Geoscience Reference
In-Depth Information
Figure 1.7 (Continued)
have been described by many workers and they are discussed in Chapters 5
10 in
great detail. The hydrothermal technique, in contrast to other conventional techni-
ques, offers several advantages:
i. Compounds with elements in oxidation states that are difficult to obtain, especially
important for transitional metal compounds, can be obtained in a closed system by the
hydrothermal method (e.g., ferromagnetic chromium (IV) oxide).
ii. The hydrothermal method is also useful for the so-called low temperature phases, e.g.,
α
-quartz,
α
-berlinite, and others.
iii. For the synthesis of metastable compounds, such as subiodides of tellurium, Te
2
I, the
hydrothermal method is unique.
Commercial production of quartz and zeolites began during 1940s, but it was
during 1940s and 1950s that the study of hydrothermal phase equilibria became
quite popular owing to the appearance of new designs in autoclaves, particularly
the Tuttle cold-cone sealed autoclaves which became popular as test tube
type
autoclaves
[110]
. The main advantage of the phase equilibria studies under hydro-
thermal conditions is that it is a closed system; one can study the influence of
temperature, pressure, and compositional variations individually. These studies
helped to understand the phase formations in many inorganic systems, which
had direct bearings on the natural systems as well. They also helped in the crystalli-
zation of several new phases which do not have natural analogues. Moreover, these
