Eucairite crystallizes as pseudo-tetragonal orthorhombic crystals having a large supercell of a = 4.105 Å, b = 20.35 Å; c = 6.31 Å based upon a tetragonal subcell whose a dimension lies somewhere between 4.07 Å and 4.105 Å and whose c dimension is equal to 6.31 Å and which has the space group symmetry of P 4/ nmm . There are 2 ( CuAgSe ) per subcell with the Ag atoms at 1/4, 1/4, .449; 3/4, 3/4, .551. The Se atoms are at 1/4, 1/4, .873; 3/4, 3/4, .127 and the Cu atoms are at 1/4, 3/4, .105; 3/4, 1/4, .895. The true cell or supercell is a stacked sequence of these subcells in two orientations. At 190–195°C there is an inversion to a high temperature modification.