Phase Relations in the System Leucite-Akermanite-Albite-SiO - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 339

(1969) observed that plagioclase and melilite have an incompatible relationship in
alkaline syenite lava. Gupta and Lidiak (1973) discussed the melilite-plagioclase
incompatibility problem and concluded that melitite and plagioclase may coexist in
some hybrid contact zone under water-saturated conditions. Schairer and Yoder
(1970) opined that akermanite could coexist with spinel, anorthite, and diopside in
the absence of forsterite. Such a calcium-rich assemblage is only found in meta-
morphic rocks, whereas an assemblage forsterite-akermanite-diopside-spinel has
representatives among the igneous rocks. Schairer and Yoder (1970) further sug-
gested that even in the absence of olivine, there is a limit to the anorthite content of
plagioclase that can coexist with melilite. They observed that volcanic centres of the
same petrographic province eject either plagioclase-bearing pyroclasts, or melilite-
bearing lavas. In natural alkaline rocks, nepheline and K-feldspar are essential
minerals of syenites and phonolites, but leucite and pure albite have not been
observed to coexist in equilibrium.
Because of the above-mentioned incompatibility between the leucite-albite pair
and akermanite-albite pair, Dwivedi et al. (2007) studied the leucite-akermanite-
albite system under atmospheric pressure first, and later added anorthite, and

completed the system leucite-akermanite-albite 50 anorthite 50 under similar condi-
tions to see the effect of anorthite on the incompatibility problem. Leucite also
breaks down under high pressures and temperatures to form orthorhombic kalsilite
and K-feldspar (Fasshauer et al. 1998). Their study showed that leucite is stable at
1 GPa at temperatures above 850

C and akermanite breaks down to form diopside
and merwinite (Yoder 1973). Therefore, the join leucite-akermanite-albite 50 anor-
thite 50 was studied under water-saturated conditions at 1 GPa to see the effect of
pressure on the leucite-plagioclase incompatibility problem in the presence or
absence of melilite.

°

11.4.1 The Join Leucite-Akermanite-Albite Under One
Atmospheric Pressure

Fourteen compositions were prepared to study this join, and the phase relations in
the join leucite akermanite-albite are summarized in Fig. 11.9 , which demonstrates
the presence of two four-phase points: (1) at the point

“ A ” (Lc 31 Ak 47 Ab 22 )at

1,275 C, where leucite ss, diopside ss and melilite are in equilibrium with liquid, and

(2) at the point

°

C, where leucite ss + ternary
feldspar + diopside ss are in equilibrium with liquid (Fig. 11.9 ). The presence of a
primary phase

“ B ” (Lc 38 Ak 22 Ab 40 ) at 1,250 ± 5 °

field of diopside ss and the appearance of nepheline ss , wollastonite ss
and alkali feldspar in the subsolidus region in the leucite- akermanite-albite join,
can be explained by the following reaction.