Geoscience Reference
In-Depth Information
4.2 Basic Principles of Phase Formation Under
Hydrothermal Conditions
A phase is defined as a part of the system that is homogeneous throughout and is
physically separable from other phases by distinct boundaries. The number of com-
ponents in a system is the minimum number of chemical constituents that must be
specified to describe the composition of all phases present. Phase relationships are
conveniently represented by means of phase diagrams. Phase diagrams and the
phase rules apply only to systems at equilibrium. These data are extremely impor-
tant to any crystal grower for successful growth of crystals. Gibbs deducted a quan-
titative relationship for phase rule:
F
5
C
2
P
1
2
where
F
5
degrees of freedom,
C
5
number of components,
P
5
number of phases.
The above relation holds true if pressure, temperature, and composition are the
only variables. If other variables such as magnetic, electric, or gravitational fields
are considered, the numerical value 2 on the right-hand side of the equation will
increase by the number of new variables considered.
The hydrothermal method is considered as one of the best methods to study
phase relationships in numerous systems, and the PTX diagrams thus obtained are
published from time to time. Phase equilibria have been described in detail by sev-
eral workers
[6,7]
. Hence, we restrict our discussion to some basic systems and
nonconventional phase diagrams.
Among the binary systems, the H
2
O
CO
2
system is important with regard to
the role of H
2
O and CO
2
vapor pressures in native mineral formation and in the
hydrothermal synthesis of inorganic compounds.
Figure 4.1
shows the phase dia-
gram for the H
2
O
a
CO
2
system
[6]
. It is found that, below the critical temperature
of water, the dependence V versus P has a gap at the pressure corresponding to the
pressure of water at its critical density. The isochores of real mixtures are linear up
to the immiscibility dome that gives a possibility to experimental data in the calcu-
lation of pressures of mineral formation from the gas
a
liquid inclusions.
The nonconventional phase diagrams plotted for equilibrium conditions are pop-
ular, especially among Russian workers. These diagrams are popularly known as
composition diagrams or NC diagrams, where N is the nutrient composition and C
is the concentration of the solvent mineralizer. Hundreds of phase diagrams of
compositions are available, especially in the Russian literature. An example of such
a diagram is given in
Figure 4.2
corresponding to the system Na
2
O
a
ZrO
2
a
SiO
2
[9]
. Similarly, the distribution of the silicon
oxygen radical in the rare earth sili-
cates is shown in the NC diagram in
Figure 4.3
corresponding to the system
Na
2
O
a
RE
2
O
3
a
SiO
2
a
H
2
O under hydrothermal conditions
[10]
. The solid lines
