In this paper we present a survey on the structure and equation of state for some silicon clathrates and their carbon analogues, as obtained by means of ab initio calculations within the Hartree-Fock approximation. We restrict our consideration to type-I clathrates, namely M-x(Si, C)(46), With M = Na, Ba. The insertion of guest species into the carbon clathrate cages promotes a significant increase in the host volume, thus reducing the bulk modulus for these compounds. In spite of that, the estimated hardness for C-46, Of about 61 GPa, constitutes an exceptionally large value for a structure with such an open framework. The issue of electronic charge transference from the guest species to the host framework and the stability of carbon and silicon clathrates relative to the diamond phase are also discussed.