Geology Reference
In-Depth Information
The P
-
T stability of leucite and sodalite group of minerals with or without
melilite (present in plagioclase-free rocks) suggest that these rocks crystallized
under volcanic to subvolcanic conditions. The study of the nepheline
kalsilite
SiO
2
-
-
system (Gupta et al. 2010) also supports this conclusion.
According to some petrologists these rocks have been formed under FMQ
NNO
buffer conditions (Turbeville 1993), but there are others suggesting their formations
between IW
-
WM buffer (Foley et al. 1986). On the basis of the chemistry of oxide
minerals, the log f(O)2-T
2
-
-
T condition of formation of these rocks from different
localities have been suggested to be as follows:
6.5 (1,300
°
C, Foley 1985),
10.4
−
−
C) and 10
−
5
(755
(815
°
C, Roden and Smith 1979),
13.5 (827
°
°
C, Ghiorso and
−
Carmichael 1981).
In his study of the system forsterite
H
2
O, Yoder showed that
orthopyroxene and phlogopite coexist together but by the time orthopyroxene
appears, leucite is silicated to form sanidine, which can be in equilibrium with
phlogopite and enstatite.
The study of the system diopside
kalsilite
SiO
2
-
-
-
sanidine at 1 and 2 GPa under P
(H
2
O) = P(total) by Gupta et al. (2006) suggests that a pyroxene-bearing nepheline
phonolite can be generated either from a trachyte or a nephelinite or a pyroxene-free
nepheline phonolite. It is interesting to note that the
-
nepheline
-
field of forsterite
ss
appearing as
a reaction product of diopside and nepheline, is eliminated at 1 and 2 GPa. Study of
the same system at 0.1 GPa shows that a nepheline leucitite can be generated from
either a leucitite, an olivine
nephelinite, or a pyroxene-free nepheline-leucite
-
phonolite.
Experimental study of the system pressure, forsterite
anorthite
under atmospheric shows that a leucite basanite).can be derived from an olivine
leucitite, a leucite tephrite, an olivine italite and a murambite. The
diopside
leucite
-
-
-
ow sheet
diagram of the system leucite
SiO
2
also studied under atmo-
spheric pressure, shows that a shoshonitic liquid can move either to a quartz latite or
towards tephri
akermanite
albite
-
-
-
phonolite in turn can be derived from either
(1) a leucite tephrite, (2) a plagioclase-bearing phonolite, (3) a pyroxene-bearing
phonolite or a shoshonite.
The paragenetic relationship amongst many leucite, nepheline and melilite-
bearing kamafujitic rocks can be adequately described from experimental study of
the
phonolite. A tephri
-
-
system forsterite
akermanite
diopside
leucite
and
diopside
nepheline
-
-
-
-
-
akermanite
leucite (Nag et al. 2007) under one atmospheric pressure.
Experimental study of the system KAlSiO
4
-
-
H
2
O at pressures
between 0.05 and 0.3 GPa (Luth 1967), at 2 GPa (Wendlendt and Eggler 1980a, b, c)
and Gupta and Green (1988) suggests that at low P-T
Mg
2
SiO
4
-
SiO
2
-
T condition (0.05 GPa) the
system cuts through the phase volumes of kalsilite, leucite, forsterite, enstatite and
quartz. Leucite-quartz, kalsilite-quartz and forsterite-quartz do not coexist. With
increase in water pressure phlogopite appears at the expense of feldspathoid and
olivine. At low pressure (<0.3 GPa) there are 35 univariant curves and nine invariant
points. At 2.0 GPa Wendtland and Eggler (1980a, b) have described 20 univariant
reactions with or without CO
2
and 24 univariant reactions in presence of CO
2
.
Various combinations of the following minerals appear in the univariant curves and
-
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