Using the Car-Parrinello method, the authors have obtained unforeseen structures for the low-lying isomers of Si45 and other midsized Si clusters. They are formed by two shells of atoms, the outer one (cage) being fullerenelike and the inner one (core) consisting of a few atoms satg. dangling bonds. These novel structures provide for the first time a consistent interpretation of the available exptl. data, including the reactivity trends and the structural transition at a size of .apprx.25 atoms. [on SciFinder (R)]