Affine coxeter group W_a(A₄), quaternions, and decagonal quasicrystals

We introduce a technique of projection onto the Coxeter plane of an arbitrary higher-dimensional lattice described by the affine Coxeter group. The Coxeter plane is determined by the simple roots of the Coxeter graph I ₂(h) where h is the Coxeter number of the Coxeter group W(G) which embeds the dihedral group D_h of order 2h as a maximal subgroup. As a simple application, we demonstrate projections of the root and weight lattices of A₄ onto the Coxeter plane using the strip (canonical) projection method. We show that the crystal spaces of the affine W_a(A ₄) can be decomposed into two orthogonal spaces whose point group is the dihedral group D₅ which acts in both spaces faithfully. The strip projections of the root and weight lattices can be taken as models for the decagonal quasicrystals. The paper also revises the quaternionic descriptions of the root and weight lattices, described by the affine Coxeter group W _a(A₃), which correspond to the face centered cubic (fcc) lattice and body centered cubic (bcc) lattice respectively. Extensions of these lattices to higher dimensions lead to the root and weight lattices of the group W_a(A_n), n ≥ 4. We also note that the projection of the Voronoi cell of the root lattice of W_a(A₄) describes a framework of nested decagram growing with the power of the golden ratio recently discovered in the Islamic arts.