The alkali feldspar group are those feldspar minerals rich in the alkali elements, potassium and sodium. The alkali feldspars include albite, anorthoclase, microcline, orthoclase and sanidine. Albite is considered as one of the alkali feldspars according to its chemical composition and it is also the alkali endmember of the plagioclase series.
Potassium and sodium feldspars are not perfectly miscible in the melt at low temperatures, intermediate compositions of the alkali feldspars therefore occur only in higher temperatures environments.
References
Alkali feldspar Wikipedia(Text) CC BY-SA