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SiO
2
90
10
P .1 G a
H
2
O
80
20
Silica
70
30
60
40
Sanidine
ss
50
50
m
NaAlSi
38
KAlSi
38
40
60
850
840
800
Leu
cite
ss
30
780
R
70
865
870
890
M750
20
80
950
10
Nepheline
ss
1200
K
alsilite
ss
1000
1100
90
10
20
30
40
50
60
70
80
90
NaAlSiO
4
KAlSiO
4
WEIGHT PERCENT
Fig. 7.2 Phase relation in the system nepheline
kalsilite
SiO
2
under 0.1 GPa [P(H
2
O) = P(Total)]
-
-
(after Hamilton and MacKenzie 1965)
that the liquid probably got frozen, as crystallization was hindered because of
eruption.
Cundari and Mattias (1974) plotted the composition of lavas of Somma-
Vesuvius and Phlegrean Field containing very low amount of MgO and other
components in terms of nepheline, kalsilite and SiO
2
(Fig.
7.3
), they found that the
most evolved rocks have their compositions clustering around M.
The Vesuvius rocks of more salic type were plotted by Cundari and Le Maitre
(1970, Fig.
7.3
) and found that they plot -near the 0.1 GPa [P(H
2
O) = P(Total)]
minimum representing phonolitic composition (Qz = 27
-
34, Ne = 35
-
47,
Ks = 25
27).
The silica
-
SiO
2
was
studied under 0.2 GPa [P(H
2
O) = P(Total)] by Taylor and MacKenzie (1975,
Fig.
7.4
) and showed that the minimum melting point occurs at Ne
51
Ks
20
Qz
29
and
710
+
7
-
undersaturated part of the system, nepheline
-
kalsilite
-
°
C. Comparison of Figs.
7.1
,
7.2
,
7.3
and
7.4
show a signi
cant reduction
of leucite
field under 0.2 GPa P(H
2
O).
Zhang and Mackenzie (1984) studied the system NaAlSiO
4
-
SiO
2
-
H
2
O under 0.5 GPa. Their results are summarized in Fig.
7.5
, which shows that in
contrast to Figs.
7.1
,
7.2
and
7.3
and
7.4
leucite
KAlSiO
4
-
cantly reduced,
because this phase produced as an incongruent melting product of sanidine, is
field has been signi
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