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mol % H 2 O
0
10
20
30
40
50
60
70
80
90
100
13
albite
leucite
pectolite
11
9
7
5
3
1
1
Fig. 1.5 Viscosity in the
albite-H 2 Osystemat
800 C and 1-2 GPa.
Modified after Audetat and
Keppler (2004). Reprinted
with permission from
AAAS.
3
5
0
10
20
30
40
50
60
70
80
90
100
wt % H 2 O
that water dissolves in silicate melts primarily as
OH groups (e.g. Burnham, 1975), according to the
reaction:
Water speciation in silicate melts may therefore
be described by the following equilibrium (as
Equation (1.4):
O melt
H 2 O gas
2OH melt ,
O melt
H 2 O melt
2OH melt ,
+
=
(1.4)
+
=
where ''O'' is some oxygen atom in the anhydrous
melt. Since even very small concentrations of
water reduce the viscosity of silicate melts by
orders of magnitude (Figure 1.5), a commonly
held view is that water ''depolymerizes'' silicate
melts by reacting with bridging oxygen atoms that
connect two neighboring SiO 4 or AlO 4 tetrahedra,
schematically:
where O melt is some bridging oxygen atom in the
silicate melt. The equilibrium constant K wm of
this reaction
a 2 OH
a O a H 2 0 ,
K wm =
(1.6)
where a are activities, implies that the concen-
tration of molecular water should increase with
the square of the OH concentration. This means
that molecular H 2 O becomes abundant in the
glass only at high water concentrations, while
at low bulk water content, water is essentially
dissolved only as OH groups. This is entirely
consistent with the observed square root de-
pendence of water solubility on water fugacity
for low water contents. However, water specia-
tion in glasses is only ''frozen in'' at the glass
H 2 O
+
Si-O-Si
=
Si-OH
+
HO-Si.
(1.5)
The simple model of water dissolution out-
lined above was modified when near infrared
spectroscopy showed that quenched hydrous
silicate glasses - and by inference, also the
corresponding silicate melts - contain physically
dissolved H 2 O molecules in addition to OH
groups (Bartholomew et al ., 1980; Stolper, 1982).
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