Geoscience Reference
In-Depth Information
Much of the recent works on the growth of calcite single crystals are being
carried out under moderate to lower temperatures (200
150
C) using several new
solvents like ammonium nitrate and ammonium acetate. The results are far better
than crystals obtained under higher temperatures because of the reduced thermal
strain. Recent workers use Teflon liners. A considerable amount of progress has
been achieved in the single crystal growth of calcite with reference to the solubil-
ity, growth rate, reaction kinetics, seed orientation, morphology, surface morphol-
ogy, optical and laser quality, and so on.
Figure 5.51
shows the schematic diagram
of the autoclave for the hydrothermal crystal growth of calcite commonly used in
recent years. It is well confirmed that the ammonium nitrate and acetate solvents
are more effective than chloride solvents from the viewpoint of calcite crystal
growth. The calcite crystals dissolve in ammonium nitrate or acetate solutions,
accompanied by the evolution of ammonia. This reaction increases the dissolution
of calcite in the hydrothermal solution which gives an advantage of lowering the
pressure, temperature conditions of the growth, and in turn facilitates the use of
Teflon liners. Great improvements are achieved by using a baffle plate and band
heater to control the temperature gradient and the transport behavior. Higher tem-
perature gradients give higher growth rates, especially at higher growth tempera-
tures. Usually the growth rate varies from 25 to 213
μ
m/day. The rhombohedral
Figure 5.50 Schematic diagram of the apparatus used in the synthesis of calcite
[178]
.
