Perthite is used to describe an intergrowth of two
feldspars: a host grain of
potassium-rich
alkali feldspar (near K-feldspar, KAlSi
3O
8, in composition) includes exsolved lamellae or irregular intergrowths of sodic alkali feldspar (near
albite, NaAlSi
3O
8, in composition). Typically the host grain is
orthoclase or
microcline, and the
lamellae are albite. If sodic feldspar is the dominant phase, the result is an antiperthite.