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as a reaction product between diopside and nepheline at low pressure in air and
0.1 GPa (Figs.
9.1
and
9.2
), but disappears below 985
±
5
°
C (point G in Fig.
9.2
).
Complete elimination of leucite
ss
occurs at 815
C, after the disappearance of
leucite and forsterite, the join behaves essentially as a ternary system. The present
experimental study of the join diopside
±
10
°
sanidine at 0.1 GPa (Fig.
9.2
)
suggest that a nepheline-bearing leucitite (point H) can be derived from a Na-rich
leucitite (curve M
nepheline
-
-
H), a nepheline-bearing italite (point E) or an olivine nephelinite
-
(point G).
Let us consider point
(Di
70
Ne
25
San
5
; Fig.
9.2
). As content of normative
pyroxene and nepheline is high, the bulk composition should correspond to mela-
nephelinite (ijolite or melteigite; also see Fig.
9.2
for mineralogical composition).
Mela-nephelinite is a melanocratic nephelinite with higher concentration of pyroxene
compared to nepheline and having dark colour. Reference to the diagram of Fig.
9.2
suggests that from such a liquid (point X), diopside
ss
should precipitate
'
X
'
rst. As the
temperature drops, the liquid composition will move along diopside-X to
'
a
'
. At point
'
forsterite
ss
will co-precipitate with diopside
ss
and the composition of the liquid
should leave the diopside
a
'
sanidine plane and move towards diop-
side
ss
+ nepheline
ss
+ forsterite
ss
liquid univariant line (olivine nephelinite). Forsterite
start to react with liquid and disappears around 985 + 5
-
nepheline
-
°
C. When the univariant line
moves to point G (Fig.
9.2
) and forsterite
ss
completely reacts out, the liquid comes
back to the diopside
-
nepheline
-
sanidine plane. After the disappearance of forsterite
ss
the liquid should move along the curve G
H with co-precipitation of nepheline
ss
and
diopside
ss
resulting in the generation of a nephelinite. At H (865
-
C), diopside
ss
,
nepheline
ss
and leucite
ss
(nepheline leucitite) are in equilibrium with liquid (Fig.
9.2
).
Leucite
ss
start to react with the liquid below 840
°
°
C, and sanidine start to appear. The
assemblage at 840
C, therefore, corresponds to a leucite and pyroxene-bearing
phonolite (Lc
ss
+Ne
ss
+San
ss
+ pyroxene L). Pargasitic amphibole precipitate just
before the solidus is reached and the assemblage at 820 + 10
°
°
C corresponds to a
potassic pargasite-bearing leucite
phonolite. The crystallization of a liquid of com-
-
position
H as mentioned in Fig.
9.2
. This should
correspond to following assemblages: Di
ss
+L(1)
'
Y
'
should follow the course Y
b
-
-
Di
ss
+Lc
ss
+ L (b, leucitite) (2) at
→
(900
°
C)
Di
ss
+Lc
ss
+Ne
ss
+ L (3) at 865
°
C (nepheline leuci-
→
tite)
Di
ss
+Lc
ss
+Ne
ss
+ San
ss
+ L at 830 + 10
°
C (A, leucite-bearing phonolite)
→
(4)
Di
ss
+Ne
ss
+ San
ss
+Lc
ss
+ amphibole + L at 820
°
C (potassic pargasite and
→
leucite-bearing phonolite) (5)
Di
ss
+Ne
ss
+ San
ss
+ Amph
ss
+ L at 815 + 10
°
C
→
(leucite-free potassic pargasite-bearing phonolite).
Edgar (1964) plotted the normative composition of 129 rocks containing alkali
pyroxene, nepheline and alkali feldspar (Fig.
9.5
). The 1 and 2 GPa isobaric four-
phase points (Di
ss
+Ne
ss
+ San
ss
+ L) in the system diopside
sanidine
are plotted in Fig.
9.5
. It is noted that the four-phase point B (1 GPa isobaric point)
is not far from the area where maximum density distribution occurs, and point C
(2 GPa isobaric four-phase point) is very close to maximum density distribution.
As the composition of the four-phase points (Di
ss
+Ne
ss
+ San
ss
+ L) for the
system diopside
-
nepheline
-
nepheline
sanidine at 1 and 2 GPa are Di
4
San
65
Ne
31
and Di
3
S-
-
-
an
73
Ne
24
, respectively, the
first liquid at those pressures should contain 4
3 wt%
-
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