Crystals of Na2Ca(CO3)2, the structural analogues of mineral nyerereite, were synthesized using hydrothermal technique at 1 kbar and 450°C. The crystals are transformational twins formed at the transition from the high-temperature hexagonal modification to the low-temperature orthorhombic modification. The structure was solved and refined to R = 0.059 in P21ca (No. 29) space group with a = 10.0713(5) Å, b = 8.7220(2) Å, and c = 12.2460(4) Å. The only structural analogue of the synthesized crystal is the high-temperature modification of K2Ca(CO3)2, which can be considered as a disordered analogue of Na2Ca(CO3)2. Structural analogues among borates and other classes of compounds have not been found. Based on group-subgroup analysis, we propose the structures of high- and intermediate-temperature modifications of Na2Ca(CO3)2. The relations of the determined structure with other polymorphs of Na2Ca(CO3)2 have also been considered.