CoCO3 A very powerful colorant and flux for glazes (<3%) and slips (<15%) typically producing deep clear blues but also pinks, mauves and greens. It decomposes to cobalt oxide on firing and can be used as a substitute for it in recipes (use 1.6 X more). It is less prone to cause specking than the more concentrated oxide.
For more information on cobalt carbonate, see the Digitalfire.com Reference Library of ceramic materials.