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as forsterite and diopside remains relatively insoluble compared to leucite. The shift
in the composition of the
five-phase point towards the feldspathoid end compared to
the ma
c end members, has also been observed by Yoder and Upton (1971).
Another interesting feature of this study is the occurrence of phlogopite
ss
at 850
°
C
or below with reference to the study of Luth (1967) on the join Fo
Lc at 0.1 Gpa [P
(H
2
O) = P(Total)], the presence of phlogopite along with forsterite and leucite
agrees well with the present study.
On the basis of composition of the glasses in equilibrium with different phases,
the projected path of the liquid in this join is shown in Fig.
10.15
. The projected line
of descent (a bold line with solid squares) is slightly off the phase boundary.
Although a liquid and a vapour phase were found to coexist with crystalline phases
in the quenching run products, the vapour phase together with the liquid probably
existed in the form of a
-
fluid phase during run. Because of the appearance of
phlogopite and presence of vapour (or
fluid), the join may be treated as a part of a
quinary system, KAlSiO
4
-
CaO
MgO
SiO
2
-
H
2
O.
-
-
10.3.3 The Join Forsterite
Leucite Studied
under 2.3 GPa and Variable Temperatures
Diopside
-
-
Seventy-two experimental runs were made by Nag et al. (2007) to study this join
under a condition of [P(H
2
O) = P(Total)] at variable temperatures. The results are
summarised in Fig.
10.16
a,
Which shows that the join penetrates the phase volumes of forsterite
ss
, phlogo-
pite
ss
diopside
ss
, K-feldspar
ss
and kalsilite. There are three
five-phase points (1)
'
B
'
(Fo
9
Di
49
Lc
42
and 1,005
C), where forsterite
ss
diopside
ss
and phlogopite
ss
coexist with liquid and vapour, (2)
±
10
º
C), where phlog-
opite
ss
, diopside
ss
and K-feldspar
ss
are in equilibrium with liquid and vapour, and (3)
'
'
C
'
(Fo
4
Di
50
Lc
46
+ 990
±
10
°
C), where phlogopite
ss
, kalsilite
ss
and K-feldspar
ss
are in
equilibrium with liquid and vapour. The presence of K-feldspar
ss
and kalsilite are
related to breakdown of leucite under high P-T
D
'
(Fo
3
Di
21
Lc
76
, 775
±
5
°
T conditions (Fashauer et al. 1998;
reaction (
10.1
)). Formation of phlogopite
ss
in this join is related to the reaction
involving forsterite, leucite and vapour to produce phlogopite
ss
and K-feldspar
ss
(reaction (
10.2
)) (Yoder 1986).
In case of the Fo
-
Di
Lc join studied at one atmosphere, the piercing point
'
I
'
-
-
(Fo
ss
+Di
SS
+Lc
SS
+ Liq), occurs at Fo
2
Di
67
Lc
31
and 1,296
C (Gupta 1972)
(Fig.
10.2
). However, in their study conducted at 2.3 GPa [P(H
2
O) = P(Total)] by
Nag et al. 2007, Fig.
10.16
a, all the three
°
five-phase points occurring at
'
B
'
(1,005
°
'
'
°
'
'
°
C),
C
(990
C) and
D
(775
C) have liquidus temperatures signi
cantly lower than
'
'
that of piercing pint
(Fig.
10.2
), because the present investigation has been
conducted in the presence of excess water [P(H
2
O) = P(total)]. Compared to the
phase equilibrium study conducted at one atmospheric pressure (Fig.
10.2
) and
0.1 GPa (Fig.
10.15
), the study of the same join at 2.3 GPa shows significant
C
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