Barium carbonate powder is dense and white and is manufactured either from the mineral barite (BaSO4) or from barium chloride. Subsequently a precipitation process is used to get the carbonate form. There are several crystalline forms of BaCO3, alpha is the most stable.
Barium carbonate is very stable thermally and does not readily disassociate unless at least some CO is available in the kiln atmosphere (i.e. reduction). BaCO3 is reduced to the unstable BaCO2 in the reaction: