Geoscience Reference
In-Depth Information
4.5 Kinetics of Crystallization Under Hydrothermal
Conditions
Hydrothermal crystallization is only one of the areas where our fundamental
understanding of hydrothermal kinetics was totally lacking until recently.
Because, in majority of the cases, we have little knowledge about the intermedi-
ate phases forming in solution, our fundamental understanding of hydrothermal
crystallization kinetics is at an early stage. In the absence of predictive models,
we must empirically define the fundamental role of temperature, pressure, precur-
sor, and time on crystallization kinetics of various compounds. Insight into this
would enable us to understand how to control the formation of solution species,
solid phases, and the rate of their formation. In this respect, the recent progress
in the area of time-resolved hydrothermal synthesis for TEDDI using synchrotron
beam and neutron beam has contributed significantly towards the understanding
of the intermediate phases and in turn the hydrothermal mechanism of formation
of various inorganic phases and their kinetics. There are several groups world-
wide working on such studies covering the hydrothermal synthesis of zeolites,
clays, and so on. In combination with the thermodynamics of the reaction system,
one can precisely understand various parameters related to the hydrothermal syn-
thesis of a desired phase. There are several groups led by professors like A.
Clearfied (USA), T. Fehr
(Germany), and G. Sankar and P. Barnes (UK)
[61
65]
.
The importance of kinetics of crystallization studies was realized with the com-
mercialization of the synthesis of zeolites during 1950s and 1960s. Following this,
a large number of publications appeared from the Soviet laboratories on kinetics
studies related to the growth of single crystals under hydrothermal conditions.
During this period, the anisotropy of the rate of growth of the faces of the crystals
during growth on a seed acquires particular importance, since it makes it possible
to orient the seed crystal platelets in such a way that the growth surface coincides
with the rapidly growing face
[66
70]
. At the same time, Laudise and coworkers
have studied the crystallization kinetics of quartz and zincite
[66
73]
.
These studies were based on the experience in the growth of crystals in aqueous
solutions which shows clearly the relationship between the anisotropy of the rates
of process: Temperature, supersaturation, and the presence of “foreign” compo-
nents, including the solvent under hydrothermal conditions. These parameters can
be varied over very wide limits.
The influence of the solvent is specific for each crystal. In the case of ZnO, for
example, the presence of NH
4
ions in the solution produces a marked increase in
the rate of growth of the prismatic faces, which under ordinary conditions does not
exceed hundredths of millimeters per day
[73]
. For calcite un
de
r condition
s
of
increased oxygen activity,
ð
Þ
ð
Þ;
the faces of the scalenohedra,
2131
and
1010
become predominant
[74]
.
Various general laws characterizing the relationship between the anisotropy of
the rates of growth of the faces and the chemical nature of the solvent have been
