Summary: The results are presented of some recent feldspar studies that relate the fundamental concepts of order-disorder series to petrological problems. New diagrams have been prepared with coordinates ‘optical properties-chemical composition-structural state’. From direct observations made on natural minerals some conclusions have been reached about the sequence in which the solid-state transformations of ordering, unmixing, and twinning occur in feldspars.

A study has been made of the geological distribution of different structural types of alkali feldspars in single intrusive bodies (Tertiary granite, Palaeozoic syenite, Pre-cambrian charnockite), in magmatic complexes, and in different complexes of the Ukrainian Shield and the Caucasus Geosynclinal Province.