Geology Reference
In-Depth Information
Table 8.2 Composition of feldspars in the join leucite
-
albite at 1 atm and 900
°
C
Bulk composition
Cell volume (a
3
)
Composition as wt%
determined from cell
volumes, following the
method of Orville
(1967)
(Ab)
(Lc)
(Ab)
(Or)
100
100
0
666.51
-
80
84.5
15.2
20
677.19
70
59
41
30
694.49
50
50.5
49.5
50
700.19
40
44
56
60
704.26
10
35
65
90
709.05
°
the 850
SiO
2
at P
(H
2
O) = 0.0256 GPa, where the stable assemblage for a bulk composition of
approximately Ab
50
Lc
50
is leucite
ss
, nepheline
ss
and feldspar of composition about
Or
60
Ab
40
. Compositions of feldspar crystallizing from different bulk compositions
in the leucite
C isothermal
section of
the NaAlSiO
4
-
KAlSiO
4
-
albite join are summarised in Table
8.2
, which suggests that maxi-
mum albite content in alkali feldspar in equilibrium with leucite is 50.5 wt%. Thus,
crystallization under moderate [P(H
2
O) = P(Total)] conditions appears to decrease
the chances of compatibility of leucite and albite. Under higher P(H
2
O) the stability
of leucite-feldspar assemblage will be restricted to a very potassium-rich compo-
sitions of feldspar.
The compositions of feldspar coexisting with leucite (Fig.
8.5
) at temperatures
above 1,000
-
C do not support the suggestions of Miyashiro (1960) and MacKenzie
and Rahman (1968), who considered that leucite and Na-feldspar may be com-
patible at high temperatures.
°
8.3 Incompatibility Between Melilite
Plagioclase
-
in Leucite-Bearing Lavas
In the lavas of alkali suites there is an incompatible relationship between melilite
and plagioclase (Yoder and Schairer 1969; Yoder 1973). From volcanic centres of
the same petrographic province, either melilite-bearing lavas are extruded or te-
phritic pyroclastics are ejected. Petrologic study of the melilite-bearing potassic
rocks and leucite-bearing basanites from the Eifel area by the present author sup-
ports the conclusion of Yoder and Schairer (1969). From his study of natural rocks
from various areas Yoder (1973) established that in alkaline lavas melilite and
plagioclase do not appear to coexist. Coexistence of these two minerals is, however
known in metamorphic rocks (Yoder 1973).
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