Au induced faceting of a 4° vicinal Si(001) surface was studied with chemical resolution using soft x-ray photoemission electron microscopy. For the first time a direct and quantitative determination of the local Au coverage in situ and during deposition was possible. Au atoms, necessary for the expansion of (001) terraces, are accummulated from a lattice gas, resulting in a phase separation between Au enriched terraces and Au depleted step bunches. During a second stage Au also adsorbs on the step bunches and transforms them into (119) facets. A simple Monte Carlo simulation shows that the initial coverage difference between terraces and bunches determines the regularity of the formed mesoscopic grating.