Geoscience Reference
In-Depth Information
In Ref.
[74]
, it has been shown that with a decrease in the values of rare earth
cations, the stability of the R
18
Si
7
O
41
decreases especially for smaller elements
(Er
Lu). If other elements like Na, Mn, Pb, and Ca go into the structure, the stabil-
ity of the structures under hydrothermal conditions again rises. However, the stabil-
ity of rare earth britolite determined mainly from the position of these other
elements is in the structures.
Transition from britolite to mixed Na R-silicates takes place during the complete
destruction of cationic motif and with a change in the coordination number of rare
earth elements.
In highly concentrated aqueous solutions NaOH (
80%) (
Table 7.11
) mixed
,
silicates like Na
2
O
Na. R[SiO
4
] are formed covering a significant
part of the rare earth series from Pr to Ho, including Y. The latter one is usually
related to the heavier (Eu
R
2
O
3
2SiO
2
5
Lu) rare earths with the coordination number 7, since
the ionic radius of Y is slightly higher (0.99) and it lies between Nd and Sm. This
emphasizes sufficiently simple means of synthesis of tetragonal modification of
NaY[SiO
4
]. The field of stability of tetragonal rare earth silicates of type NaR
[SiO
4
] lies between Ho and Er. This is the traditional boundary for the morpho-
trophic series of rare earth elements. Its existence is usually based on the geometric
representation of the stability of the rare earth octahedra, since the significant part
of the R
75%). However, under the conditions of high
potential elements with lower values of electronegativity, as Na
1
, a better model
could be obtained giving the influence and an overall covalent bonding in the
nature of R
a
O bonding is ionic (
B
O bond. It can be assumed that in the aqueous solvents, with higher
concentration of Na
1
, the capability of internal f-orbit for hybridization, which is
for Er
a
Lu group in
NaOH aqueous solvent forms only silicates with octahedral coordination of the rare
earth: NaR[SiO
4
] and Na
3
R[Si
2
O
7
].
Reduction in the influence of f-orbit at higher potential Na
1
can be seen as the
possibility for the distortion of rare earth motifs, particularly leading to the decrease
in the coordination number of rare earth elements up to 6 (trigonal prism). In this
case, the rare earth polyhedra form discrete groupings of rare earth prism. It is
Lu, indistinctly becomes closer to zero. Therefore, the Eu
Table 7.11 Phase Formation in the System Na
2
O
a
R
2
O
3
a
SiO
2
a
H
2
O in the Region
of Surplus R
2
O
3
Depending upon the NaOH Concentration
[35]
Rare Earth Elements
CNaOH (%) La Ce Pr Nd
Sm Eu Gd TB Dy Ho Er Tu Yb Lu Y Sc
80 A G C C C C C C C C F F F F C G
80
2
55 A G A C C C C C C C E E E E H H
55
2
25 A A A A
1
CCCCCCCDDDDDG
25
2
5 BBBB CCCCCCDDDDDG
5
BBBB BBBBBBBBBBBH
A, NaRSiO
4
(orthorhombic I); C, NaRSiO
4
(tetragonal); D, NaRSiO
4
(orthorombic II); E, Na
3
Rsi
2
O
7
; B, britolites
R
4.67
(SiO
4
)
3
O; F, unknown phase; G, experiments not carried out; H, NaScSi
2
O
6
.
