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et al., it is not a saturation surface projected from the apex of a tetrahedron as in the
H
2
O and CO
2
system, but the differences in the projection angles are not large,
allowing comparisons with CO
2
and H
2
O-bearing kalsilite
forsterite
quartz system
-
-
at 2.8 GPa (Gupta and Green 1988).
In the dry system, kalsilite
quartz at 2.8 GPa, MgO-rich liquid crys-
tallises forsterite and enstatite and the composition moves toward a forste-
rite + enstatite + sanidine + liquid peritectic, and then to a phlogopite +
sanidine + kalsilite + sanidine + liquid eutectic, as in the case of silica-undersat-
urated compositions (Fig.
12.12
). These two four-phase points occur in the silica
undersaturated portion of the forsterite-sanidine join. The crystallisation path in the
kalsilite
forsterite
-
-
quartz system at 2.8 GPa is similar to those in the same join at
3.0 GPa except for the presence of leucite at low pressures. Compositions de
forsterite
-
-
ned
by enstatite-sanidine-phlogopite crystallize through a peritectic, where quartz +
phlogopite + enstatite + liquid are at equilibrium at all pressures from 1 to 2.8 GPa.
Phase volume of phlogopite is enlarged greatly with increasing pressure and the
enstatite + phlogopite + forsterite + liquid peritectic lies at much more Mg-rich
compositions at pressures equivalent to that in the mantle.
Foley et al. observed that the phase diagram for 4 % F
2
O
−
1
at 2.8 GPa
(Fig.
12.14
) broadly resembles the water-saturated,
fluorine-free system of Gupta
and Green (1988) (Fig.
12.13
) in having a large primary phase volume for
phlogopite plus primary phase volumes for the same six minerals (enstatite, for-
sterite, phlogopite, kalsilite, sanidine and quartz). Foley et al. (1986, Fig.
12.15
)
compared the shift in the enstatite-forsterite phase boundary in presence of CH
4
,
CO
2
and F with respect
to the system forsterite
kalsilite
SiO
2
-
H
2
O system
-
-
investigated by Gupta and Green (1988). However the
fluorine-bearing system
differs in that the
fluorphlogopite has a much greater thermal stability (maximum
1,490
1,500
°
C) than hydroxyl-phlogopite (<1,200
°
C: Gupta and Green in 1988).
-
This may be attributed to the lack of K
fluorphlogopite. This
repulsion exists in hydroxy-phlogopite due to orientation of the O
H repulsion in
-
H bond directly
away from neighbouring octahedral cations and toward the interlayer potassium
cations (McCauley et al. 1973). The enstatite-phlogopite phase boundary is not a
peritectic reaction, despite its extension, apparently lying outside the join enstatite-
phlogopite. According to Foley et al. (1986), this is an artefact of the projection due
to phlogopite
ss
and liquid composition lying outside the plane of projection. As in
the H
2
O-saturated system, the intersection of forsterite + phlogopite phase boundary
with the extension of forsterite
-
phlogopite join forms a thermal maximum. Liquids
with compositions to the silica-rich side of this divide will fractionate either through
the enstatite + forsterite + phlogopite + liquid peritectic point or across the
phlogopite phase
-
field to either the phlogopite + sanidine + quartz + liquid or
phlogopite + sanidine + kalsilite + liquid eutectics. Compositions to the silica- poor
side of the phlogopite
forsterite join and its extension will fractionate through the
kalsilite + forsterite + phlogopite + liquid, peritectic point or across the phlogopite
phase
-
field toward the kalsilite + sanidine + phlogopite eutectic.
In the
fluorine-bearing system the primary phase
field of enstatite relative to
forsterite
is
enlarged compared
to the
volatile-free
join,
so that
the
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