hydromica

from The Century Dictionary.

• noun A variety of potashmica which contains more water than ordinary muscovite, and is less elastic. See mica.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Min.) A variety of potash mica containing water. It is less elastic than ordinary muscovite.
• noun (Min.) a mica schist characterized by the presence of hydromica. It often has a silky luster and almost soapy feel.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun mineralogy An alternate name for illite.