Geoscience Reference
In-Depth Information
4.0
100 MPa
100 MPa
1.0
(a)
3.0
(a)
5.0
2.0
0
(b)
(b)
1.0
-1.0
0
0
350 400
Temperature (
°
C)
450
500
140
160
180
200
1.2
1.3
1.4
1.5
Temperature (
°
C)
1/
T
(10
-3
K
-1
)
Figure 5.48 Solubility curves for calcite in different solvents: I. a) in 1.5 M Ca(NO3)2
solution; b) in 3M NaCl solution; II. in 3M NH4NO3 solution; III. Log S (solubility) as a
function of 1/T: a) in 1.5M Ca(NO3)2; b) 3M NaCl
[180]
.
This work strongly confirms that a small amount of additives, such as phos-
phate, sulfate, borate, and ethylene glycol, effectively arrested the growth of cal-
cite. Similarly, the use of some organic additives like polypeptides is becoming
very popular. Nefyodova et al. (2003)
[188]
have studied the optical properties of
hydrothermal calcite crystals with respect to the dopant metals and dislocation
density.
5.7.1 Crystal Growth
Solubility of calcite can be both positive and negative for increasing CO
2
pressure
and increasing temperatures, respectively. Therefore, the growth of calcite crystals
insists on a special experimental setup.
Figure 5.49
shows the arrangement of the
apparatus designed by Ikornikova for growing calcite crystals in aqueous solutions
of CO
2
with a negative TC of solubility, together with curves illustrating the height
distribution of temperature and CO
2
concentration
[178]
. The crystals obtained by
this means, with seed crystals at the bottom and nutrient at the top followed by a
temperature gradient between the two zones separated through a baffle, have rhom-
bohedral faces covered with stepped pyramids. Similarly, Ikornikova has designed
another apparatus for synthesizing calcite crystals in aqueous solutions of carbonic
acid under isothermal conditions. The autoclave used is a rocking autoclave.
Figure 5.50
shows the schematic diagram of the apparatus used in the synthesis of
calcite. The autoclaves in both the cases are either glass or Teflon lined
[178,181]
.
In this case, usually at pressure under 150 kg/cm
2
, rhombohedral calcite crystals
without any pinacoid are formed. The pinacoid appears on crystals formed at pres-
sures greater than 200 kg/cm
2
.
