title: Communication between cation environments in aluminosilicate frameworks: incommensurately modulated crystal structure of an e-plagioclase.
abstract: Despite being one of the most common minerals in the earth's crust the crystal structure of intermediate e-plagioclase remains only partially understood, due in a large part to its complex diffraction patterns including satellite reflections. In this article we present a detailed analysis of the structure of e-plagioclase (An44) using single-crystal X-ray diffraction measured at ambient and low temperature (T = 100 K), in which the full modulated structure is successfully refined. As in earlier studies, the diffraction pattern exhibits strong main a-reflections and weak e-satellite reflections. The average structure could be solved in terms of an albite-like basic cell with the triclinic centrosymmetric and non-centrosymmetric space groups P \bar 1 and P1 (treated in its C \bar 1 and C1 setting, respectively, to follow conventions in the literature), while the incommensurately modulated structure was modeled in (3 + 1)D superspace, employing both the centro- and non-centrosymmetric superspace groups X \bar 1(αβγ)0 and X1(αβγ)0, where X refers to a special (3 + 1)D lattice centering with centering vectors (0 0 ½ ½), (½ ½ 0 ½), and (½ ½ ½ 0). Individual positional and occupational modulations for Ca/Na were refined with deeper insights being revealed in the non-centrosymmetric structure model. Through the structural details emerging from this model, the origin of the modulation can be traced to the communication between Ca/Na site positions through their bridging aluminosilicate (Si/Al)O4tetrahedra.
authors.affiliation: Department of Geoscience, University of Wisconsin, 1215 W Dayton Street, Madison, WI 53706, USA.
authors.forename: Rie T
authors.initials: RT
authors.lastname: Fredrickson
authors.affiliation: Department of Chemistry, University of Wisconsin, 1101 University Ave, Madison, WI 53706, USA.
authors.forename: Daniel C
authors.initials: DC
authors.lastname: Fredrickson
mesh_terms: D017607:Aluminum Compounds / Q000737:chemistry*; D000538:Aluminum Silicates / Q000737:chemistry; D002412:Cations / Q000737:chemistry; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D017640:Silicates / Q000737:chemistry*
chemical_list: D017607:Aluminum Compounds; D000538:Aluminum Silicates; D002412:Cations; D017640:Silicates; C000600851:plagioclase; C049037:aluminosilicate
keywords: incommensurability; modulated structure; plagioclase; single-crystal X-ray diffraction; superspace groups