The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. (Chem.) A compound of manganese dioxide with a metallic
      oxide; so called as though derived from the hypothetical
      manganous acid.
      [1913 Webster]

WordNet (r) 3.0 (2006):

manganite
    n 1: a black mineral consisting of basic manganese oxide; a
         source of manganese