Geoscience Reference
In-Depth Information
As the OH
2
concentration is raised, the right-hand sides of
Eqs (7.4) and (7.5)
are increasingly favored, Si
Si bonding decreases, the fraction of oxygen
atoms bonded to only one silicon increases, and the average size of the dissolved
silica complexes decreases. During hydrothermal growth, it is expected that these
complexes act as precursors to the crystalline precipitate such that, as the aqueous
concentration of a particular species is increased, the likelihood of a structure into
which it is easily incorporated will precipitate correspondingly increases. It is in
this manner “an increase in the alkali content in the liquid phase causes a decrease
in the connectivity of the silica framework in the resultant crystalline phase.”
The anionic species present in the solvent plays a decisive role in determining
the solution molarity required for the synthesis of the various compounds. Much
lower concentrations are required to obtain K
8
Nd
3
Si
12
O
32
OH from K
2
B
4
O
7
. For
example, concentration less than 1 M K
2
B
4
O
7
provides twice as many potassium
ions as 1 M KOH. The absence of KNd
9
(SiO
4
)
6
O
2
from K
2
CO
3
experiments, even
at very high concentrations, suggests that, in accordance with reactions shown
above, pH is also an important factor. The higher pH values cause not only greater
SiO
2
dissolution but also the breakup of dissolved silica complexes into smaller
species. The reasons that the fluorine compounds gave rise to entirely different
phases remain unclear. The low concentrations at which these fairly pH neutral
compounds caused the formation of K
3
NdSi
7
O
17
rather than K
3
NdSi
6
O
15
(derived
when pure water served as the solvent) indicates that some variable other than pH,
possibly the high reactivity of F
2
with Si
4
1
, is playing a significant role.
Higher temperatures slightly favored the formation of low Si:O phases. This was
true in spite of the fact that at high temperatures (at constant molarity), the % fill
of liquid was lowered to compensate for the decrease in the density of water, lead-
ing to a systematic reduction in the total alkali content. If such a change in experi-
mental conditions was to occur alone, that should result in the synthesis of the high
Si:O phases. If we accept that changes in the structure of silica in solution are
responsible for changes in the structure of the precipitate, the result implies that
high temperatures are particularly effective in shifting the above two reactions to
the right, thereby causing dissolved silica complexes to break apart into smaller
species.
Higher pressures also favor the low Si:O phases. In this case, however, experi-
ments performed at 1400 bar pressure were given higher percent than those per-
formed at 825 bar (constant molarity), to reflect the increase in solvent density at
the higher pressure. It is, therefore, not clear whether the change in crystallized
phase resulted from an increase in the pressure or increase in the total alkali
content.
Lastly, although only a limited number of experiments have been performed
with variation in the crystallization time, a tendency toward the formation of the
low Si:O ratio apatite-type phase with increasing time has been observed in the
5 M KOH experiments. The reason may be that the glass, with a relatively high sol-
ubility, quickly supersaturates in the solution with respect to the crystalline phases.
As the initial silica content in the solution is high, dissolved species are large,
and the higher silica content phase—K
8
Nd
3
Si
12
O
32
OH—precipitates preferentially
a
O
a
