Perthite
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A perthite is a structural (perthitic structure) variety of the potassium feldspars microcline and orthoclase (KAlSi3O8). It usually manifests itself in the form of a series of perpendicular vertical and horizontal lines, giving it a woven or cross hatched appearance under the petrographic microscope. When megascopically developed it consists of distinct pink and white lamellae which formed from exsolved white albite (NaAlSi3O8) in pink K-feldspar.
Alkali feldspar formed at relatively high temperature (above 660 °C) is a solid solution of potassium and sodium feldspar. As the system cools the solid solution becomes unstable and the two feldspars separate by exsolution into lamellae of albite and microcline. If sodium feldspar is the dominant phase the result is an antiperthite.
The gem varieties of potassium feldspar, amazonite and moonstone are variant colored perthites.
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