The Alkali-Feldspar Join in the System NaAlSiO₄-KAlSiO₄-SiO₂

Abstract

A study, of the alkali-feldspar join $$KAlSi_{3}O_{8}-NaAlSi_{3}O_{8}$$ in the system $$NaAlSiO_{4}-KAlSiO_{4}-SiO_{2}$$ shows an unbroken series of solid solutions between these alkali feldspars with a minimum on the melting and freezing curves at $$1,063^{\circ} \pm 3^{\circ} C$$. at 35 weight per cent potash feldspar. All alkali-feldspar compositions with more than 49 weight per cent potash feldspar melt incongruently and have leucite crystals present, which disappear only at higher temperatures. A revised equilibrium diagram for the system $$NaAlSiO_{4}-KAlSiO_{4}-SiO_{2}$$ is given; the data on which it is based will be published in full later.