Title: Incommensurately modulated twin structure of nyerereite Na1.64K0.36Ca(CO3)2,
Abstract: Incommensurately modulated twin structure of nyerereite Na1.64K0.36Ca(CO3)2
has been first determined in the (3+1)D symmetry group Cmcm({\alpha}00)00s with
modulation vector q = 0.383a*. Unit-cell values are a = 5.062(1), b = 8.790(1),
c = 12.744(1) {\AA}. Three orthorhombic components are related by threefold
rotation about [001]. Discontinuous crenel functions are used to describe
occupation modulation of Ca and some CO3 groups. Strong displacive modulation
of the oxygen atoms in vertexes of such CO3 groups is described using
x-harmonics in crenel intervals. The Na, K atoms occupy mixed sites whose
occupation modulation is described by two ways using either complementary
harmonic functions or crenels. The nyerereite structure has been compared both
with commensurately modulated structure of K-free Na2Ca(CO3)2 and with widely
known incommensurately modulated structure of {\gamma}-Na2CO3.