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piercing point occurs at Fo
8
Di
50
Ak
42
, where forsterite
ss
+diopside
ss
+liquidcoexistat
1,357
°
C. In this join monticellite
ss
reacts with the liquid to form akermanite
ss
and the
final crystalline assemblage is forsterite
ss
+diopside
ss
+akermanite
ss
.
10.1.1.5 The Join Diopside
Akermanite
Leucite with 3 wt% Forsterite
-
-
Study of the bounding joins of the tetrahedron suggests that the invariant point,
where forsterite
ss
+ akermanite
ss
+ diopside
ss
+ leucite
ss
+ liquid are in equilibrium,
lies very close to the basal plane diopside
leucite, probably containing
less than 5 wt% forsterite. Accordingly, the join diopside
akermanite
-
-
leucite with
3 wt% forsterite was studied to determine the temperature of the invariant point.
The equilibrium diagram for the join is represented by Fig.
10.4
.AtK
(Fo
3
Di
55
Ak
9
Lc
33
), forsterite
ss
+ diopside
ss
+ leucite
ss
+ liquid are in equilibrium at
1,295
akermanite
-
-
C, and at point L (Fo
3
Di
41
Ak
24
Lc
32
), diopside
ss
+ akermanite
ss
+ leu-
cite
ss
+ liquid coexist in equilibrium at 1,280
±
3
°
°
C, below which there is a liquid
phase, which does not freeze until 1,255
C. As the compositions of the crystalline
phases lie outside the join and there is a liquid phase below 1,274
°
°
C, the system
forsterite
leucite is not quaternary. Optical and X-ray studies,
however, suggest that although they are chemically nonstoichiometric, all the
phases in this join contain only small amounts of solid solution and the behaviour of
the join thus approximates that of a quaternary system and points K and L behave as
piercing points. From these two points liquid continues to crystallise along two
different paths and at 1,274
-
diopside
-
akermanite
-
five-phase assemblage of forsterite
ss
+ akerma-
nite
ss
+ leucite
ss
+ liquid, is reached. Below 1,274
°
Ca
°
C, crystallization continues as
Fig. 10.4 Phase relation
in the join diopside
97% CaMgSi
26
(Diopside)
-
akermanite
leucite with 3 %
forsterite (after Gupta 1972)
-
87%
77%
67%
Di
ss
Fo
ss
K(1295
0
57%
47%
L(1280
0
37%
27%
17%
Lc
ss
Ak
ss
7%
v
v
v
v
v
v
v
v
v
97% KAlSi
26
(Leucite)
97% Ca MgSi O
2 2 7
(Akermanite)
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