On Two Highly Symmetric Stencils for Crystal Structures of Inorganic Compounds

A crystallographic analysis of hexagonal poudretteite (KNa₂B₃Si₁₂O₃₀, symmetry P6/mcc) and cubic pollucite (CsAlSi₂O₆, Ia(Formula presented.)d) has revealed characteristic features of two versions of energetically stable atomic configurations, which are abundant among natural and synthesized inorganic compounds. These are structures with a minimum number of parameters (not fixed by the symmetry of basis-atom coordinates) and minimum volumes V* of independent parts of unit cells. The structures of silicates with a large number of tetrahedral cations exhibit a symmetry of glide planes and screw axes, which provides both stability and close packing of topologically connected fragments (templates) of these structures.