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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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