Geoscience Reference
In-Depth Information
insignificant changing of redox potential in the solution can result in the “decompo-
sition” of HgS.
As noted above, the differences in the stability of CdS and ZnS, on the one
hand, and PbS and HgS, on the other hand, are very important for choosing the
growth conditions of these substances. It is evident that the stability of CdS and
ZnS in a wide range of Eh and pH magnitudes gives the possibility to use various
media for its crystallization. In the case of lead sulfide, the growth of crystals in
alkaline solutions is only possible in certain interval of Eh values. The narrow field
of stability of mercury sulfide makes great demands on growth conditions.
The growth of CdS crystals was realized in acid (H
3
PO
4
), chlorine (NH
4
Cl) and
alkaline (KOH, NaOH) solutions. In acid and chloride solutions (H
3
PO
4
5
10
30 wt
%, NH
4
Cl
25 wt%), very intensive transfer of CdS takes place at temperatures
above 400
C and spontaneous crystals of about 3
1
5
5 mm in size were grown
[100]
.In
alkaline solution, the intensive transfer of the sulfide was observed at higher tempera-
tures (480
500
C), but at lower temperatures partial hydrolysis takes place. At 500
C
and KOH, NaOH concentrations of 30
50 wt%, spontaneous CdS crystals reached
1.0 mm
2
in size.
The intensity of hydrolysis of the sulfides is distinctly inversely proportional to their
crystallization. Thus, in KOH solutions, crystallization of CdS takes place stably only
at 500
C, and hydrolysis is largely suppressed above 500
C. At 530
10
20
3
0.5
535
C, a most
intense transfer of the substance from the lower to the upper zone of the autoclave was
observed, and circular CdS crystals up to 8
10 mm long are readily formed.
In NaOH solutions, owing to the weaker hydrolysis of CdS and PbS, their crys-
tallization takes place more intensely at lower temperatures than in KOH solutions.
Transfer of considerable amounts of substance takes place, even at 400
450
C.
However, as in the case of KOH solutions, maximum transfer of the substance was
observed at 500
535
C, when sulfide hydrolysis in NaOH was practically absent.
The properties of crystals obtained in alkaline and acid solutions strongly differ.
The hydrothermal method is the only technique to obtain the large-size single-
crystal cinnabar, HgS. Toudic et al.
[103]
obtained large-size HgS crystals using
Na
2
S solutions at 265
C, considerably below the temperature of the
-HgS
transition (344
C). These authors have found that, at temperatures above 200
C,
cinnabar is unstable in solutions with the high pH values characteristic of Na
2
S,
hence, it is desirable to crystallize HgS from weak alkaline solutions with lower
pH values in which the cinnabar is more stable.
The experiments have been carried out in autoclaves provided with Teflon liners.
The pressure in these experiments is maintained at between 100 and 200 atm and tem-
perature between 230 and 265
C. The pH is usually maintained at 9
α!β
13,
40
C. The crystals are usually prismatic and often needle like. The growth
rate is nearly 0.1 mm/day in the pinnacoidal face and less than 0.02 mm/day on the
prism face. Cambi and Elli
[96]
have studied in detail the morphological aspects of
HgS with reference to the growth conditions.
The sphalerite ZnS crystals grown by different methods differ from one another
in concentration and, in this respect, the hydrothermal sphalerites are more perfect.
The growth of ZnS has been carried out with varying parameters, like temperature,
Δ
T
5
28
