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

TL;DR

This study characterizes the incommensurately modulated twin structure of nyerereite using advanced crystallographic methods, revealing detailed atomic modulations and comparing it with related carbonate structures.

Contribution

First determination of nyerereite's incommensurate twin structure in (3+1)D symmetry, detailing atomic modulation functions and structural relationships.

Findings

Identified incommensurate modulation vector q = 0.383a*

Described occupation and displacive modulations of atoms and groups

Compared nyerereite structure with related carbonate structures

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.