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determined by electron microprobe analyses). With further lowering of temperature
the liquid moves along the curve U
G to G. Tridymite co-precipitates with feldspar
-
along the curve U
G. Final composition of the liquid is represented by the mini-
-
mum point G.
When the liquid composition lies in the silica-undersaturated portion of the
system within the triangle Ab
nepheline (say point D), with lowering of tem-
perature, the liquid composition should move along the curve E
Or
-
-
M to M (the
minimum point). The point E represents the composition of feldspar in equilibrium
with the liquid D. The point E (the composition of feldspar) has to be determined by
electron microprobe analysis. The curved path D
D
P
-
-
-
P is obtained by determining the
-
compositions of liquids by microprobe analyses.
When the bulk composition of the liquid lies within this triangle Lc
Or
Ne
-
-
(say V), leucite should precipitate
first and the composition of the liquid should
move along Lc
W. At W co-precipitation of alkali feldspar and leucite should
take place. When the liquid composition reaches the invariant point R, nepheline
ss
should also appear. Leucite reacts out at R and then the liquid composition should
move along R
V
-
-
M to M (Ne
52
Ks
15
Qz
33
), where nepheline
ss
will co-precipitate with
alkali feldspar. If the liquid composition lies within the triangle nepheline-Lc-
kalsilite (at point N), with lowering of temperature, leucite should precipitate
-
rst.
The liquid composition then moves along the line Lc
Z
leucite and orthorhombic kalsilite should co-precipitate from the liquid. At Z
(1460
-
N
-
T
-
T-Z Along the line T
-
C) crystallization should stop and the liquid should be exhausted. In the
above discussion it has been assumed that leucite contains little or no solid solution
at one atmospheric pressure.
Hamilton and MacKenzie (1965) studied the system nepheline
°
SiO
2
in
presence of excess water under 0.1 GPa (Fig.
7.2
). Their study shows that at 0.1
GPa,
kalsilite
-
-
field of leucite, produced due to incongruent melting of sanidine,
decreases. Hamilton and MacKenzie noted that although the temperature of the
minimum melting point is lowered from 1,050 (established under atmospheric
pressure) to 750
the
C under 0.1 GPa P (H
2
O), the composition of the minimum
(Ne
50
Ks
19
Qz
31
) is very similar to that found by Schairer (1950).
Luhr and Giannetti (1987) recalculated the composition of Brown Leucitic Tuffs
in terms of nepheline
°
SiO
2
system (Fig.
7.3
). The asterisk symbols rep-
resent compositions, which are phonolitic with low CaO content (<5.6 wt%). The
compositions with CaO content (>5.6 wt%) are indicated by open circle (>5.6 wt%,
Fig.
7.3
). Luhr and Giannetti (1987) also plotted the composition of primitive lava
(marked by stars) in the same diagram. The 0.1 GPa cotectic line is shown by solid
line but dashed line indicates one atmosphere cotectic line. When the anorthite
content is high the liquid composition should not lie in the ternary join and the
course of crystallization of liquid cannot be depicted in this join. Assuming that
before eruption, the melt equilibrated at shallow depth, it should precipitate leucite
kalsilite
-
-
lled triangles. With progressive
decrease in temperature, alkali feldspar should coprecipitate with leucite, as the
liquid reaches the cotectic line. As nepheline did not appear, it may be interpreted
first from the bulk composition denoted by the un
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