Source : Webster's Revised Unabridged Dictionary (1913)

Manganite \Man"ga*nite\, n.
   1. (Min.) One of the oxides of manganese; -- called also
      {gray manganese ore}. It occurs in brilliant steel-gray or
      iron-black crystals, also massive.

   2. (Chem.) A compound of manganese dioxide with a metallic
      oxide; so called as though derived from the hypothetical
      manganous acid.

Source : WordNet®

manganite
     n : a black mineral consisting of basic manganese oxide; a
         source of manganese