The Collaborative International Dictionary of English v.0.48:

Hydromica \Hy`dro*mi"ca\, n. [Hydro-, 1 + mica.] (Min.)
   A variety of potash mica containing water. It is less elastic
   than ordinary muscovite.
   [1913 Webster]

   Hydromica schist (Min.), a mica schist characterized by the
      presence of hydromica. It often has a silky luster and
      almost soapy feel.
      [1913 Webster]