Ab-Or TX Diagram. 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. This diagram is based on the experimental work of Schairer (1950) and Tuttle and Bowen (1958).