High-resolution photoelectron spectroscopy with synchrotron radiation and magnetic linear dichroism in Co 3p core-level photoemission have been used to study the initial stages of formation and ferromagnetic ordering of  Co/Si(100) and Co/Si(111) interfaces. The correlation between the phase composition, electronic structure and magnetic behavior of the interfaces has been established during Co deposition on Si surface and subsequent sample annealing. It is shown that ferromagnetic ordering has a threshold nature and arises after the deposition of 6 Å of Co in both systems. At higher Co coverages a continuous film of a Si solid solution in cobalt is found to develop.  Further increase of ferromagnetic ordering of the interface is caused by the growth of pure metal film. Annealing of the samples covered with a Co film of few nm thickness leads to the gradual disappearance of the metal film and  formation of four silicide phases: a metastable ferromagnetic Co3Si silicide obtained at room temperature for the first time and three stable non-magnetic cobalt silicides: Co2Si, CoSi and CoSi2. It is shown that solid-phase reactions  start at ~250 °С and ~320 °С in Co/Si(100) and Co/Si(111) systems respectively.