Geoscience Reference
In-Depth Information
5.7.2 HHP of Calcite
The HHP is being applied in recent years for the solidification of CaCO
3
. The HHP
technique is a method by which hard solid bodies of powders can be produced in a
short time and at a relatively low temperature under saturated vapor pressure. This
process is very similar to diagenesis
[190]
in geology; therefore, a diagenesis pro-
cess, such as for limestone, may be resurrected by using this technique. The results,
using the HHP technique, may bring in some new information to the sedimentary
process and a new technology of forming fabrication processes. Yamasaki et al.
(1993)
[175]
have obtained CaCO
3
by HHP with seawater, and they have designed
a “quasi-diagenesis” experiment. Initially, the CaCO
3
powder was synthesized in
seawater, and this powder was then solidified by the HHP technique. Various ions
of seawater, Cl
2
,Na
1
,K
1
, and SO
2
4
;
were absorbed on the surface of the synthe-
sized CaCO
3
sample powder during the precipitation process, and these ions
formed soluble salts such as NaCl and CaSO
4
under HHP conditions, i.e., the solu-
bility of the synthetic CaCO
3
powder surface may have increased rapidly in the
presence of CaSO
4
and NaCl solution under hot pressing conditions. Consequently,
it was revealed that the small grains were dissolved, and then recrystallization
occurred, or the grains adhered to the surface of larger grains and formed large
grain growth during HHP. The solidification process was carried out by a dissolu-
tion
recrystallization mechanism during the hot pressing process.
Yamasaki et al. (1993)
[175]
have solidified the powders of calcium carbonate
with sodium carbonate by mechanical compression under hydrothermal conditions.
The effects of hydrothermal conditions such as temperature, pressure, and ratio of
Ca/Sr on the reaction process and microstructure of the solidified bodies have been
studied.
The growth of calcite single crystals is becoming very popular in recent years
with the fast decrease in the availability of natural calcite. Although the growth of
calcite has been better understood in the last 5
6 years, still there is no unanimity
with reference to the solvent and experimental conditions. There is no doubt that
calcite crystal growth is one of the flourishing and challenging topics.
5.7.3 Growth of Related Carbonates
There are several works in the literature dealing with the growth of other carbo-
nates like MnCO
3
(rhodochrosite), FeCO
3
(siderite), CaCO
3
(spherocobaltite),
CdCO
3
(otavite), and NiCO
3
. Most of these are the important carbonate minerals,
and their growth carries a great significance to the geological literature.
MnCO
3
(rhodochrosite). The crystallization of rhodochrosite crystals takes place
in LiCl solutions (1
450
C, pressure 680
25 wt%) at temperatures 400
2500 atm,
Δ
50
C
[191]
.
and
T
5
10
In the experiments with 20
25% LiCl solutions
Δ
10
C, T
430
C, and P
at
960 atm, Ikornikova has obtained transparent
crystals of rhodochrosite with a size of 5
T
5
5
5
7 mm.
FeCO
3
(siderite). The synthesis of siderite is slightly more difficult than MnCO
3
having to do with the stability during the variation in the oxygen potential.
