  A quark-cluster state, rather than the color-super-conductivity state, may
appear in matter with low-temperature but high density, since the phase
transition of chiral symmetry broken and that of color-confinement could not
occur imultaneously. Such quark clusters might be stable in strange quark
matter. Quark stars would have temperatures to be low enough to freeze by
residual color interaction between the clusters, and become then solid ones.The
charge numbers and the sizes of quark clusters, as well as the residual
interactions in-between are investigated. The solid state properties of quark
stars are constrained by observations.
