Geoscience Reference
In-Depth Information
1.6.2 Fluorine and Chlorine
Fluorine, as H
2
O lowers solidus and liquidus temperatures, enhances cation diffusiv-
ities and decreases melt viscosities
[53
55]
. One important difference between F and
H
2
O is that the DCF (Determination of Moisture Content) vapor/melt is L1 in metalu-
minous and peraluminous granitic bulk compositions
[56,57]
. Fluorine decreases the
solubility of the melt
[58,59]
so that, at equal H
2
O content of volatile-undersaturated
magma at the same pressure and temperature, H
2
O is higher in F-bearing melts than
in F-absent melt. Metal
fluoride complexing in aqueous vapor may be important in
rare-metal transport and the formation of hydrothermal ores
[60,61]
.
1.6.3 Boron
The pronounced effects of boron in hydrous silicate melt are well known. Boron lowers
the solidus and it increases the solubility of H
2
O in silicate melts. Lewis acid
base
properties suggest that the solubility of H
2
O/mole B
2
O
3
in melts should increase with
increasing boron content because of changes in B
O coordination. Boron also
decreases the viscosity of silicate melts, presumably through melt depolymerization
caused by the synergistic network-modifying effects of boron and water
[62]
.
In silicate melts, boron forms two common oxyanions: trigonal planar BO
3
3
a
and
tetrahedral BO
5
4
BO
5
4
BO
3
3
ratio increases with melt alkalinity
[63,64]
and boron content
[65]
. Although boron exhibits strong interaction with silicate
melt, nonideal mixing between borate and silicate melt components is reflected by
stable liquid
:
The
½
=
glass immiscibility over a wide range of
bulk compositions in a hydrous silicate system
[66]
. The miscibility gap between
metal-rich borate and essentially pure silica liquids increases with the increasing
field strength of added metal cations, with a consequent rise in the consolute tem-
perature. Limited experimental evidence also indicates that the solubilities of other
high charge-density cations (e.g., Group IV and Group V elements) are signifi-
cantly higher in borosilicate melts than in simple aluminosilicate melts
[67]
.
As in fluorine-bearing systems, the addition of boron to silicate melts leads to
an expansion of the liquidus field of quartz
[68]
. This behavior may reflect the
removal of cations from coordination with SiO
4
4
of the melt framework, resulting
in higher SiO
2
through increased polymerization of SiO
4
4
tetrahedra
[69]
. Among
Group I cations, boron has a tendency to depress the activities of the higher field-
strength ions. The increased solubility of H
2
O in borosilicate melts corresponds to
a lower H
2
O that may stem from direct hydrolysis of borate oxyanions.
liquid or metastable glass
1.6.4 Phosphorus
Phosphorus exhibits limited solubility in silicate melt
[70
73]
, hence the addition
of phosphorus promotes phosphate-silicate liquid immiscibility. The phase equi-
libria data in the systems, SiO
2
a
P
2
O
5
,P
2
O
5
a
M
x
O
y
, and P
2
O
5
a
M
x
O
y
a
SiO
2
, show
that phosphorus has an affinity for H, and it increases the solubility of H
2
O in sili-
cate melts, possibly by P
Q
O
H
2
O
!
(HO)
a
P
a
(OH)
[74]
. Although experimental
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