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

A study, of the alkali-feldspar join KAlSi₃O₈-NaAlSi₃O₈ in the system NaAlSiO₄-KAlSiO₄-SiO₂ shows an unbroken series of solid solutions between these alkali feldspars with a minimum on the melting and freezing curves at 1,063∘ ± 3∘ 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 NaAlSiO₄-KAlSiO₄-SiO₂ is given; the data on which it is based will be published in full later.