The Collaborative International Dictionary of English v.0.48:

Carnallite \Car"nal*lite\, n. [G. carnallit, fr. Von Carnall, a
   Prussian.] (Min.)
   A hydrous chloride of potassium and magnesium, sometimes
   found associated with deposits of rock salt.
   [1913 Webster]

WordNet (r) 3.0 (2006):

carnallite
    n 1: a white or reddish mineral consisting of hydrous chlorides
         of potassium and magnesium; used as a fertilizer and as a
         source of potassium and magnesium