# Truncated tetrahedron

Template:Semireg polyhedron stat table In geometry, the truncated tetrahedron is an Archimedean solid. It has 4 regular hexagonal faces, 4 equilateral triangle faces, 12 vertices and 18 edges (of two types). It can be constructed by truncating all 4 vertices of a regular tetrahedron at one third of the original edge length.

A deeper truncation, removing a tetrahedron of half the original edge length from each vertex, is called rectification. The rectification of a tetrahedron produces an octahedron.[1]

A truncated tetrahedron is the Goldberg polyhedron GIII(1,1), containing triangular and hexagonal faces.

A truncated tetrahedron can be called a cantic cube, with Coxeter diagram, Template:CDD, having half of the vertices of the cantellated cube (rhombicuboctahedron), Template:CDD. There are two dual positions of this construction, and combining them creates the uniform compound of two truncated tetrahedra.

## Area and volume

The area A and the volume V of a truncated tetrahedron of edge length a are:

${\displaystyle A=7{\sqrt {3}}a^{2}\approx 12.12435565a^{2}}$
${\displaystyle V={\frac {23}{12}}{\sqrt {2}}a^{3}\approx 2.710575995a^{3}.}$

## Densest Packing

The densest packing of the Archimedean truncated tetrahedron is believed to be Φ = 207/208, as reported by two independent groups using Monte Carlo methods.[2][3] Although no mathematical proof exists that this is the best possible packing for those shapes, the high proximity to the unity and independency of the findings make it unlikely that an even denser packing is to be found. In fact, if the truncation of the corners is slightly smaller than that of an Archimedean truncated tetrahedron, this new shape can be used to completely fill space.[2]

## Cartesian coordinates

Cartesian coordinates for the 12 vertices of a truncated tetrahedron centered at the origin, with edge length √8, are all permutations of (±1,±1,±3) with an even number of minus signs:

• (+3,+1,+1), (+1,+3,+1), (+1,+1,+3)
• (−3,−1,+1), (−1,−3,+1), (−1,−1,+3)
• (−3,+1,−1), (−1,+3,−1), (−1,+1,−3)
• (+3,−1,−1), (+1,−3,−1), (+1,−1,−3)
 Orthogonal projection showing Cartesian coordinates inside it bounding box: (±3,±3,±3). The hexagonal faces of the truncated tetrahedra can be divided into 6 coplanar equilateral triangles. The 4 new vertices have Cartesian coordinates: (-1,-1,-1), (-1,+1,+1),(+1,-1,+1), (+1,+1,-1). The set of vertex permutations (±1,±1,±3) with an odd number of minus signs forms a complementary truncated tetrahedron, and combined they form a uniform compound polyhedron.

Another simple construction exists in 4-space as cells of the truncated 16-cell, with vertices as coordinate permutation of:

(0,0,1,2)

## Orthogonal projection

Orthogonal projection
Centered by Edge normal Face normal Edge Face/vertex
Image
Dual image
Projective
symmetry
[1] [1] [3] [4]

## Spherical tiling

The truncated tetrahedron can also be represented as a spherical tiling, and projected onto the plane via a stereographic projection. This projection is conformal, preserving angles but not areas or lengths. Straight lines on the sphere are projected as circular arcs on the plane.

Orthographic projection Stereographic projections triangle-centered hexagon-centered

### Friauf polyhedron

A lower symmetry version of the truncated tetrahedron (a truncated tetragonal disphenoid with order 8 D2d symmetry) is called a Friauf polyhedron in crystals such as complex metallic alloys. This form fits 5 Friauf polyhedra around an axis, giving a 72 degree dihedral angle on a subset of 6-6 edges.[4] Its named after J. B. Friauf and his 1927 paper The crystal structure of the intermetallic compound MgCu2. [5]

## Use in architecture

Giant truncated tetrahedra were used for the "Man the Explorer" and "Man the Producer" theme pavilions in Expo 67. They were made of massive girders of steel bolted together in a geometric lattice. The truncated tetrahedra were interconnected with lattice steel platforms. All of these buildings were demolished after the end of Expo 67, as they had not been built to withstand the severity of the Montreal weather over the years. Their only remnants are in the Montreal city archives, the Public Archives Of Canada and the photo collections of tourists of the times.[6]

## Truncated tetrahedral graph

Template:Infobox graph In the mathematical field of graph theory, a truncated tetrahedral graph is a Archimedean graph, the graph of vertices and edges of the truncated tetrahedron, one of the Archimedean solids. It has 12 vertices and 18 edges.[7] It is a connected cubic graph,[8] and connected cubic transitive graph.[9]

Circular Orthographic projections

4-fold symmetry

3-fold symmetry

## Related polyhedra and tilings

Dimensional family of cantic polyhedra and tilings: 3.6.n.6
Symmetry
*n32
[1+,2n,3]
= [(n,3,3)]
Spherical Planar Hyperbolic...
*332
[1+,4,3]
Td
*333
[1+,6,3]
P3m1
*433
[1+,8,3]...

*∞33
[1+,∞,3]

Cantic
figure

3.6.2.6

3.6.3.6

3.6.4.6

3.6.∞.6
Coxeter
Schläfli
Template:CDD
h2{4,3}
= Template:CDD
Template:CDD
h2{6,3}
= Template:CDD
Template:CDD
h2{8,3}
= Template:CDD
Template:CDD
h2{∞,3}
= Template:CDD
Dual figure
V3.6.2.6

V3.6.3.6

V3.6.4.6

V3.6.∞.6
Coxeter Template:CDD Template:CDD Template:CDD Template:CDD

This polyhedron is topologically related as a part of sequence of uniform truncated polyhedra with vertex configurations (3.2n.2n), and [n,3] Coxeter group symmetry.

## References

1. Template:Cite web
2. Template:Cite web
3. Template:Cite web
4. http://met.iisc.ernet.in/~lord/webfiles/clusters/polyclusters.pdf
5. Friauf, J. B. The crystal structure of the intermetallic compound MgCu2 (1927) J. Am. Chem. Soc. 19, 3107-3114.
6. http://expo67.ncf.ca/man_the_producer_p1.html
7. An Atlas of Graphs, page=267, truncated tetrahedral graph
8. An Atlas of Graphs, page=130, connected cubic graphs, 12 vertices, C105
9. An Atlas of Graphs, page=161, connected cubic transitive graphs, 12 vertices, Ct11
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