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to pure kalsilite (Smith and Tuttle 1957); indicate that both these minerals are solid
solutions. However, the exact compositions of these two phases were not deter-
mined. Appearance of kalsilite
ss
and nepheline
ss
in the leucite
albite join can be
explained, if it is considered that leucite contains small amounts of excess silica and
NaAlSiO
4
in solid solution, as is usually observed in nature. If the crystallization of
a liquid of bulk composition J is considered (Fig.
7.1
, Chap. 7), with drop in
temperature, leucite
ss
of composition Lc
1
should crystallise and the liquid should
move along the line J-T
-
orthorhombic kalsilite
ss
boundary
and at T, orthorhombic kalsilite
ss
should crystallise. If the bulk composition lies at
K, with temperature decrease leucite
ss
of composition Lc
1
should again appear as
the primary phase and the composition of the liquid should move along the line
Lc
1
-
T towards the leucite
ss
-
-
K and at L, nepheline
ss
should appear. With further drop in temperature, liquid
should move to R and following phases should appear: Lc
ss
+Ne
ss
+Na
K
-
feldspar.
Compositions of nepheline
ss
, crystallised at the same temperature from the bulk
compositions Ab
60
Lc
40
and Ab
50
Lc
50
(where leucite is an additional phase) was
found to be Ne
73
Ks
25
Qz
3
. Nepheline
ss
coexisting with feldspar at 960
C in the
mixture Ab
70
Lc
30
has following composition: Ne
68
Ks
30
Qz
2
. When the composition
is plotted along with coexisting feldspar composition (Ab
59
Or
4
) in the system
NaAlSiO
4
-
°
SiO
2
the tie line passes very close to the bulk composition
Ab
70
Lc
30
indicating the absence of any other phase.
KAlSiO
4
-
8.2.2 The Leucite
-
Albite
-
Anorthite Join
Phase relations in this pseudo-ternary join of the system KAlSiO
4
-
NaAlSiO
4
-
CaAl
2
Si
2
O
8
-
SiO
2
are shown in Fig.
8.4
. Only the liquidus relations within the
system have been studied. However, it appears from the bulk compositions lying
close to the KAlSi
2
O
6
-
CaAl
2
Si
2
O
8
join leucite
ss
and plagioclase should be the only
two crystalline phases. Figure
8.4
, however, suggests that with the increase in the
NaAlSi
3
O
8
content in the bulk composition, nepheline
ss
and alkali feldspar should
be additional phases at low temperatures and when the starting material is con-
siderably rich in the albite content, leucite should cease to appear.
Due to the pseudoternary nature of this join compositions of liquids do not lie in
the leucite
anorthite join. Estimation of the nature of these liquids can be
obtained from the determination of the composition of feldspars, crystallised at or
near the boundary line A
albite
-
-
B (Fig.
8.4
). Microprobe analyses of feldspar were made
on four bulk compositions, crystallised at temperatures close to liquidus, ranging
from 1,320 to 1,180
-
°
C (Table
8.1
). For each feldspar the contents of CaO, Al
2
O
3
,
and SiO
2
were determined by microprobe, assuming that Na
2
O+K
2
O form the
remainder. In Fig.
8.5
tie lines have been drawn between the bulk compositions and
the corresponding feldspar compositions projected on to the leucite
albite
anorthite
-
-
join. These tie lines show progressive enrichment in the albite
orthoclase contents
-
of the feldspar with decreasing temperature, but
indicate that
the plagioclase,
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