Geology Reference
In-Depth Information
Fig. 11.1 The leucite-
nepheline-akermanite-SiO
2
tetrahedron with 25 and 40 wt%
albite plane (after Gupta and
Gupta 1985, 1997)
Ne
Ab
e
SiO
2
40 wt.% Plane
f
c
d
b
a
25 wt.% Plane
Kf
Di+Wo
Lc
Ak
point occurs at Lc
50.5
Ak
49.5
and 1,297
±
3
°
C. The presence of liquid below 1,297
°
C
further con
rms its pseudobinary nature.
The join leucite-SiO
2
was studied by Schairer and Bowen (1955), who found
that in this join K-feldspar forms an intermediate compound. They further estab-
lished that it melts incongruently to produce leucite
ss
and a SiO
2
-saturated melt. The
eutectic between K-feldspar and tridymite occurs at Lc
45.6
Q
54.5
and 990
C.
The phase diagram of the leucite-albite join was obtained by Gupta and Edgar
(1975), who found it to be pseudobinary as nepheline
ss
, kalsilite
ss
and a liquid phase
appear in addition to leucite
ss
and feldspar below the pseudoeutectic point
(Lc
41
Ab
59
, 1,068
±
20
°
±
°
C).
The join albite-akermanite was studied by Yoder and Schairer (1969). It is
pseudobinary and may be considered as a binary join of the system NaAlSiO
4
-
CaO
-
MgO
-
SiO
2
. Diopside
ss
, wollasto-nite
ss
and nepheline
ss
appear as additional
phases with melilite or Na-feldspar. It has two piercing points:
5
(1) One at Ab
91
Ak
9
and 1,138
C, and
(2) The other at Ab
33
Ak
67
and 1,274
°
°
C.
The join albite-SiO
2
was studied by Schairer and Bowen (1935). This join is a
simple binary with a eutectic point at Ab
32
Q
68
and 1,062
±
3
°
C, where Na-feldspar
and tridymite coexist with liquid.
The akermanite-SiO
2
join was investigated by Gupta and Gupta (1985), who
found it to be pseudobinary with two piercing points: (1) one at Ak
87
Q
13
and
1,355
°
C, and the other at Ak
72
Q
28
and 1,320
°
C. The assemblage of diopside
ss
-
tridymite-liquid occurs at Ak
68
Q
32
and 1,325
°
C. The pseudobinary nature was
con
rmed by the presence of diopside
ss
and wollastonite
ss
in addition to
Search WWH ::
Custom Search