The Collaborative International Dictionary of English v.0.48:

Saponite \Sap"o*nite\, n. [Sw. saponit, fr. L. sapo, -onis,
   soap.] (Min.)
   A hydrous silicate of magnesia and alumina. It occurs in
   soft, soapy, amorphous masses, filling veins in serpentine
   and cavities in trap rock.
   [1913 Webster]