Geoscience Reference
In-Depth Information
over KNd
9
(SiO
4
)
6
O
2
in the solution. If more time is given, K
8
Nd
3
Si
12
O
32
OH
slowly redissolves, but the total silica content in the solution remains low.
Therefore, dissolved species are reduced in size, and the apatite-type phase eventu-
ally precipitates.
The conditions (average temperature, estimated pressure, solvent type and
molarity, solid-to-liquid ratio, total alkali content, and time) that gave rise to
K
3
NdSi
3
O
8
(OH)
2
growth using the temperature gradient technique broadly corre-
spond to those that caused K
8
Nd
3
Si
12
O
32
OH and KNd
9
(SiO
4
)
6
O
2
co-crystallization
isothermally. As K
3
NdSi
3
O
8
(OH)
2
has a silicon to (bonding) oxygen ratio of 0.3, a
structure based on trisilicate chains
[50]
and a composition that closely resembles
the combined stoichiometry of the nutrient and the mineralizer is formed.
The reasons for the appearance of the different phases under different conditions
have been explained in terms of their differing structures, rather than differing stoi-
chiometries, and in terms of silicate solution chemistry. It is generally accepted
that during hydrothermal growth, the nature of the dissolved species determines the
nature of the crystalline product. We have observed a decrease in the connectivity
of the silicate framework in the product phase, as measured by the Si:O ratio (or,
alternatively, SiO
2
mol%), with increasing solution molarity (alkali content), tem-
perature and, to some extent, pressure. Therefore, it is concluded that higher solubi-
lities under these conditions coincide with a decrease in the average size of the
dissolved complexes and is responsible for the change in the structural nature of
the crystalline product. Longer times also appear to favor the crystallization of low
Si:O ratio phases. These observations provide guidelines by which new phases with
some desired structural feature might be synthesized. Stoichiometric parameter
other than the Si:O ratio and the SiO
2
mol% are not easily correlated to the experi-
mental variables. The mole percent alkali in the product phase, for example, does
not monotonically increase as the molarity of the solution is increased. Lastly, it is
observed that when phases of low silica content form, overall mass balance is
maintained via the presence of a high silica content gel. It is most probable that
this gel precipitates out of solution as the autoclave is quenched.
Such studies greatly help in understanding the hydrothermal chemistry of sili-
cates and their crystallization mechanism. Hitherto, only zeolites have such a vast
amount of data on their hydrothermal chemistry. Another approach to the hydro-
thermal chemistry of silicates has been treated in the forthcoming sections under
degree of silification.
7.5 Crystal Chemical Significance of Phase Formation
The study of the crystallization of various rare earth silicates and their correlation
with the crystal chemistry has not been carried out effectively. In laboratory condi-
tions, it is interesting to understand the conditions of synthesis and the structure
with the change in the ionic radii of rare earths. Such studies have revealed the
mechanism of the formation of some of the fine crystal structures.
