The Collaborative International Dictionary of English v.0.48:

Glauconite \Glau"co*nite\, n. [Cf. F. glauconite, glauconie, fr.
   L. glaucus. See Glaucous.] (Min.)
   The green mineral characteristic of the greensand of the
   chalk and other formations. It is a hydrous silicate of iron
   and potash. See Greensand.
   [1913 Webster]

WordNet (r) 3.0 (2006):

glauconite
    n 1: a green mineral consisting of hydrated silicate of
         potassium or iron or magnesium or aluminum; found in
         greensand