Grand Antiprism and Quaternions

Vertices of the 4-dimensional semi-regular polytope, the \textit{grand antiprism} and its symmetry group of order 400 are represented in terms of quaternions with unit norm. It follows from the icosian representation of the \textbf{$E_{8}$} root system which decomposes into two copies of the root system of $H_{4}$. The symmetry of the \textit{grand antiprism} is a maximal subgroup of the Coxeter group $W(H_{4})$. It is the group $Aut(H_{2} \oplus H'_{2})$ which is constructed in terms of 20 quaternionic roots of the Coxeter diagram $H_{2} \oplus H'_{2}$. The root system of $H_{4}$ represented by the binary icosahedral group \textit{I}of order 120, constitutes the regular 4D polytope 600-cell. When its 20 quaternionic vertices corresponding to the roots of the diagram $H_{2} \oplus H'_{2}$ are removed from the vertices of the 600-cell the remaining 100 quaternions constitute the vertices of the\textit{grand antiprism}. We give a detailed analysis of the construction of the cells of the\textit{grand antiprism} in terms of quaternions. The dual polytope of the \textit{grand antiprism} has been also constructed.