Geology Reference
In-Depth Information
SiO
2
1
713
0
C
Cristobalite
1470
0
C
1470
0
C
Tridymite
990
0
C
1062
0
C
H
G
U
F
11
50
0
C
B
50
Y
50
C
E
Ab
A
1118
0
C
Or
X
D
W
Lc
1
1068
0
C
M
K
0
R (1020 C)
J
P
Lc
V
1686
0
C
1280
0
C
1460
0
C
N
161
5
0
C
Z
T
T
L
Z
L
Orthorho
mbic
Na-K Ne
pheline
[(K,Na)A
lSiO
4 ss
Carnegieite
ss
1526
0
C
1730
0
C
V
1404
0
C
1588
0
C
K
AlSiO
4
NaAlSiO
4
50
Fig. 7.1 Phase relation in the system nepheline
kalsilite
SiO
2
under atmospheric pressure (after
-
-
Schairer 1950)
undersaturated potassium-rich rocks, further discussion on the silica-saturated
portion of the system will not be made.
In the silica-undersaturated portion of the system, nepheline
SiO
2
,
there is also a temperature minimum M, and a reaction point R. If the composition
of an initial liquid
kalsilite
-
-
Or (Fig.
7.1
), after crystal-
lization of leucite, the liquid composition should cross over the thermal barrier
Ab
'
X
'
lies within the triangle, Lc
Ab
-
-
R at Y (the point Y is determined
by joining the initial bulk composition leucite and X). Feldspar will co-precipitate
with leucite at point Y and then the composition of liquid should move along Y
-
Or, till it reaches the univariant curve B
-
Y
-
-
W
to R. At R leucite will co-precipitate with Na
K feldspar. Crystallization should
continue till the liquid is exhausted at the point R. Thus, even though the liquid
composition lying within the triangle, Lc
-
Or
-
Ab, crosses over temporarily to the
silica-saturated side, they ultimately return to the silica-de
-
cient portion. In case the
liquid composition lies above the thermal barrier Ab
Or, but within the leucite
eld
-
(say point A), the liquid should yield leucite
first and its composition should move
along A
B once again has been derived by joining the composition
of leucite with point A, and extending Lc
B. The line A
-
-
A toward the B
Y
W
R curve. At B,
-
-
-
-
leucite and K
Na feldspar co-precipitate from the liquid. Then, with lowering of
temperature the liquid composition moves along the path B
-
C with gradual elim-
ination of leucite, which reacts with the liquid. At C, all leucite is exhausted and the
liquid composition moves along the curve C
-
U (the composition of the path can be
-
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