What is a perthite in geology ?
Perthite : definition
Perthites are intergrowths of sodium feldspar (albite) in potassium feldspar (orthose or microcline). They are roughly oriented parallel to the (001) planes and can take the form of films, spots, or veinlets, usually visible under a microscope and sometimes with the naked eye.
It is more precisely about exsolutions formed by cooling of a solid solution dominant orthoclase - albite. At high temperature K and Na are interchangeable and the compound of formula (K,Na)AlSi3O8 is single-phase; by cooling, this phase separates into two distinct and immiscible mineral phases : albite and orthoclase. Minority albite then appears in fine oriented inclusions : perthites.
It is more precisely about exsolutions formed by cooling of a solid solution dominant orthoclase - albite. At high temperature K and Na are interchangeable and the compound of formula (K,Na)AlSi3O8 is single-phase; by cooling, this phase separates into two distinct and immiscible mineral phases : albite and orthoclase. Minority albite then appears in fine oriented inclusions : perthites.