The Collaborative International Dictionary of English v.0.48:

Lignitic \Lig*nit"ic\ (l[i^]g*n[i^]t"[i^]k), a.
   Containing lignite; resembling, or of the nature of, lignite;
   as, lignitic clay.
   [1913 Webster]

   Lignitic group. See Laramie Group.
      [1913 Webster]