in which M is a metal of main groups I to VIII or of transition groups II to VIII of the Periodic Table of the Elements including boron, X is as defined in formula (I), where two X groups may be replaced by one oxo group, and a corresponds to the valence of the element, where the molar ratio of silicon compounds used to metal compounds used is in the range from 8000:1 to 8:1, is infiltrated or inj