What is "bitter spar"

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.

n. dolomite