pertite

Pertite is a textural term used in petrology to describe a specific intergrowth of two feldspar minerals. It is characterized by the lamellar or granular exsolution of one feldspar within the other, typically occurring as the host crystal cools slowly from a solid solution. The most common type of pertite involves the intergrowth of alkali feldspar (a solid solution of potassium feldspar and albite) into distinct lamellae or patches within a host feldspar crystal. This intergrowth is a result of the immiscibility of the alkali feldspar system at lower temperatures, causing the initially homogeneous solid solution to separate into potassium-rich and sodium-rich phases.

The appearance of pertite can vary significantly. It can manifest as fine, hair-like lamellae, broader veins, or irregular patches. The orientation and distribution of these intergrowths are often diagnostic of the cooling history of the rock. Pertites are commonly found in various igneous rocks, particularly granites, syenites, and pegmatites, as well as in some metamorphic rocks. The study of pertite textures provides valuable insights into the formation temperatures, pressures, and cooling rates experienced by these rocks. It is a visual indicator of subsolidus processes, where mineral phases separate after the initial crystallization of a melt.