Nyerereite has the idealized composition (Na 0.82 K 0.18 ) 2 Ca(CO 3 ) 2 with significant minor substitution of SO 4 2− , PO 4 3− , F − for CO 3 2− , optical properties α = 1.5112, β = 1.5333, γ = 1.5345, 2 V α = 29°, and density 2.541 g cm −3 . The average unit cell has space group Cmc 2 1 , dimensions a = 5.044, b = 8.809, c = 12.743 Å and contains four formula units. Non-Bragg reflexions have h = n + mδ where n and m are integers and δ = 0.383 and indicate a structural modulation. The natural mineral is an inversion product from a high-temperature hexagonal polymorph, high nyerereite with a = 5.05, c = l2.85 Å and space group P 6 3 mc . The average structure of the natural mineral (low nyerereite) is discussed and the structure of high nyerereite is inferred. The high-nyerereite structure is the aristotype for all phases except buetschliite in the system Na 2 Ca(CO 3 ) 2 –K 2 Ca(CO 3 ) 2 . Mineralogical nomenclature in this system is discussed.