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Their experimental results indicate that in presence of nepheline, leucite and spinel,
olivine and melilte are stable at temperatures >1,090
C. A peritectic relationship
occurs among olivine, melilite, and liquid to produce either a olivine + high calcium
pyroxene-bearing assemblage or a melilite and high-calcium pyroxene-bearing
assemblage with decreasing temperature. The preferred path of liquid divergence is
contingent upon bulk composition. A similar peritectic relationship has also been
inferred from studies in simple systems (e.g. Onuma and Yagi 1967; Gupta et al.
1973). Relative to these simple systems, the peritectic is depressed by 30
°
Cin
the natural system because of the presence of other oxides. The course of crys-
tallization of liquid, established by Gee and Seck (1988) is very similar to that
determined by Gupta et al. (1973; Fig.
10.9
).
Melluso et al. (1996) plotted the Mt. Vulture melilite-bearing rocks in the
diopside
50
°
-
leucite pseudoternary diagram of Gupta and Lidiak (1973).
They noted that the bulk compositions of these rocks plot in the central part of their
diagram (10.7), and the Paragenesis of the rocks is very similar to that predicted by
Gupta and Lidiak.
nepheline
-
-
10.2.1.3 The Join Nepheline
-
Akermanite
-
Leucite
The join was studied by Gupta et al. (1973); and the phase diagram is shown in
Fig.
10.10
.AtpointS(Ne
34.5
Ak
20.5
Lc
45
) and 1,170
C, nepheline
ss
,leucite
ss
melilite, and liquid are in equilibrium. Onuma and Yagi (1967) showed that in the join
diopside
±
3
°
akermanite, forsterite
ss
appears as the last solid phase. This phase
also appears as a subliquidus phase within the join nepheline
nepheline
-
-
akermanite
leucite, but it
-
-
reacts with the liquid and disappears at 1,100
C. As melilite contains soda-melilite,
and forsterite
ss
appears as an additional phase, the join is pseudoternary.
±
5
°
Fig. 10.10 Phase relation in
the join nepheline
AKERMANITE
-
akermanite
-
leucite at 1
atmosphere (after Gupta et al.
1973)
90
80
70
60
Mel
50
50
40
30
S
20
Lc
ss
Cg
ss
Ne
ss
10
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