Geoscience Reference
In-Depth Information
Table 7.5 Solubility of Na
2
CO
3
in Water
[33]
P
(atm) Na
2
CO
3
in wt%
475
C
500
C
540
C
900
1.5
2
2
1735
55
1200
,
1.0
1300
3.5
2
4
1905
88
1300
1.0
1550
16
520
C
1400
1.0
1625
28
1200
1.5
1637
37
1400
3.5
1833
44
1500
3.5
1955
49
2
50
For example, rare earth elements with carbonate ions would form complex
anions of [R(CO
3
)
3
] type, which can migrate and in the presence of alkali elements
would form Na
3
[R(CO
3
)
3
]
3
. From Lu to La, the stability of such complexes would
increase with an increase in the alkali concentration in the medium. Thus, in natu-
ral processes, rare earth elements must have migrated as the soluble complexes of
carbonates, fluorides, chlorides, and maybe sulfates. Such complexes, in turn, par-
ticipate in chemical reactions to form more stable complexes with additional anions
of CO
3
, F, Cl, O
4
, and OH, thus, the study of rare earth silicates in aqueous solu-
tions of Na
2
CO
3
,Na
2
SO
4
, NaCl, and NaF. In fact, the solubility of Na
2
CO
3
,
Na
2
SO
4
and NaF in water decreases with an increase in temperature and increases
with an increase in pressure. The solubility of NaCl in water increases with an
increase in temperature and pressure. These studies on the behavior of solvents
with temperature and pressure have helped in the understanding of the phase for-
mations in various rare earth silicates and analogous systems.
Although the study of various rare earth silicate systems began during the 1970s
extensively, the objective was to understand the distribution and mobility of rare
earth elements in natural systems. The importance of the addition of alkali elements
into the systems was realized in the 1980s, because the alkali and rare earth metals
together play an important role in the formation of various phases. Several new
phases were discovered and with the availability of a vast amount of structural data
on these alkali rare earth silicates, their physical properties became known to the
scientific community. Some of these alkali rare earth silicates exhibit very interest-
ing mixed framework structures. Among the cations in the alkali rare earth silicates
with mixed framework structures, the alkali ion can serve as a mobile charge car-
rier. It provides interesting opportunities for the development of potentially new
fast ion conductors because of the extensive possibilities for crystal chemical tailor-
ing. The chemical stability for silicates and a large number of related phases are
available for investigation
[36,37]
. High Ionic conductivity has been reported in
Na
5
RSi
4
O
12
(where R
La
Lu, Sc, Y) ring silicates, almost simultaneously from
5
three different
laboratories
[38
43]
. Hydrothermal synthesis of Na
5
YSi
4
O
12
,
