetical factors). In the case of the lower concentration of NaOH, the
monocrystals can be obtained in the presence of additional anions like CO
3
, Cl, F,
and SO
4
. Dimitrova
[35]
has studied the degree of silification for several rare earth
silicate systems and found S
3 only for one silicate and for a majority of the sili-
cates S
1 with a higher association of cationic motif. Corresponding to the
increase in the concentration of Si from 0.27% to 16.7%, the following phase
changes have been observed:
,
R
42
=
3
½
SiO
4
3
O
!
Na
3
R
½
Si
6
O
15
2H
2
O
!
NaR
½
Si
6
O
14
ð
7
:
7
Þ
S
0
:
64
1
:
5
3
:
0
5
The other example for silification of rare earth silicates in highly concentrated
solution of NaOH (30
45%) is the (Ho
Lu) series:
½
SiO
4
!
½
SiO
7
!
½
Si
12
O
36
NaR
NaR
Na
15
R
ð
:
Þ
7
8
:
:
:
S
5
0
5
0
5
0
66
In highly concentrated solutions, the change in the concentration of Si from
48% to 60% does not influence on the degree of silification. For example, at a con-
stant concentration of Si (48%) and increase in NaOH concentration, the following
chain sequence is observed:
i. For lighter lanthanides:
R
42
=
3
½
SiO
4
3
O
!
NaR
½
SiO
4
!
Na
2
R
½
SiO
4
ð
OH
Þ
ð
7
:
9
Þ
S
5
0
:
64
0
:
5
0
:
5
