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(1965). Melting temperatures of sanidine (1,115
°
C), diopside (1,295
°
C) and neph-
eline (1,370
C) at 0.1 GPa in the presence of excess water were estimated from
Hamilton andMacKenzie (1965, Luth andBoettcher (1986) and Boettcher andWyllie
(1969), respectively. Comparison of the two-phase diagrams studied at 1 bar under dry
condition (Platt and Edgar 1971; Fig.
9.1
) and 0.1 GPa (Fig.
9.2
) further shows
considerable shrinkage in the leucite and olivine
°
fields at 0.1 GPa [P(H
2
O) = P
(Total)]. The temperature of appearance of the four-phase point at 865
°
C is also
considerably less than that established by Platt and Edgar (1,120 + 5
C) at 1 atm. It
was observed that although in this system the subsolidus assemblage under 0.1 GPa at
830
°
°
C comprises Di
ss
+Ne
ss
+Lc
ss
+ San
ss
+ L + V at 815 + 5
°
C leucite is absent
because of
its
reactions with liquid and the
final assemblage comprises
Di
ss
+Ne
ss
+ San
ss
+L+V.
9.2.2 Experimental Study of the System at 1 and 2 GPa
[P(H
2
O) = P(Total)]
It was noted by Sigh et al. (2000) that in the join diopside
-
nepheline at 1 GPa in the
presence of excess water, there is no primary phase
field of forsterite
ss
. The present
result indicates that at 1 and 2 GPa isobaric conditions, there is a systematic shift in
the positions of the Di
ss
-
C, the
four-phase points is located at Di
11
Ne
31
San
58
(Fig.
9.2
, Chattopadhyay et al. 1999).
At 1 GPa and 670
Ne
ss
-
San
ss
-
l four-phase point. At 0.1 GPa and 865
°
C, the assemblage Di
ss
+Ne
ss
+ San
ss
, co-exists with liquid in
the composition Di
4
San
65
Ne
31
(Fig.
9.3
). The four-phase point at 2 GPa and 640
°
C
is located at Di
3
San
73
Ne
24
(Fig.
9.4
). It may be observed that four-phase point
(Di
ss
+Ne
ss
+San
ss
+ L) shifts towards sanidine with increasing pressure.
It is interesting to observe that if comparison of Fig.
9.1
(Platt and Edgar 1971) is
made with respect to Fig.
9.2
the four-phase (Point B) under atmospheric pressure is
shifted more towards the sanidine-rich side (point H at 0.1 GPa in the presence of
excess water). The invariant points determined at 1 GPa (Point T, Fig.
9.3
) and 2 GPa
(Point P, Fig.
9.4
) are shifted even farther towards the sanidine-rich side and away
from the diopside end due to extensive solubility of nepheline and sanidine in the
alkali silicate aqueous
°
fluid.
Yoder (1958) studied the system nepheline
water, whereas Yoder and Upton
-
(1971) investigated the system diopside
sanidine. These studies also show rela-
tively high degree of dissolution of sanidine compared to nepheline with the
increase of pressure. Thus shift in the composition of the four-phase points
(Di
ss
+Ne
ss
+ San
ss
+ L) towards sanidine (Figs.
9.2
,
9.3
and
9.4
) in the join
diopside
-
sanidine with the increase of pressure is expected. The liquid
composition of the four-phase points in case of 1 and 2 GPa [P(H
2
O) = P(Total)]
isobaric diagram should correspond to an extremely pyroxene-poor phonolite.
-
nepheline
-
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