Geoscience Reference
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obtained. This compound belongs to the structure-type olivine. At lower pressures
and temperatures (P
5
650
C) in the NaOH rich region
3
20 atm and T
5
558
(
Eu, phases belong to the tetragonal modification and
the crystals are plate like, thick, 1
80 wt%) for groups Nd
.
2 mm in size, and twinning is common.
Na
2
RGeO
4
(OH) (where R
La
Nd) crystals form at GeO
2
/R
2
O
3
5
1:2 and
5
concentration of NaOH
6 wt%. Crystals have equidimensional habit. Output of
100% was obtained in solutions when H
2
O
.
10%. Na
2
R[GeO
4
](OH) belongs to
,
the olivine-type structures. Thus, for La
Gd group of germanates of olivine-type
structures in NaOH solution, the following scheme with an increase in the concen-
tration of NaOH has been proposed:
NaR
3
½
GeO
4
2
ð
OH
Þ
2
!
NaR
½
GeO
4
!
Na
2
R
½
GeO
4
ð
OH
Þ
ð
7
:
13
Þ
In spite of the rearrangement of Na
1
and rare earth cations, the common olivine-
type motif is retained over a wide interval not only on the concentration of NaOH
but also on the ratio of GeO
2
/R
2
O
3
. When GeO
2
/R
2
O
3
.
90 wt%, NaOH
solutions' complex germanates like Na
2
R
6
Ge
8
O
26
appear for Nd and Pr representa-
tives. However, the output is less than 10% and crystals are small (
3:1 in 20
,
1 mm).
Lu are used, a slightly different picture is seen. NaR
2
Ge
4
O
10
(OH),
NaRGeO
4
, and Na
4
R
2
Ge
4
O
13
are the characteristic phases. NaR
2
Ge
4
O
10
(OH) phase
crystallizes in less than 10 wt% NaOH with GeO
2
/R
2
O
3
5
When Dy
1:3.
NaRGeO
4
-orthogermanates crystallize in the R
2
O
3
rich region in the system
Na
2
O
1:2.5
H
2
O.
In the GeO
2
-rich region, for Dy
a
R
2
O
3
a
GeO
2
a
Lu groups, Na
4
R
2
Ge
4
O
13
is the characteristic
phase with 100% output in 30
60 wt% NaOH solutions at ratio GeO
2
/R
2
O
3
5
2.5:1
3:1. The crystals are up to 2 mm in size.
Similarly, the rare earth germanates have been synthesized using aqueous solu-
tions of KOH and KF
[44]
. Unfortunately, output of 100% has not been obtained
for several germanates. Quite often two or three phases crystallize simultaneously.
All the KR-germanates have been grouped into two structure types: K
2
R
4
[Ge
4
O
13
]
(OH,F)
4
and K
4
R
2
[Ge
4
O
13
](OH,F)
4
.
Figure 7.47
shows the phase formation of rare
N
(GeO
2
/R
2
O
3
)
K
4
Ge
9
O
20
1:1
K
2
R
4
[Ge
4
O
13
](OH, F)
4
K
4
R
2
[Ge
8
O
20
](OH, F)
2
1:2
K
2
RF
5
Pr
Sm
Gd
Dy
Er
Yb
La
Nd
Eu
Tb
Ho
Tu
Lu
Figure 7.47 Phase formation of rare earth germanates in the aqueous solution of KF
[44]
.