An icosahedron flake, or sierpinski icosahedron, is formed by successive flakes of twelve regular icosahedra. They have elongated, geminate capsids with two incomplete T=1 icosahedra joined at the missing vertex. Carborane acids also have molecular structures approximating regular icosahedra. The unit cell is orthorhombic and its most salient feature is four boron-containing icosahedra. Eight more boron atoms connect the icosahedra to the other elements in the unit cell. The plural can be either "icosahedra" () or "icosahedrons". This structural transformation occurs at pressures at which theory predicts the icosahedra will dissociate. The plural can be either "icosahedrons" or "icosahedra" (). These arrangements are now called Mackay icosahedra. Examples include "Circoporus octahedrus", "Circogonia icosahedra", "Lithocubus geometricus" and "Circorrhegma dodecahedra". Taking all permutations (not just cyclic ones) results in the Compound of two icosahedra. The centers of the icosahedra are located at the fcc positions of the lattice. The 1938 book on the fifty-nine icosahedra resulted, written by Coxeter and Patrick du Val. These are not strictly icosahedra, although they are often referred to as such.