KAl3Si3O10(OH)2

from The Collaborative International Dictionary of English v.0.48

Muscovite \Mus"co*vite\, n. [See {Muscovy glass}.]
   1. A native or inhabitant of Muscovy or ancient Russia;
      hence, a Russian.
      [1913 Webster]

   2. An inhabitant of Moscow.
      [PJC]

   3. (Min.) Common potash mica, essentially {KAl3Si3O10(OH)2}.
      It is used as an electrical insulator. See {Mica}.
      [1913 Webster]