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The incommensurately modulated twin structure of nyerereite Na 1.64 K 0.36 Ca(CO 3 ) 2 has been first determined in the (3 + 1)-dimensional symmetry group Cmcm(α00)00s with modulation vector q = 0.383a*. Unit-cell values are a = 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 CO 3 groups. The strong displacive modulation of the O atoms in vertexes of such CO 3 groups 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 Na 2 Ca(CO 3 ) 2 and with the widely known incommensurately modulated structure of γ-Na 2 CO 3 .