Geoscience Reference
In-Depth Information
Figure 8.29 Spahalerite crystals grown under
hydrothermal conditions
[98]
.
the solvent composition, and pH. The experiments are carried out in autoclaves
provided with platinum linings in KOH solutions and the growth temperature varies
from 355
C to 400
C; however, the sphalerite crystals of lowest degree of perfec-
tion are grown under hydrothermal condition at high temperature (
400
C,
Figure 8.29
). Bryatov and Kuzmina
[104]
and Ikornikova
[105]
have studied the
hydrothermal growth of ZnS crystals in acidic chloride solutions. When 25 wt%
ammonium chloride aqueous solution was used in the experiments, intensive recrys-
tallization of sphalerite takes place at 400
C and
Δ
T
5
2
C. Mininzon et al.
[106]
have studied in detail the solubility of sphalerite in phosphoric acid solutions and
the morphology of the crystals. It is interesting to note that the solubility of sphaler-
ite is highest in phosphoric acid solutions.
Greigite, Fe
3
S
4
, has been synthesized under hydrothermal conditions using Na
2
S
solution
[107]
. Mohr's salt solution is mixed with Na
2
S solution at room tempera-
ture, to form black particles of iron sulfide precipitates, and upon heating together
with the mother solution yields greigite (Fe
3
S
4
, cubic). The optimum heating tem-
perature for preparing greigite depends on the pH of the solution; it is about 180
C
for a solution with pH 9 and lowers to 80
C with a decrease in pH. Above these
temperatures, greigite particles disappear by decomposition and alternatively, pyr-
rhotite (Fe
1
B
XS
1
hexagonal) or pyrite (FeS
2
, cubic) forms, depending on the pH of
the solution.
8.11.3 Hydrothermal Synthesis of Complex Sulfides
The complex sulfides include compounds containing multiple cations. The difficul-
ties in obtaining these complex sulfides are connected to the existence of a large
number of stable phases in these systems; hence, the earliest hydrothermal experi-
ments usually do not cover these complex sulfides. However, in the last few
decades, much work has been done on the synthesis of these natural analogs of
complex sulfides
[108]
. For obtaining these complex sulfides with 100% output,
synthesis through ion exchange reactions is the most useful technique. The more
complex the cationic composition, the more difficult it is to fix the stoichiometry
of the nutrient material. Under such circumstances, the trial-and-error experiments
are more useful.
