phengite

Phengite is a mica mineral in the phyllosilicate group, recognized as a high-silica variety within the muscovite–phlogopite lineage. It is a potassium-rich sheet silicate whose tetrahedral framework contains more silicon relative to aluminum than in muscovite, giving it a distinct compositional signature within the mica family. In hand specimens, phengite forms plate-like crystals with perfect basal cleavage, a vitreous to pearly luster, and a range of colors from colorless to pale greenish or brownish. It is monoclinic and typically has a hardness around 2–3 on the Mohs scale.

Formation and occurrence: Phengite forms mainly through metamorphism, especially in pelitic rocks subjected to medium- to high-pressure conditions, such as those associated with subduction zones. It is common in schists and gneisses and also occurs in higher-grade rocks such as granulites and eclogites. Phengite frequently coexists with minerals such as quartz, garnet, biotite, and chlorite, and a ferric- or ferro-phengite variety can be iron-rich and common in high-pressure assemblages.

Geologic significance: The silicon content of phengite increases with pressure, a relationship exploited in geobarometry to estimate the pressure conditions during metamorphism. Consequently, phengite chemistry is widely used to interpret metamorphic histories and tectonic settings in subduction-related and high-grade terranes.

See also: mica, muscovite, biotite, metamorphic rocks.