Geology Reference
In-Depth Information
Fig. 8.4 Phase diagram
of the join
leucite
-
albite
-
anorthite at
1 atm (after Gupta and Edgar
1975)
KAlSi
26
(Leucite)
Lc
ss
B
(1413 C)
0
A
0
(1068 C)
Plag.
V
V
VV
V
V
VVVVVVVV
V
VVVV
NaAlSi
38
CaAl Si
28
2
(Albite)
(Anorthite)
Table 8.1 Composition of ternary feldspars crystallizing in the system leucite
albite
anorthite
-
-
Composition of
mixtures
Temp
(
o
C)
SiO
2
(wt%)
Na
2
O+K
2
O l
2
O
3
CaO
(wt
%)
An
(wt
%)
Ab + Or
(wt%)
Lc
Ab
An
43
31
26
1,320
45.92
4.54
33.32
16.22
80.40
9.60
40
50
10
1,180
51.87
5.72
29.64
12.77
63.15
36.85
53
42
5
1,180
50.35
7.05
30.29
12.31
60.93
39.07
30
60
10
1,200
57.09
5.44
28.23
9.24
45.63
54.37
coexisting with a liquid low in anorthite content (Lc
53
Ab
42
An
5
), is rich in anorthite
(An
61
,Ab
Or
39
). This implies that the residual liquid after crystallization of such a
feldspar, must be enriched in NaAlSi
3
O
8
, although small amounts of soda may be
incorporated in leucite. These results demonstrate that the addition of anorthite does
not change the leucite
-
albite incompatibility.
-
8.2.3 Petrological Implications
Study of phase relations in the join leucite
albite and leucite
albite
anorthite at
-
-
-
atmospheric pressure supports the conclusion of
field studies that leucite and albite
are incompatible. In the absence of the anorthite molecule, the most sodium-rich
feldspar that can coexist with leucite contains, 46 wt% orthoclase molecule. In the
presence of the anorthite molecule,
leucite coexists with a ternary feldspar,
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