Figure 5.07. NaAlSi3O₈-KAlSi3O₈ TX Diagram with Pressure Slider. The liquidus and solidus for the alkali feldspar solid solution meet at an azeotrope minimum. This relationship is typical for minerals that have a solvus at lower temperatures. The alkali feldspar melting diagram is complicated by the incongruent melting of K-rich compositions to leucite and liquid, neither of which plot on the Ab-Or binary.

Use the pressure slider to increase the water pressure on the system. Increasing the pressure with P(H₂O)=P(total) significantly lowers the liquidus and solidus for the alkali feldspars (and makes leucite unstable). The presure changes are shown without leucite, using the (metastable) alkali feldspar data of Waldbaum and Thompson (1969). At 0.5 GPa, the solidus meets the solvus, eliminating the azeotrope and forming a eutectic. These diagrams are based on the experimental work of Schairer (1950), Tuttle and Bowen (1958), and Morse (1970).    Return to Text