Phengite
Phengite is a series name for dioctahedral micas of composition K224O10 it is common for Mg or Fe2+ to substitute for Al on the Y site and a corresponding increase in Si on the Z site in the mica group formula X2Y4–6Z8O204. 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.
