Platonic solids [1] are the five polyhedrons with equivalent facets composed of congruent convex regular polygons, in which the same number of facets meet at every vertex. The octahedron, with eight facets, is a platonic shape composed of triangles, and the dodecahedron, with 12 facets, is the only one with pentagonal facets. In nature, pyrite frequently occurs as octahedral or dodecahedral crystals. Certain viruses and radiolaria also routinely take the form of these regular polyhedral shapes. [2] A number of synthetic inorganic crystals with platonic shapes have been reported recently, such as gold nanocrystals, [3a] mesoporous silicate dodecahedra and octahedra, [3b,c] SnO 2 octahedra, [3d] In 2 O 3 and Cu 2 O octahedra, [3e,f] as well as metallic alloy quasi-crystals. [3g-i] For the trigonal CaCO 3 calcite crystal structure, the formation of platonic shapes is, owing to a too-low symmetry, not to be expected. Calcite pseudo-dodecahedra, nevertheless, were synthesized with a variety of additives, such as Mg 2+ , [4a-c] Co 2+ , [4d] simple organic acids, [4e] phosphate, [4f] carboxylate-rich polymers, [4g] amino acids, [4h] calcite-binding helical peptide, [4i] globular proteins, [4j] protein extracts isolated from aragonitic abalone shell nacre [4k] and from sea urchins. [4l,m] The mechanisms of crystal growth of these calcite pseudo-dodecahdra are additive adsorption on the {011} faces, [4l, n] the inhibiting effect of additives on step-growth, [4c] or the combination of the two functions. [4] Calcite mesocrystals with pseudo-octahedral morphology [5a,b] and otoconia-like morphology [5c] were also described.
However, a unified route towards more than one platonic calcite shape based on one same mechanistic approach has not yet been revealed. Herein, we present a nonclassical crystallization approach towards calcite with a platonic shape based on nanoparticle aggregation. We show that a commercial co-polyelectrolyte can be used to manoeuvre the crystal-