Small rhombicosidodecahedron

The small rhombicosidodecahedron, or srid, also commonly known as simply the rhombicosidodecahedron, is one of the 13 Archimedean solids. It consists of 20 triangles, 30 squares, and 12 pentagons, with 1 triangle, 2 squares, and 1 pentagon meeting at each vertex. It can be obtained by cantellation of the dodecahedron or icosahedron, or equivalently by expanding either polyhedron's faces outward.

Vertex coordinates
A small rhombicosidodecahedron of edge length 1 has vertex coordinates given by all permutations of along with all even permutations of
 * $$\left(±\frac{2+\sqrt5}{2},\,±\frac12,\,±\frac12\right),$$
 * $$\left(0,\,±\frac{3+\sqrt5}{4},\,±\frac{5+\sqrt5}{4}\right),$$
 * $$\left(±\frac{1+\sqrt5}{4},\,±\frac{1+\sqrt5}{2},\,±\frac{3+\sqrt5}{4}\right).$$

Representations
A small rhombicosidodecahedron has the following Coxeter diagrams:


 * x5o3x (full symmetry)
 * oxxFofxx5xxfoFxxo&#xt (H2 axial, pentagon-first)
 * xx(oF)fVxF(Vx)fo3of(Vx)FxVf(oF)xx&#xt (A2 symmetry, triangle-first)

Semi-uniform variant
The small rhombicosidodecahedron has a semi-uniform variant of the form x5o3y that maintains its full symmetry. This variant has 12 pentagons of side length x, 20 triangles of size length y, and 30 rectangles as faces.

With edges of length a (of pentagons) and b (of triangles), its circumradius is given by $$\sqrt{\frac{9a^2+5b^2+8ab+(3a^2+b^2+4ab)\sqrt5}{8}}$$ and its volume is given by $$\frac{15a^3+45a^2b+15ab^2+5b^2}{4}+(21a^3+45a^2b+45ab^2+5b^3)\frac{\sqrt5}{12}$$.

Related polyhedra
The small rhombicosidodecahedron is the colonel of a three-member regiment that also includes the small dodecicosidodecahedron and the small rhombidodecahedron.

It is possible to cut off a pentagonal cupola cap from the rhombicosidodecahedron to diminish it, or to gyrate any such cap by 36° (so squares connect to other squares, and triangles connect to pentagons). The various combinations of diminishings and gyrations lead to a total of 12 Johnson solids:


 * Gyrate rhombicosidodecahedron - One cap rotated
 * Parabigyrate rhombicosidodecahedron - Two opposite caps rotated
 * Metabigyrate rhombicosidodecahedron - Two non-opposite, non-adjacent caps rotated
 * Trigyrate rhombicosidodecahedron - Three non-adjacent caps rotated
 * Diminished rhombicosidodecahedron - One cap removed
 * Paragyrate diminished rhombicosidodecahedron - One cap removed, the opposite cap rotated
 * Metagyrate diminished rhombicosidodecahedron - One cap removed, another non-opposite, non-adjacent cap rotated
 * Bigyrate diminished rhombicosidodecahedron - One cap removed, two more non-adjacent ones rotated
 * Parabidiminished rhombicosidodecahedron - Two opposite caps removed
 * Metabidiminished rhombicosidodecahedron - Two non-adjacent, non-opposite caps removed
 * Gyrate bidiminished rhombicosidodecahedron - Two non-opposite caps removed, a third non-adjacent one rotated
 * Tridiminished rhombicosidodecahedron - Three non-adjacent caps removed