The Plutino 28978 Ixion, one of the largest objects in the Kuiper Belt, is measured by R filter photometry and linear polarimetry over the phase angle range as well as by BVRI colours and visible spectroscopy. Ixion is a medium red object with spectral slope of 17.7%/100 nm (400-900 nm). While its opposition phase curve surge (0.2 mag/deg) is within the typical range found for other solar system objects, it displays unusually high negative polarization (min. -1.3%). Comparison with model spectra and numerical modeling of the opposition brightening and polarization effects suggest an areal mixture of absorbing (dark) and icy (bright) compounds on its surface: the mixing ratio is about 6:1 for dark and bright material of 0.21 and 0.79 single-scattering albedo and of 250 and 33 dimensionless mean free path (length multiplied by wave number), respectively.Based on observations collected at the ESO VLT within programs 69.C-0133 and 167.C-0340.