Rhombic dodecahedron

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Rhombic dodecahedron
Rhombicdodecahedron.jpg
(Click here for rotating model)
Type Catalan solid
Coxeter diagram CDel node f1.pngCDel 3.pngCDel node.pngCDel 3.pngCDel node f1.png
CDel node.pngCDel 4.pngCDel node f1.pngCDel 3.pngCDel node.png
Conway notation jC
Face type V3.4.3.4
DU07 facets.png

rhombus
Faces 12
Edges 24
Vertices 14
Vertices by type 8{3}+6{4}
Symmetry group Oh, B3, [4,3], (*432)
Rotation group O, [4,3]+, (432)
Dihedral angle 120°
Properties convex, face-transitive isohedral, isotoxal, parallelohedron
Cuboctahedron.png
Cuboctahedron
(dual polyhedron)
Rhombic dodecahedron Net
Net
3d model of a rhombic dodecahedron

In geometry, the rhombic dodecahedron is a convex polyhedron with 12 congruent rhombic faces. It has 24 edges, and 14 vertices of two types. It is a Catalan solid, and the dual polyhedron of the cuboctahedron.

Properties[edit]

The rhombic dodecahedron is a zonohedron. Its polyhedral dual is the cuboctahedron. The long diagonal of each face is exactly 2 times the length of the short diagonal, so that the acute angles on each face measure arccos(1/3), or approximately 70.53°.

Being the dual of an Archimedean polyhedron, the rhombic dodecahedron is face-transitive, meaning the symmetry group of the solid acts transitively on the set of faces. In elementary terms, this means that for any two faces A and B there is a rotation or reflection of the solid that leaves it occupying the same region of space while moving face A to face B.

The rhombic dodecahedron can be viewed as the convex hull of the union of the vertices of a cube and an octahedron. The 6 vertices where 4 rhombi meet correspond to the vertices of the octahedron, while the 8 vertices where 3 rhombi meet correspond to the vertices of the cube.

The rhombic dodecahedron is one of the nine edge-transitive convex polyhedra, the others being the five Platonic solids, the cuboctahedron, the icosidodecahedron and the rhombic triacontahedron.

The rhombic dodecahedron can be used to tessellate three-dimensional space. It can be stacked to fill a space much like hexagons fill a plane.

This polyhedron in a space-filling tessellation can be seen as the Voronoi tessellation of the face-centered cubic lattice. It is the Brillouin zone of body centered cubic (bcc) crystals. Some minerals such as garnet form a rhombic dodecahedral crystal habit. As Johannes Kepler noted in his 1611 book on snowflakes (Strena seu de Nive Sexangula), honey bees use the geometry of rhombic dodecahedra to form honeycombs from a tessellation of cells each of which is a hexagonal prism capped with half a rhombic dodecahedron. The rhombic dodecahedron also appears in the unit cells of diamond and diamondoids. In these cases, four vertices (alternate threefold ones) are absent, but the chemical bonds lie on the remaining edges.[1]

The graph of the rhombic dodecahedron is nonhamiltonian.

A rhombic dodecahedron can be dissected with its center into 4 trigonal trapezohedra. These rhombohedra are the cells of a trigonal trapezohedral honeycomb. This is analogous to the dissection of a regular hexagon dissected into rhombi, and tiled in the plane as a rhombille.

The collections of the Louvre include a die in the shape of a rhombic dodecahdron dating from Ptolemaic Egypt. The faces are inscribed with Greek letters representing the numbers 1 through 12: Α Β Γ Δ Ε Ζ Ϛ Η Θ Ι ΙΑ ΙΒ. The function of the die is unknown.[2]

Dimensions[edit]

If the edge length of a rhombic dodecahedron is a, the radius of an inscribed sphere (tangent to each of the rhombic dodecahedron's faces) is

OEISA157697

and the radius of the midsphere is

OEISA179587.

Area and volume[edit]

The area A and the volume V of the rhombic dodecahedron of edge length a are:

Orthogonal projections[edit]

The rhombic dodecahedron has four special orthogonal projections along its axes of symmetry, centered on a face, an edge, and the two types of vertex, threefold and fourfold. The last two correspond to the B2 and A2 Coxeter planes.

Orthogonal projections
Projective
symmetry
[4] [6] [2] [2]
Rhombic
dodecahedron
Dual cube t1 B2.png Dual cube t1.png Dual cube t1 e.png Dual cube t1 v.png
Cuboctahedron
(dual)
3-cube t1 B2.svg 3-cube t1.svg Cube t1 e.png Cube t1 v.png

Cartesian coordinates[edit]

Pyritohedron animation.gif
Pyritohedron variations between a cube and rhombic dodecahedron
R1-R3.gif
Expansion of a rhombic dodecahedron

The eight vertices where three faces meet at their obtuse angles have Cartesian coordinates:

(±1, ±1, ±1)

The coordinates of the six vertices where four faces meet at their acute angles are:

(±2, 0, 0), (0, ±2, 0) and (0, 0, ±2)

The rhombic dodecahedron can be seen as a degenerate limiting case of a pyritohedron, with permutation of coordinates (±1, ±1, ±1) and (0, 1 + h, 1 − h2) with parameter h = 1.

Topologically equivalent forms[edit]

Parallelohedron[edit]

The rhombic dodecahedron is a parallelohedron, a space-filling polyhedron, dodecahedrille, being the dual to the tetroctahedrille or half cubic honeycomb, and described by two Coxeter diagrams: CDel node f1.pngCDel 3.pngCDel node.pngCDel split1-43.pngCDel nodes.png and CDel node f1.pngCDel split1.pngCDel nodes.pngCDel split2.pngCDel node.png. With D3d symmetry, it can be seen as an elongated trigonal trapezohedron.

Rhombic dodecahedra.png
The rhombic dodecahedron can tessellate space by translational copies of itself. So can the stellated rhombic dodecahedron.
Parallelohedron edges rhombic dodecahedron.png
The rhombic dodecahedron can be constructed with 4 sets of parallel edges.

Dihedral rhombic dodecahedron[edit]

Other symmetry constructions of the rhombic dodecahedron are also space-filling, and as parallelotopes they are similar to variations of space-filling truncated octahedra.[3]

For example, with 4 square faces, and 60-degree rhombic faces, and D4h dihedral symmetry, order 16. It can be seen as a cuboctahedron with square pyramids augmented on the top and bottom.

Squared rhombic dodecahedron.png Squared rhombic dodecahedron net.png
Net
Coordinates
(0, 0, ±2)
(±1, ±1, 0)
(±1, 0, ±1)
(0, ±1, ±1)

Bilinski dodecahedron[edit]

Bilinski dodecahedron.png
Bilinski dodecahedron with edges and front faces colored by their symmetry positions.
Bilinski dodecahedron parallelohedron.png
Bilinski dodecahedron colored by parallel edges

In 1960 Stanko Bilinski discovered a second rhombic dodecahedron with 12 congruent rhombus faces, the Bilinski dodecahedron. It has the same topology but different geometry. The rhombic faces in this form have the golden ratio.[4][5]

Faces
First form Second form
DU07 facets.png GoldenRhombus.svg
2:1 5 + 1/2:1

Deltoidal dodecahedron[edit]

Example net (3/4,3/2)

Another topologically equivalent variation, sometimes called a deltoidal dodecahedron[6] or trapezoidal dodecahedron,[7][8] is isohedral with tetrahedral symmetry order 24, distorting rhombic faces into kites (deltoids). It has 8 vertices adjusted in or out in alternate sets of 4, with the limiting case a tetrahedral envelope. Variations can be parametrized by (a,b), where b and a depend on each other such that the tetrahedron defined by the four vertices of a face has volume zero, i.e. is a planar face. (1,1) is the rhombic solution. As (a) approaches 1/2, (b) approaches infinity. Always holds 1/a + 1/b = 2, with a,b > 1/2.

(±2, 0, 0), (0, ±2, 0), (0, 0, ±2)
(a, a, a), (−a, −a, a), (−a, a, −a), (a, −a, −a)
(−b, −b, −b), (−b, b, b), (b, −b, b), (b, b, −b)
(1,1) (7/8,7/6) (3/4,3/2) (2/3,2) (5/8,5/2) (9/16,9/2)
Rhombic dodecahedron.png Skew rhombic dodecahedron-116.png Skew rhombic dodecahedron-150.png Skew rhombic dodecahedron-200.png Skew rhombic dodecahedron-250.png Skew rhombic dodecahedron-450.png

Related polyhedra[edit]

Spherical rhombic dodecahedron

When projected onto a sphere (see right), it can be seen that the edges make up the edges of two tetrahedra arranged in their dual positions (the stella octangula). This trend continues on with the deltoidal icositetrahedron and deltoidal hexecontahedron for the dual pairings of the other regular polyhedra (alongside the triangular bipyramid if improper tilings are to be considered), giving this shape the alternative systematic name of deltoidal dodecahedron.

*n42 symmetry mutation of dual expanded tilings: V3.4.n.4
Symmetry
*n32
[n,3]
Spherical Euclid. Compact hyperb. Paraco.
*232
[2,3]
*332
[3,3]
*432
[4,3]
*532
[5,3]
*632
[6,3]
*732
[7,3]
*832
[8,3]...
*∞32
[∞,3]
Figure
Config.
Spherical trigonal bipyramid.png
V3.4.2.4
Spherical rhombic dodecahedron.png
V3.4.3.4
Spherical deltoidal icositetrahedron.png
V3.4.4.4
Spherical deltoidal hexecontahedron.png
V3.4.5.4
Tiling Dual Semiregular V3-4-6-4 Deltoidal Trihexagonal.svg
V3.4.6.4
Deltoidal triheptagonal tiling.svg
V3.4.7.4
H2-8-3-deltoidal.svg
V3.4.8.4
Deltoidal triapeirogonal til.png
V3.4.∞.4

This polyhedron is a part of a sequence of rhombic polyhedra and tilings with [n,3] Coxeter group symmetry. The cube can be seen as a rhombic hexahedron where the rhombi are squares.

Similarly it relates to the infinite series of tilings with the face configurations V3.2n.3.2n, the first in the Euclidean plane, and the rest in the hyperbolic plane.

Rhombicdodecahedron net2.png
V3.4.3.4
(Drawn as a net)
Tile V3636.svg
V3.6.3.6
Euclidean plane tiling
Rhombille tiling
Uniform dual tiling 433-t01.png
V3.8.3.8
Hyperbolic plane tiling
(Drawn in a Poincaré disk model)

Stellations[edit]

Like many convex polyhedra, the rhombic dodecahedron can be stellated by extending the faces or edges until they meet to form a new polyhedron. Several such stellations have been described by Dorman Luke.[9]

This animation shows the construction of a stellated rhombic dodecahedron by inverting the center-face pyramids of a rhombic dodecahedron.

The first stellation, often simply called the stellated rhombic dodecahedron, is well known. It can be seen as a rhombic dodecahedron with each face augmented by attaching a rhombic-based pyramid to it, with a pyramid height such that the sides lie in the face planes of the neighbouring faces:

Luke describes four more stellations: the second and third stellations (expanding outwards), one formed by removing the second from the third, and another by adding the original rhombic dodecahedron back to the previous one.

Second Third
Stellated rhombic dodecahedron.png
Stellated rhombic dodecahedron
Great rhombic dodecahedron.png
Great stellated rhombic dodecahedron

Related polytopes[edit]

In a perfect vertex-first projection two of the tesseract's vertices (marked in pale green) are projected exactly in the center of the rhombic dodecahedron

The rhombic dodecahedron forms the hull of the vertex-first projection of a tesseract to three dimensions. There are exactly two ways of decomposing a rhombic dodecahedron into four congruent rhombohedra, giving eight possible rhombohedra as projections of the tesseracts 8 cubic cells. One set of projective vectors are: u=(1,1,-1,-1), v=(-1,1,-1,1), w=(1,-1,-1,1).

The rhombic dodecahedron forms the maximal cross-section of a 24-cell, and also forms the hull of its vertex-first parallel projection into three dimensions. The rhombic dodecahedron can be decomposed into six congruent (but non-regular) square dipyramids meeting at a single vertex in the center; these form the images of six pairs of the 24-cell's octahedral cells. The remaining 12 octahedral cells project onto the faces of the rhombic dodecahedron. The non-regularity of these images are due to projective distortion; the facets of the 24-cell are regular octahedra in 4-space.

This decomposition gives an interesting method for constructing the rhombic dodecahedron: cut a cube into six congruent square pyramids, and attach them to the faces of a second cube. The triangular faces of each pair of adjacent pyramids lie on the same plane, and so merge into rhombuses. The 24-cell may also be constructed in an analogous way using two tesseracts.[10]

Practical usage[edit]

In spacecraft reaction wheel layout, a tetrahedral configuration of four wheels is commonly used. For wheels that perform equally (from a peak torque and max angular momentum standpoint) in both spin directions and across all four wheels, the maximum torque and maximum momentum envelopes for the 3-axis attitude control system (considering idealized actuators) are given by projecting the tesseract representing the limits of each wheel's torque or momentum into 3D space via the 3 × 4 matrix of wheel axes; the resulting 3D polyhedron is a rhombic dodecahedron.[11] Such an arrangement of reaction wheels is not the only possible configuration (a simpler arrangement consists of three wheels mounted to spin about orthogonal axes), but it is advantageous in providing redundancy to mitigate the failure of one of the four wheels (with degraded overall performance available from the remaining three active wheels) and in providing a more convex envelope than a cube, which leads to less agility dependence on axis direction (from an actuator/plant standpoint). Spacecraft mass properties influence overall system momentum and agility, so decreased variance in envelope boundary does not necessarily lead to increased uniformity in preferred axis biases (that is, even with a perfectly distributed performance limit within the actuator subsystem, preferred rotation axes are not necessarily arbitrary at the system level).

See also[edit]

References[edit]

  1. ^ Dodecahedral Crystal Habit Archived 2009-04-12 at the Wayback Machine. khulsey.com
  2. ^ Perdrizet, Paul. (1930). "Le jeu alexandrin de l'icosaèdre". Bulletin de l'Institut français d'archéologie orientale. 30: 1–16.
  3. ^ Order in Space: A design source book, Keith Critchlow, p.56–57
  4. ^ Branko Grünbaum (2010). "The Bilinski Dodecahedron and Assorted Parallelohedra, Zonohedra, Monohedra, Isozonohedra, and Otherhedra" (PDF). 32 (4): 5–15. Archived from the original (PDF) on 2015-04-02. Cite journal requires |journal= (help)
  5. ^ H.S.M Coxeter, "Regular polytopes", Dover publications, 1973.
  6. ^ Economic Mineralogy: A Practical Guide to the Study of Useful Minerals, p.8
  7. ^ http://mathworld.wolfram.com/Isohedron.html
  8. ^ http://loki3.com/poly/transforms.html
  9. ^ Luke, D. (1957). "Stellations of the rhombic dodecahedron". The Mathematical Gazette. 41 (337): 189–194. doi:10.2307/3609190. JSTOR 3609190.
  10. ^ https://www.youtube.com/watch?v=oJ7uOj2LRso
  11. ^ Markley, F. Landis (September 2010). "Maximum Torque and Momentum Envelopes for Reaction-Wheel Arrays". ntrs.nasa.gov. Retrieved 2020-08-20.

Further reading[edit]

External links[edit]

Computer models[edit]

Paper projects[edit]

Practical applications[edit]