Perthite

Perthite or perthitic texture is used to describe an intergrowth of two feldspars: a host grain of potassium-rich alkali feldspar (near K-feldspar, KAlSi3O8, in composition) includes exsolved lamellae or irregular intergrowths of sodic alkali feldspar (near albite, NaAlSi3O8, in composition).

Typically, the host grain is orthoclase or microcline, and the lamellae are albite.

[1] The intergrowth forms by exsolution due to cooling of a grain of alkali feldspar with a composition intermediate between K-feldspar and albite.

If cooling is sufficiently slow, the alkali feldspar may exsolve to form separate grains with near-endmember albite and K-feldspar compositions.

[2] The gem varieties of potassium feldspar, amazonite and moonstone are variant colored perthites.

Perthitic textures: antiperthite (1), perthite (2), and mesoperthite (3)
A photomicrograph of a perthite part of a feldspar grain in thin section as viewed through a petrographic microscope and with a first-order red plate. The K-feldspar host ( orthoclase ) appears orange, and albite exsolution lamellae appear yellow. The longer dimension of the field is 0.4 mm.