magnesium carbonate

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A very light, odorless, white powdery compound, MgCO₃, used in a wide variety of manufactured products including inks, glass, dentifrices, and cosmetics.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun inorganic chemistry The magnesium salt of carbonic acid, MgCO3; it is a white powdery solid that occurs as magnesite and other minerals; it used as a mild base and in numerous industrial applications

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a very white crystalline salt that occurs naturally as magnesite or as dolomite