Geoscience Reference
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Fig. 1.6
Complete miscibility in the albite-H
2
O system, as seen in an externally heated diamond anvil cell at 1.45
GPa and 763-766
◦
C. The optical contrast between a droplet of hydrous albite melt and the surrounding fluid
disappears as the compositions of the coexisting phases approach each other. After Shen & Keppler (1997).
Reproduced with permission of Nature.
transformation temperature, which may be very
low for these water-rich systems. To obtain equi-
librium constants for reaction (1.4), direct infrared
spectroscopic measurements by water speciation
at magmatic temperatures are necessary. Such
measurements were first carried out by Nowak
and Behrens (1995) and by Shen and Keppler
(1995). They show that equilibrium (1.4) shifts
to the right side with increasing temperature, so
that at typical magmatic temperatures of 1000
◦
C
or higher, OH groups dominate in the melts even
if they contain several wt % of water. At pressures
approaching 100 GPa, hydrogen becomes bonded
to several oxygen atoms and forms extended
structures in hydrous silicate melts (Mookherjee
et al
., 2008).
The commonly held view that water depoly-
merizes silicate melts and glasses was challenged
by Kohn
et al
. (1989) on the basis of NMR
studies of hydrous albite glasses that failed to
detect clear signs of depolymerization. The work
by Kohn
et al
. has fostered intense research in
the structural role of water in silicate melts
and glasses. However, several more recent stud-
ies (e.g. K ummerlen
et al
., 1992; Malfait & Xue,
2010), often using more sophisticated NMR meth-
ods, appear to fully confirm the traditional view
that water depolymerizes the structure of sili-
cate melts and glasses by forming Si-OH and
Al-OH groups, as indicated by Equation (1.5). In
aluminosilicate glasses, however, the effects of
depolymerization are not easily detected due to
complications arising from Al/Si disorder in the
glass and melt structure.
Water speciation in silicate melts is essential
for understanding the strong effect of water on the
physical properties of silicate melts; water does
not only reduce viscosities by many orders of mag-
nitude (e.g. Hess & Dingwell, 1996; Audetat &
Keppler, 2005), it also greatly enhances electrical
conductivity (e.g. Gaillard, 2004; Ni
et al
., 2011)
and diffusivity (e.g. Nowak & Behrens, 1997).
While the effect on viscosity is likely due to de-
polymerization, i.e. due to the formation of OH
groups, molecular water appears to be the main
diffusing hydrous species. Water also reduces the
density of silicate melts, but this effect appears
to be rather insensitive to speciation and can be
described by one nearly constant partial molar
volume of water (Richet & Polian, 1998; Bouhifd
et al
., 2001).
Since water solubility in silicate melts in-
creases with pressure and since at the same time,
the solubility of silicates in aqueous fluids also
increases, the miscibility gap between water and
silicate melts may ultimately disappear. This
effect was already considered by Niggli (1920).
Kennedy
et al
. (1962) showed that in the system
SiO
2
-H
2
O the compositions of water-saturated
melt and coexisting fluid approach each other
close to 1 GPa. The first direct observation of
complete miscibility in a silicate-H
2
Osystem
was reported by Shen and Keppler (1997), see
Figure 1.6.
If complete miscibility between melt and fluid
occurs, a water-saturated solidus cannot be de-
fined any more. At low pressures, melting in
a binary silicate-H
2
O system in the presence
