Formation and modification of metastable intermediate sodium potassium mica, paragonite, and muscovite in hydrothermally altered metabasites from northern Wales

Metastable intermediate sodium potassium mica (Mu₄₇Pa₅₂Ma₁ ) is intergrown with paragonite (Mu₆Pa₉₁Ma₃) and muscovite (Mu₈₈Pa₁₂), forming aggregates of randomly oriented crystals in the margin of the Ordovician Caradoc metabasites, northern Wales. The cross sections of the mica aggregates resemble the morphology of euhedral plagioclase. The intermediate sodium potassium mica formed as hydrothermally derived pseudomorphs after plagioclase microphenocrysts. Subsequent interactions between the intermediate sodium potassium mica and hydrothermal fluids resulted in the redistribution of Na and K in micas and formation of paragonite and muscovite. Redistribution of Na and K of the micas occurred in two ways: (1) dissolution of the intermediate sodium potassium mica and crystallization of muscovite and paragonite, resulting in lamellar intergrowths of parallel or subparallel packets of micas, (2) alkali exchange between fluids and micas and diffusion within micas without significant dissolution and crystallization, giving rise to inclined irregular interfaces between micas. -from Authors