What is "phengite"

Phengite is a series name for dioctahedral micas of composition K(AlMg)(OH)(SiAl)O, similar to muscovite but with addition of magnesium. It is a non- IMA recognized mineral name representing the series between muscovite and celadonite.

The silica content of phengite has been proposed as a geobarometer for the metamorphism of low grade schists.