Geoscience Reference
In-Depth Information
way nearer to the fabrication of devices. The most significant problem for the
growth of calcite crystals is that the growth techniques are restricted due to the
dissociation of CaCO
3
at high temperatures. Several techniques have been
employed to grow calcite single crystals: like low-temperature solutions
[166,167]
, gels
[168,169]
, fluxes or eutectics
[170,171]
, top seeded solution
growth
[172]
, traveling solvent zone melting
[173]
, HHP
[174
176]
, and hydro-
thermal
[162,163,177
182]
. Calcite crystals grown by most of these methods
encounter several problems including the presence of large thermal stress, CO
2
dissociation, and alkaline carbonate flux inclusions which cause serious defects
and contamination by metal ions. On the other hand, the hydrothermal method is
one of the most promising techniques for growing unstable crystals such as a car-
bonate compound, with the advantage of suppressing the dissociation of CO
2
and
yielding high-quality crystals in a homogeneous ambient at relatively low tem-
peratures
[183]
. Also, this technique is closer to the conditions in which calcite
grows in nature.
Many researchers have tried to grow calcite single crystals at relatively low tem-
peratures since calcite crystals dissociate to form CaO and CO
2
above 900
C under
atmospheric pressure. Varieties of solvents have been used for hydrothermal
growth of calcite, but none of them have succeeded in growing large single crys-
tals. The solubility of calcite is very interesting. It has been studied in water under
various CO
2
pressures
[184,185]
. The CO
2
pressure would be concerned with the
dissolution and precipitation of calcite in nature. Thus, hydrothermal growth under
variable CO
2
pressures is also very interesting from the point of view of geochem-
istry. The solubility is positive for increasing CO
2
pressure and negative for
increasing temperature. This behavior of calcite has posed a real problem in the
search for a suitable solvent to obtain higher/optimum growth rates. Despite this, a
large number of reports concerning calcite growth, hydrothermal reaction of calcite
in chloride, and other chloride solutions have appeared. Because natural calcite
crystals are formed in both chloride and carbonate hydrothermal solutions, these
chloride solutions are almost analogous to natural carbonate thermal springs.
Ikornikova
[178]
has done an extensive work on the aspects of solubility, designing
an apparatus to grow calcite crystals with changing CO
2
concentration as the pres-
sure is reduced at constant temperature, crystal growth kinetics, and mechanism.
For hydrothermal crystal growth of calcite, several inorganic solvents like NaCl,
LiCl, CaCl
2
, NaNO
3
, Ca(NO
3
)
2
,NH
4
NO
3
,K
2
CO
3
, and carbonic acid have been
employed as mineralizers. The growth conditions vary from 150
C and 15 MPa, to
600
C and 200 MPa. No one has used organic salt solutions to grow calcite crystals
under hydrothermal conditions. Yamasaki and coworkers
[182]
have tested the sol-
ubility of calcite in ammonium acetate solution. However, there is no unanimity
with respect to solubility owing to the change in the sign of the TC of CO
2
solubil-
ity in H
2
O, as shown in the isobaric diagram (
Figure 5.46
). The diagram clearly
shows two regions: I and II, lying on the two sides of the 200 bar isobar, which is
close to the isobar of the critical pressure of water. The pH of the system falls with
increasing P
CO
2
under standard conditions. This is connected to the fact that, with
increasing CO
2
solubility, the concentration of both the hydrocarbonate and the
