Incompatible Mineral Pairs in K-Rich Rocks - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 283

diopside ss + nepheline ss + melilite + anorthite ss + liquid + vapour. Above 0.25 GPa
phlogopite appeared as an important phase and leucite ss disappeared, but melilite
coexisted with anorthite ss , phlogopite ss , nepheline ss , liquid and vapour. Above
0.28 GPa in presence of water, melilite disappeared and garnet appeared. The
equilibrium assemblage at 700 and 750

C and 0.5 GPa consists of large quantities
of grossularite ss , phlogopite ss , small amounts of diopside ss and rare anorthite ss .
Yoder (1969) studied the join Ab 50 Ak 50 -

°

An 50 Ak 50 (Fig. 8.7 ). It may be
observed that in the anorthite-rich portion of the join at low temperature (assem-
blage 4 and 6), calcium-rich plagioclase and melilite can coexist in equilibrium but
in the albite-rich portion this two minerals do not coexist.
Schairer and Yoder (1970) studied the system CaO

MgO Al 2 O 3 - SiO 2 , and

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found that in the volume enclosed by anorthite akermanite diopside forsterite

-
spinel, akermanite coexists with spinel, anorthite and diopside in the absence of
forsterite. Such a calcium-rich assemblage is found only in metamorphic rocks,
whereas the assemblage forsterite

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spinel has representatives
among igneous rocks. They indicated that olivine may be a deciding factor in the
melilite

- akermanite - diopside -

plagioclase incompatibility in the lavas. Thus, in the absence of olivine,
melilite and plagioclase may coexist in some hybrid contact zone, and even in the
absence of olivine and presence of water, there is a limit to the anorthite content of
plagioclase that can coexist with melilite.

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