Geoscience Reference
In-Depth Information
ambient temperatures. By analogy, one might therefore expect the formation of
such polynuclear ions or clusters in near-critical region and at supercritical tem-
peratures and pressures in aqueous electrolyte solutions. The molecular dynamics
simulations of ion pairing and cluster formation in a 1 M NaCl solution at 380
C
and near-critical pressures indicating the presence of simple monatomic ions and
ion pairs together with triple ions such as Na
2
Cl
1
and NaCl
2
as well as the Na
2
Cl
2
and more complicated polynuclear species. Combined molecular dynamics simula-
tions and EXAFS measurements on 1 M SrCl
2
solutions up to 300
C also indicate
the presence of Sr
2
1
,Cl
2
, SrCl
1
, SrCl
2
as well as cluster species such as Sr
2
Cl
2
2
and Sr
2
Cl
2
[52]
. Insight into ion pair and cluster formation is of importance to the
understanding of many phenomena operating in hydrothermal systems in the
earth's crust, including mineral equilibria, stable isotope fractionation, and element
transport.
Benning and Seward
[53]
have studied the stepwise metal complex formation
of the AuHS
0
Þ
2
complexes and their thermodynamic data in sulfide
aqueous solutions up to 400
C and 1500 bar. However, a major source of uncer-
tainty in the study of metal hydrosulfide complexes at high temperatures and high
pressures has been the lack of reliable data for K
1
, the first ionization constant for
H
2
S. There is a major lack of experimental data pertaining to metal complex equi-
libria in supercritical aqueous systems as well as in binary solvent systems such
as H
2
O
and Au
ð
HS
CO
2
. Walther and Schott
[54]
applied the dielectric-constant approach
to study the speciation and ion pairing at high temperature and high pressure.
These authors have proposed many theoretical and empirical relations to account
for various aspects of solubility behavior and speciation. The calculation of the
dielectric-constant dependence of ion pairing at high temperatures and high pres-
sures allows the prediction of speciation in complex fluids at high temperature
and high pressure for reduced activities of H
2
O. Using this approach, the behavior
of aqueous silica in complex solutions at high temperatures and high pressures has
been studied. Such data are of enormous importance to understanding the geo-
chemistry of element transport by hydrothermal fluids migrating in the earth's
crust
[43]
.
In the hydrothermal growth of crystals, the PVT diagram of water proposed
by Kennedy
[55]
is very important (Figure 2.6). Usually, in most routine hydro-
thermal experiments, the pressure prevailing under the working conditions is
determined by the degree of filling and the temperature. When concentrated solu-
tions are used, the critical temperature can be several hundred degrees above
that of pure water
[56]
. The critical temperatures are not known for the usually
complex solutions at hand; hence, one cannot distinguish between sub- and super-
critical systems for reactions below 800
C. Although the temperature in the
growth zone and the actual vapor pressure are not known to 100% accuracy, the
PVT diagram of Kennedy is used by most hydrothermal crystal growers routinely.
The PVT relations in AlPO
4
and SiO
2
systems have been reviewed
[57
a
59]
.
Also, the P
T behavior of the quartz
water system has been well discussed by
Brebrick
[60]
.
