The Collaborative International Dictionary of English v.0.48:

Kainite \Kai"nite\, n. [Gr. ? recent.] (Min.)
   A compound salt consisting chiefly of potassium chloride and
   magnesium sulphate, occurring at the Stassfurt salt mines in
   Prussian Saxony.
   [1913 Webster]

WordNet (r) 3.0 (2006):

kainite
    n 1: a white mineral consisting of magnesium sulphate and
         potassium chloride; a source of potassium salts; used as a
         fertilizer