Geoscience Reference
In-Depth Information
The first reaction is characteristic of alkali solutions, and the second reaction is
characteristic of acids. In both the cases, hydrogen helps in establishing equilib-
rium. As a result of this, the desired metal crystallizes. If the temperature gradient
is maintained in the autoclave, in one case, the crystallization of native metal
crystals takes place in the hotter zone, and in the other case, in the cooler zone
depending upon the hydrogen concentration (Exception handling (Eh)), of the sys-
tem. Thus, the hydrothermal crystallization of metals is possible in both acid and
alkali solutions. It is possible to obtain the entire series of metals as single crystals
when an oxidizing agent like halogenide is used
[3]
. The crystallization takes place
in quartz ampoules with external control of
the CO
2
gas at
temperatures
120
C in aqueous solutions of HCl, HI,
HBr and the crystals obtained are Au, Ag, Pt, Co, Ni, Te, and As. In this regard, it
is appropriate to emphasize the work of Rabenau and Rau
[4]
on the hydrothermal
synthesis of gold crystals. In aqueous solutions of HI at 500
C, the well-soluble
[AuI
4
]
2
complexes are formed, their stability depends upon the temperature, and it
falls with destabilization of [AuI
4
] complexes and the formation of gold crystals:
600
C and temperature gradients 30
440
AuI
4
2
1
Au
1
4HI
!½
2H
2
m
ð
9
:
3
Þ
The first attempt of gold crystallization from aqueous solutions was demon-
strated by Bujor
[5]
under hydrothermal conditions at 390
C and 300 atm and cubic
crystals of gold were obtained.
Honma et al.
[6]
have studied, in detail, the dissolution and precipitation of gold
and silver at 150
C in various strong acid solutions and aqueous NaHS over periods
of 60
120 days. The alloy Au
50
Ag
50
was used as the starting material. For the first
time, millimeter-sized free-growing enhedral crystals of pure gold were obtained
from 6 N HCl solution after 120 days, and no precipitation of either gold or silver
was obtained from the sulfuric acid and nitric acid solutions. Small crystals of both
electrum (Au
71
Ag
29
) and argentite were precipitated onto the starting alloy sample
after 120 days in 5.2 M aqueous NaHS
[6]
.
Gold solubility has been studied extensively by several authors in HCl aque-
ous solution and others
[7
14]
have done excel-
lent work on the complexing and solubility of gold and other noble metals under
hydrothermal conditions, especially in the aqueous sulfide solutions at tempera-
tures between 150
C and 500
C and pressure of 500
12]
. Seward and coworkers
[12
1500 bar over a wide range
of pH. They have derived stability constants for
the complexes AuHS and
Þ
2
:
Au
It can be used to quantitatively model the transport of gold in natural
hydrothermal fluids to understand the mechanisms and conditions of gold deposi-
tion. It has been shown that, in sulfide solutions at near neutral pH, the complex
Au
ð
HS
Þ
2
is the dominant gold species. The solubility of gold reaches a maxi-
mum where pH
ð
HS
pKofH
2
S. However, with increasing temperatures, the pK
1
of
H
2
S shifts toward more alkaline values
[15]
and, therefore, the maximum solubil-
ity due to this species also shifts to more alkaline pH. In acidic pH high-
temperature solutions, the neutral species AuHS
, therefore, will be the dominant
gold transporting species and, although the gold solubility may be 1
5
2 orders of
