Geoscience Reference
In-Depth Information
The synthesis in the Na
2
O
a
Nd
2
O
3
a
SiO
2
a
H
2
O system is much more sensitive
to the basic alkali content than in the K
2
O
H
2
O system, with the
apatite-type phase forming at much lower hydroxide and carbonate concentrations.
High SiO
2
solubility in the presence of Na
2
O is also responsible for the much
greater quantity of gelatinous material that resulted from these experiments.
It is interesting to note that the crystallization of various phases and the stoichio-
metries of the phases depend mainly on the type and concentration of the alkali
metals. The apatite-type phases have been obtained at the highest KOH molarity.
The two quantities, Si:O and mol% SiO
2
, are necessarily related in a system where
the nonsilicon cations are mono- and trivalent. The former has in addition a very
direct relationship to the linkage of tetrahedra in the silicate framework. A low Si:
O ratio implies that if any or some oxygen atoms are bonded to more than one sili-
con, the connectivity of the framework is low. The change observed in the nature
of the crystallized phase upon an increase in the potassium content, corresponding
to a lowering of the connectivity of the structure. The phase encountered at the
lowest KOH concentration is an ideal Si
2
O
5
layered silicate (K
3
NdSi
6
O
15
,P
bam
form) with all SiO
4
tetrahedra sharing apices with three others. The phase
encountered at the highest KOH molarity is an orthosilicate (KNd
9
(SiO
4
)
6
O
2
)
structurally composed of isolated SiO
4
tetrahedra. Similarly, K
3
NdSi
7
O
17
,pro-
duced at lower KHF
2
concentrations, has an interconnected, 3d silica framework,
whereas K
2
NdSi
4
O
10
OH, obtained at higher concentrations, is based on infinite
(Si
2
O
5
) tubular units.
The concept of classifying compounds as “chain breakers” (e.g., K
2
O) and
“chain formers” (e.g., P
2
O
5
) has been well developed in silicate glass technology,
yet rarely used in the context of silicate crystal growth. As the hydrothermal growth
involves a solution in addition to the growing silicate phase, it is by no means obvi-
ous that such an approach should be applicable in this context. Ikornikova
[49]
has
studied aqueous solutions of SiO
2
and Na
2
O, and has observed that at low alkali to
silica ratios in the solute, but high overall solute concentrations, silica is dissolved
as long chain polymers or colloidal particles (5
a
Nd
2
O
3
a
SiO
2
a
20
˚
in radius) that have an over-
all composition ranging between Si
2
O
2
5
and SiO
2
3
As the alkali to silica ratio in
the liquid is increased, such agglomerates break apart, coordination decreases, and
the predominant species become more negatively charged pyrosilicate
:
Si
2
O
6
7
ð
Þ
and
SiO
4
4
orthosilicate
ð
Þ
groups. In this particular system, we expect reactions such as
2Si
6
O
6
4OH
2
Si
12
O
16
ð
Þ
1
ð
Þ
1
ð
:
Þ
aq
aq
2H
2
O
7
4
15
32
Si
3
O
4
8
8OH
2
3SiO
4
4
ð
aq
Þ
1
ð
aq
Þ
1
4H
2
O
ð
7
:
5
Þ
to take place. The dissolved species on the left-hand side of
Eq. (7.4)
has Si:O ratio
0.4 and comprises the basic unit of K
3
NdSi
6
O
15
, while that on the right-hand side
has a ratio of 0.375 and it corresponds to the silicate layer in K
8
Nd
3
Si
12
O
32
OH.
Similarly,
Eq. (7.5)
represents the equilibrium between the aqueous counterparts
of K
8
Nd
3
Si
12
O
23
OH and KNd
9
(SiO
4
)
6
O
2
(Si:O
0.25 on the right).
5
