We have obtained new nucleation and crystal property data for sodium carbonate-sulfate salts that crystallize during evaporation of paper industry spent pulping liquors. Earlier lab-scale evaporation experiments at 115C indicated that a new species, referred to as dicarbonate with approximate composition 2Na2CO3-Na2SO4, precipitated when the liquid phase was in a narrow range of carbonate-to-sulfate ratio. Our work also suggested that heat exchanger surface fouling was more likely to occur in evaporation equipment when the liquid composition fell in this narrow range. In the present work, we have verified that a dicarbonate-rich crystal phase is formed from supersaturated solutions when the mole fraction x=CO3/(CO3+SO4) of the liquid phase falls between 0.833 and 0.889 and the temperature is between 125 and 135C. For temperatures of 110-115C, there is a sharp transition from burkeite to dicarbonate crystallization between x=0.833 and 0.840. At temperatures below 110C, no evidence of dicarbonate was found in our experiments; the crystal phase was burkeite for x less than 0.834 or sodium carbonate monohydrate for x greater than 0.835. Dicarbonate solubility decreased slightly (5% of maximum value) from 110 to 135C. The dicarbonate crystals were found to be stable at temperatures up to 140C, and progress is being made towards growing and isolating single dicarbonate crystals for high-resolution diffraction analysis. This new data has enhanced our ability to predict fouling conditions in paper industry spent pulping liquor evaporators.