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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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