New Data on Astrophyllite Supergroup Minerals: Crystall Chemical Mechanisms of the Formation of Triclinic and Monoclinic Structures

Abstract

Crystall chemical mechanisms of the formation of triclinic and monoclinic species of astrophyllite supergroup minerals have been considered using the concept of mixed anion complexes and modular approach, as well as taking into account the specific features of the composition, structure, and symmetry of complexes with 1D or 2D periodicity. The structures of astrophyllite supergroup minerals are based on three-layer HOH-packages, where O is a layer consisting of octahedra and H are layers consisting of $[{\text{S}}{{{\text{i}}}_{{\text{4}}}}{{{\text{O}}}_{{{\text{12}}}}}]_{\infty }^{{8-}}$ chains and D-polyhedra (^[6,5]D = Ti, Nb, Zr, Fe³⁺). Large cations (mainly K⁺ and Nа⁺) are located in the interpackage space. The complexes exhibiting 1D periodicity along the shortest (~5.3 Å) parameter are selected in the HOH-packages; their peripheral parts are formed by (I) $({\text{S}}{{{\text{i}}}_{{\text{2}}}}{{{\text{O}}}_{{\text{6}}}})_{\infty }^{{4-}}$ and (II) $({\text{TiS}}{{{\text{i}}}_{{\text{2}}}}{{{\text{O}}}_{{\text{6}}}}{\text{O}})_{\infty }^{{2-}}$ blocks, and the central part is formed by ribbons composed of octahedra. The formation of HOH-packages of two types, which differ radically in the triclinic and monoclinic structures, is determined by the way in which the octahedra of O-layer are combined with blocks I and II of H-layers.