Phengite
Phengite is a series name for dioctahedral micas of composition K(AlMg)2(OH)2(SiAl)4O10, 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.