Perthite

Perthite is an intergrowth of sodium-rich albite within potassium-rich feldspar, produced when a once-homogeneous feldspar separates into two phases during cooling.

Alkali feldspar: A feldspar group whose principal members contain potassium or sodium, or both. Perthite forms within alkali feldspar as its potassium- and sodium-rich components separate.
Feldspar: A major mineral group of framework aluminosilicates containing calcium, sodium, potassium, or combinations of these elements. Perthite is an intergrowth within this abundant mineral group.
Antiperthite: An intergrowth of potassium-rich feldspar within a sodium-rich feldspar host. It reverses perthite’s usual relation between the host and the intergrown phase.
Geothermometry: The estimation of geological temperatures from mineral compositions or equilibria. Feldspar compositions and exsolution textures can constrain temperatures during cooling.
Graphic granite: A quartz–feldspar intergrowth whose interlocking shapes resemble written characters. It is a visually patterned intergrowth formed during crystallization, not feldspar unmixing.
Albite: A sodium-rich feldspar with composition near NaAlSi₃O₈. Albite is the sodium-rich phase that forms the intergrowth’s lamellae or patches.
Albite–anorthite series: A plagioclase feldspar series spanning sodium-rich albite and calcium-rich anorthite compositions. Its sodium-rich end member, albite, also forms the sodium-rich component of perthite.
Mesoperthite: An approximately equal intergrowth of sodium-rich and potassium-rich feldspar phases. It contrasts with perthite, where the potassium-rich phase is the host.
Granite: A coarse-grained intrusive igneous rock composed mainly of quartz and feldspar. Perthitic alkali feldspar is common in many granites.
Albitization: The replacement of feldspar or other minerals by albite through sodium-rich fluid alteration. Albitization can add albite to feldspar, unlike perthite’s separation from a solid solution.

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