A series of layers that occurs as intergrowths in alkali feldspars which form at high temperatures and cool slowly, unmixing as they cool. Most alkali feldspars in such slowly cooled rocks (e.g. plutonic and igneous) consist of an alkali feldspar host with an exsolved, sodium-rich, plagioclase feldspar phase that segregated from the alkali feldspar during cooling. Perthitic intergrowths have textures ranging from macroperthite (visible to the naked eye) through microperthite (visible under a microscope) to cryptoperthite (which can be detected only by X-ray diffraction and other techniques). The nature of the perthite depends upon the ordering of the silicon and aluminium atoms achieved by the mineral lattice as it cools.
Subjects: Earth Sciences and Geography.