Any of the oriented intergrowths of potassium- and sodium-rich feldspars, (K,Na)AlSi₃O₈, whose proportions are determined in part by the initial composition of the alkali feldspar from which they exsolved and whose physical properties are thus somewhat variable. The early stages of perthite formation from homogeneous, usually monoclinic (K,Na)AlSi₃O₈ may be observed experimentally by high-magnification electron microscopy.