Incommensurately modulated twin structure of nyerereite Na1.64K0.36Ca(CO3)2

Abstract

The incommensurately modulated twin structure of nyerereite Na1.64K0.36Ca(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.