The Collaborative International Dictionary of English v.0.48:

Bitter spar \Bit"ter spar"\
   A common name of dolomite; -- so called because it contains
   magnesia, the soluble salts of which are bitter. See
   Dolomite.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

dolomite \dol"o*mite\ (d[o^]l"[-o]*m[imac]t), n. [After the
   French geologist Dolomieu.] (Geol. & Min.)
   A mineral consisting of the carbonate of lime and magnesia in
   varying proportions. It occurs in distinct crystals, and in
   extensive beds as a compact limestone, often crystalline
   granular, either white or clouded. It includes much of the
   common white marble. Also called bitter spar.
   [1913 Webster]

WordNet (r) 3.0 (2006):

bitter spar
    n 1: a light colored mineral consisting of calcium magnesium
         carbonate; a source of magnesium; used as a ceramic and as
         fertilizer [syn: dolomite, bitter spar]