title: 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 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 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.
authors.affiliation: Shubnikov Institute of Crystallography, Federal Scientific Research Centre `Crystallography and Photonics' of Russian Academy of Sciences, Leninskii prosp. 59, 119333 Moscow, Russian Federation.
authors.forename: Nadezhda B
authors.initials: NB
authors.lastname: Bolotina
authors.affiliation: Sobolev Institute of Geology and Mineralogy, Siberian Branch of Russian Academy of Sciences, prosp. acad. Koptyuga 3, 630090 Novosibirsk, Russian Federation.
authors.forename: Pavel N
authors.initials: PN
authors.lastname: Gavryushkin
authors.affiliation: Sobolev Institute of Geology and Mineralogy, Siberian Branch of Russian Academy of Sciences, prosp. acad. Koptyuga 3, 630090 Novosibirsk, Russian Federation.
authors.forename: Andrey V
authors.initials: AV
authors.lastname: Korsakov
authors.affiliation: Sobolev Institute of Geology and Mineralogy, Siberian Branch of Russian Academy of Sciences, prosp. acad. Koptyuga 3, 630090 Novosibirsk, Russian Federation.
authors.forename: Sergey V
authors.initials: SV
authors.lastname: Rashchenko
authors.affiliation: Sobolev Institute of Geology and Mineralogy, Siberian Branch of Russian Academy of Sciences, prosp. acad. Koptyuga 3, 630090 Novosibirsk, Russian Federation.
authors.forename: Yurii V
authors.initials: YV
authors.lastname: Seryotkin
authors.affiliation: Sobolev Institute of Geology and Mineralogy, Siberian Branch of Russian Academy of Sciences, prosp. acad. Koptyuga 3, 630090 Novosibirsk, Russian Federation.
authors.forename: Alexander V
authors.initials: AV
authors.lastname: Golovin
authors.affiliation: Laboratoire Magmas et Volcans, UMR6524 CNRS Université J. Monnet, 23 rue du Dr. P. Michelon, F-42023 Saint-Etienne, France.
authors.forename: Bertrand N
authors.initials: BN
authors.lastname: Moine
authors.affiliation: Department of Mineralogy, St. Petersburg State University, University Emb. 7/9, 199034 St Petersburg, Russian Federation.
authors.forename: Anatoly N
authors.initials: AN
authors.lastname: Zaitsev
authors.affiliation: Sobolev Institute of Geology and Mineralogy, Siberian Branch of Russian Academy of Sciences, prosp. acad. Koptyuga 3, 630090 Novosibirsk, Russian Federation.
authors.forename: Konstantin D
authors.initials: KD
authors.lastname: Litasov
mesh_terms: D018360:Crystallography, X-Ray; D008903:Minerals / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation
chemical_list: D008903:Minerals; C000626345:nyerereite
keywords: X-ray analysis; carbonates; incommensurately modulated structures; minerals; nyerereite; twinning