Abstract

The structure of svyatoslavite, a pseudo-orthorhombic polymorph of CaAl2Si2O8, has been solved from a crystal twinned on (100) and refined to an R1 value of 0.024, calculated for the 1788 unique observed (|Fo| ≥ 4sF) reflections. The structure is monoclinic, P1211, a 8.220(5), b 8.951(5), c 4.828(5) Å, β 90.00(5)°, V 355.2(5) Å3. The structure of svyatoslavite is based on a three-dimensional framework of SiO4 and AlO4 tetrahedra with Ca2+ ions at the interstitial sites. There are two Ca sites, Ca1 and Ca2, with occupancy factors of 0.919(4) and 0.081(4), respectively. The Ca1 site is coordinated by six O atoms with Ca1–O bond lengths in the range 2.417–2.599 Å, with one long seventh Ca1–O bond of 3.068 Å. The Ca2 site is 6-coordinated with Ca2–O bond lengths in the range 2.380–2.775 Å. Framework of tetrahedra in svyatoslavite, as well as tetrahedral frameworks in other M2+[Al2Si2O8] polymorphs (M2+ = Ba2+, Ca2+), is based on an orthogonal network, i.e., a network with the angles between adjacent edges equal to either 90 or 180°. Growth of orthogonal nets is modeled using structural automata, which are finite automata adapted for the description of crystal structures. State diagrams for svyatoslavite and dmisteinbergite automata consist of four states each. The anorthite automaton is more complex as it contains eight states. The paracelsian automaton is remarkable in that it consists of 16 states and its state diagram has the topology of a four-dimensional cube (hypercube). During crystallization of the Ca[Al2Si2O8] melt, metastable phases with the svyatoslavite and dmisteinbergite topologies form first and then either dissolve or transform to anorthite. In terms of complexity of structural automata, this means that the less complex phases (svyatoslavite and dmisteinbergite) evolve into more complex anorthite structures. The observed sequence of phases corresponds to the increasing structural complexity of the solid system.

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