The incommensurately modulated twin structure of nyerereite Na_1.64K_0.36Ca(CO₃)₂has been first determined in the (3 + 1)-dimensional symmetry groupCmcm(α00)00swith modulation vectorq= 0.383a*. Unit-cell values area= 5.062 (1),b= 8.790 (1),c= 12.744 (1) Å. Three orthorhombic components are related by threefold rotation about [001]. Discontinuous crenel functions are used to describe the occupation modulation of Ca and some CO₃groups. The strong displacive modulation of the O atoms in vertexes of such CO₃groups is described using x-harmonics in crenel intervals. The Na, K atoms occupy mixed sites whose occupation modulation is described in two ways using either complementary harmonic functions or crenels. The nyerereite structure has been compared both with the commensurately modulated structure of K-free Na₂Ca(CO₃)₂and with the widely known incommensurately modulated structure of γ-Na₂CO₃.