Sodium calcium carbonate -- an equimolar double salt (Na2CO3ĚCaCO3) never described in the literature -- forms by a novel solid state reaction when intimately mixed Na2CO3 and CaCO3 powders are heated. The thermal synthesis and the properties of this compound are evaluated by thermal analysis using an instrument yielding simultaneous DTA, TG, and EGA data. The compound forms slowly at temperatures above 300 degrees C with no gas evolution and no weight loss. This product's X-ray diffraction pattern, unique and reproducible, is not included in the ASTM X-ray data card file. Stable at room temperature, the compound undergoes four crystal inversions in the temperature range from 390 to 450 degrees C. Easily prepared from common laboratory chemicals, this compound may be useful as a temperature and DTA resolution standard.