It has long been recognized that crystallization from solutions of sodium carbonate and sodium sulfate can produce anhydrous or hydrated forms of the pure species or, in certain composition ranges, burkeite-a crystalline double salt of sodium carbonate and sodium sulfate having the nominal stoichiometry Na2CO3.2Na(2)SO(4). The present work examines and explains the observed variation of burkeite composition and identifies a new double-salt species whose composition has been determined to contain approximately 2 moles of sodium carbonate per mole of sodium sulfate. The new species was formed at 115 degreesC from solutions whose compositions range from approximately 5 moles of sodium carbonate per mole of sodium sulfate to 7 moles of sodium carbonate per mole of sodium sulfate.