Paragonite
Paragonite, also known as Natron-Glimmer, is a mineral, related to muscovite. Its empirical formula is NaAl2[(OH)2|AlSi3O10]. A wide solvus separates muscovite from paragonite, such that there is little solid solution along the vector NaK and apparent micas of intermediate composition is most commonly a microscopic (or even sub-microscopic) intergrowth of two distinct micas, one rich in K, and the other in Na. Paragonite is a common mineral in rocks metamorphosed under blueschist facies conditions along with other sodic minerals such as albite, jadeite and glaucophane. During the transition from blueschist to greenschist facies, paragonite and glaucophane are transformed into chlorite and albite.