Some dodecahedra have the same combinatorial structure as the regular dodecahedron (in terms of the graph formed by its vertices and edges), but their pentagonal faces are not regular:Thepyritohedron, a common crystal form inpyrite, haspyritohedral symmetry, while thetetartoid hastetrahedral symmetry.
While the regular dodecahedron shares many features with other Platonic solids, one unique property of it is that one can start at a corner of the surface and draw an infinite number of straight lines across the figure that return to the original point without crossing over any other corner.[3]
Theregular dodecahedron is a convex polyhedron with regular pentagonal faces, three meeting at each vertex. It has 12 faces, 30 edges, and 20 vertices.[4] It is one of the five regularPlatonic solids, named afterPlato who described them and considered the other four to symbolize theclassical elements; he assigned the regular dodecahedron to the cosmos.[5] Itsdual is theregular icosahedron.[6]
Left to right: Regular dodecahedron, small stellated dodecahedron, great dodecahedron, great stellated dodecahedron
The regular dodecahedron has threestellations, all of which are regular star dodecahedra. They form three of the fourKepler–Poinsot polyhedra. They are thesmall stellated dodecahedron, thegreat dodecahedron, and thegreat stellated dodecahedron.[7] The small stellated dodecahedron and great dodecahedron are dual to each other; the great stellated dodecahedron is dual to thegreat icosahedron. All of these regular star dodecahedra have regular pentagonal orpentagrammic faces. The convex regular dodecahedron and great stellated dodecahedron are different realisations of the sameabstract regular polyhedron; the small stellated dodecahedron and great dodecahedron are different realisations of another abstract regular polyhedron.
While the three star forms satisfy the requirements for regularity and have twelve faces, and therefore any of them could also be referred to as a "regular dodecahedron", this specific term is exclusively reserved for the first, convex form. A fifth shape which could be classified as a regular dodecahedron, namely thedodecagonalhosohedron, is also not referred to as such, in part because it exists only as aspherical polyhedron and is degenerate in Euclidean space.
Apyritohedron (or pentagonal dodecahedron) is a dodecahedron withpyritohedral symmetry Th. Like theregular dodecahedron, it has twelve identicalpentagonal faces, with three meeting in each of the 20 vertices. However, the pentagons are not constrained to be regular, and the underlying atomic arrangement has no true fivefold symmetry axis. Its 30 edges are divided into two sets, containing 24 and 6 edges of the same length. The only axes ofrotational symmetry are three mutually perpendicular twofold axes and four threefold axes.[8]
The namecrystal pyrite comes from one of the two commoncrystal habits shown bypyrite (the other one being thecube). In pyritohedral pyrite, the faces have aMiller index of (210), which means that thedihedral angle is 2·arctan(2) ≈ 126.87° and each pentagonal face has one angle of approximately 121.6° in between two angles of approximately 106.6° and opposite two angles of approximately 102.6°. The following formulas show the measurements for the face of a perfect crystal (which is rarely found in nature).
The pyritohedron has a geometric degree of freedom withlimiting cases of a cubicconvex hull at one limit of collinear edges, and arhombic dodecahedron as the other limit as 6 edges are degenerated to length zero. The regular dodecahedron represents a special intermediate case where all edges and angles are equal.
It is possible to go past these limiting cases, creating concave or nonconvex pyritohedra. Theendo-dodecahedron is concave and equilateral; it can tessellate space with the convex regular dodecahedron. Continuing from there in that direction, we pass through a degenerate case where twelve vertices coincide in the centre, and on to the regulargreat stellated dodecahedron where all edges and angles are equal again, and the faces have been distorted into regularpentagrams. On the other side, past the rhombic dodecahedron, we get a nonconvex equilateral dodecahedron with fish-shaped self-intersecting equilateral pentagonal faces.
Special cases of the pyritohedron
Versions with equal absolute values and opposing signs form a honeycomb together. (Comparethis animation.) The ratio shown is that of edge lengths, namely those in a set of 24 (touching cube vertices) to those in a set of 6 (corresponding to cube faces).
Atetartoid (alsotetragonal pentagonal dodecahedron,pentagon-tritetrahedron, andtetrahedric pentagon dodecahedron) is a dodecahedron with chiraltetrahedral symmetry (T). Like theregular dodecahedron, it has twelve identicalpentagonal faces, with three meeting in each of the 20 vertices. However, the pentagons are not regular and the figure has no fivefold symmetry axes.
Although regular dodecahedra do not exist in crystals, the tetartoid form does. The name tetartoid comes from the Greek root for one-fourth because it has one fourth of full octahedral symmetry, and half of pyritohedral symmetry.[11] The mineralcobaltite can have this symmetry form.[12]
Abstractions sharing the solid'stopology and symmetry can be created from the cube and the tetrahedron. In the cube each face is bisected by a slanted edge. In the tetrahedron each edge is trisected, and each of the new vertices connected to a face center. (InConway polyhedron notation this is a gyro tetrahedron.)
Orthographic projections from 2- and 3-fold axes
Cubic and tetrahedral form
Relationship to the dyakis dodecahedron
A tetartoid can be created by enlarging 12 of the 24 faces of adyakis dodecahedron.(The tetartoid shown here is based on one that is itself created by enlarging 24 of the 48 faces of thedisdyakis dodecahedron.)
Chiral tetartoids based on the dyakis dodecahedron in the middle
Crystal model
Thecrystal model on the right shows a tetartoid created by enlarging the blue faces of the dyakis dodecahedral core. Therefore, the edges between the blue faces are covered by the red skeleton edges.
Theregular dodecahedron is a tetartoid with more than the required symmetry. Thetriakis tetrahedron is a degenerate case with 12 zero-length edges. (In terms of the colors used above this means, that the white vertices and green edges are absorbed by the green vertices.)
Therhombic dodecahedron can be seen as a degeneratepyritohedron where the 6 special edges have been reduced to zero length, reducing the pentagons into rhombic faces.
Another important rhombic dodecahedron, theBilinski dodecahedron, has twelve faces congruent to those of therhombic triacontahedron, i.e. the diagonals are in the ratio of thegolden ratio. It is also azonohedron and was described byBilinski in 1960.[16] This figure is another spacefiller, and can also occur in non-periodicspacefillings along with the rhombic triacontahedron, the rhombic icosahedron, and rhombic hexahedra.[17]
There are 6,384,634 topologically distinctconvex dodecahedra, excluding mirror images—the number of vertices ranges from 8 to 20.[18] Two polyhedra aretopologically distinct if they have intrinsically different arrangements of faces and vertices, such that it is impossible to distort one into the other simply by changing the lengths of edges or the angles between edges or faces.
Topologically, notably distinct dodecahedra (excluding pentagonal and rhombic forms) include:
Decagonal prism, aprism consisting of ten squares and two decagonal bases. Its symmetry group isD10h symmetry of order 40.[19]
^Brigaglia, Aldo; Palladino, Nicla; Vaccaro, Maria Alessandra (2018). "Historical notes on star geometry in mathematics, art and nature". In Emmer, Michele; Abate, Marco (eds.).Imagine Math 6: Between Culture and Mathematics. Springer International Publishing. pp. 197–211.doi:10.1007/978-3-319-93949-0_17.hdl:10447/325250.ISBN978-3-319-93948-3.
^Koca, Mehmet; Ozdes Koca, Nazife; Koc, Ramazon (2010). "Catalan Solids Derived From 3D-Root Systems and Quaternions".Journal of Mathematical Physics.51 (4).arXiv:0908.3272.doi:10.1063/1.3356985.