This paper demonstrates phase transition phenomena of a Cd-based approximant Cd6Ca for an icosahedral quasicrystal. A salient anomaly has been observed at 100 K in the magnetic susceptibility as well as in the electrical resistivity, which is attributed to temperature variation of the density of states at EF, possibly caused by lattice distortion. Diffraction experiment together with a group theoretical analysis based on the Landau theory suggests the space group symmetry of the low temperature phase is C2/c, and this result allows to propose possible superstructure with an antiparallel ordering of a Cd4 tetrahedron located inside the icosahedral cluster, along a [110] direction of the high temperature phase.
