Gilbertite

(named for the English antiquarian and economist D. Gilbert, 1767–1839), a squamulose, sometimes dense cryptocrystalline variety of muscovite, usually partially hydrated and converted to hydromuscovite. Squamulose aggregates of nacrite and kaolinite are sometimes called gilbertite. Gilbertite is often formed by the replacement of feldspars and topaz (for example, in pegmatites and greisens); it is also found in clays formed by the decomposition of mica-rich rocks (micaceous slates and gneisses).