Au-induced faceting on 4° vicinal Si(001) has been studied with in situ low energy electron microscopy and scanning tunneling microscopy; the surface decomposes into areas that are perfectly flat with a (001)-orientation and (119)-facets. Above a critical Au coverage extremely elongated (001) super-terraces propagate with a speed of crystallisation of up to 100 mm s−1. The width and area of (001) terraces increase linearly with additional coverage. Conservation of the macroscopic miscut requires step bunching and facet formation. The driving force for the phase separation is an Au-induced reconstruction on Si(001).