Using ab initio calculations, we show that the encapsulation of Y, La and Ac metal (M) atom stabilizes the dodecahedral fullerene anion M@ Si20- in the icosahedral symmetry. Similar to C60, it is the ideal cage of silicon and the largest that can be stabilized by an M atom. Doping of other rare earths is further shown to stabilize magnetic dodecahedral fullerenes Pa@ Si20, Sm@ Si20, Pu@ Si20 and Tm@ Si20 with 1 $\mu$B, 4 $\mu$B, 4 $\mu$B and 3 $\mu$B spin magnetic moments, respectively, in contrast to most previous studies on M -encapsulated Si clusters in which the magnetic moment is completely quenched. The highest spin magnetic moment of 7 $\mu$B is achieved for Gd@ Si20- with half-filled 4f states. The orbital magnetic moment is also calculated and it is ∼1 $\mu$B in most cases. Neutral M@ Si20 (M=Y, La, Ac and Gd) behaves like superhalogen and interaction with a noble or alkali metal atom leads to salt like behavior. These findings could pave way for the realization of silicon fullerenes by doping of several elements. {\textcopyright} 2006 The American Physical Society.