| Edge-contracted icosahedron | |
|---|---|
 | |
| Type | Octadecahedron |
| Faces | 18 triangles |
| Edges | 27 |
| Vertices | 11 |
| Vertex configuration | 2 (34) 8 (35) 1 (36) |
| Symmetry group | C2v, [2], (*22), order 4 |
| Properties | Convex, deltahedron |
| Net | |
 | |
In geometry, an edge-contracted icosahedron is a polyhedron with 18 triangular faces, 27 edges, and 11 vertices.
It can be constructed from the regular icosahedron, with one edge contraction, removing one vertex, 3 edges, and 2 faces. This contraction distorts the circumscribed sphere original vertices. With all equilateral triangle faces, it has 2 sets of 3 coplanar equilateral triangles (each forming a half-hexagon), and thus is not a Johnson solid.
If the sets of three coplanar triangles are considered a single face (called a triamond [1] ), it has 10 vertices, 22 edges, and 14 faces, 12 triangles and 2 triamonds.
It may also be described as having a hybrid square-pentagonal antiprismatic core (an antiprismatic core with one square base and one pentagonal base); each base is then augmented with a pyramid.
The dissected regular icosahedron is a variant topologically equivalent to the sphenocorona with the two sets of 3 coplanar faces as trapezoids. This is the vertex figure of a 4D polytope, grand antiprism. It has 10 vertices, 22 edges, and 12 equilateral triangular faces and 2 trapezoid faces. [2]
In chemistry, this polyhedron is most commonly called the octadecahedron, for 18 triangular faces, and represents the closo-boranate [B11H11]2−. [3]
 Ball-and-stick model of the closo-undecaborate ion, [B11H11]2− |  closo-boranate [B11H11]2− |  Net |