A new family of modifications of silica is obtained by thermal dehydration of crystalline silicic acids with layer structures. Examples are SiO2 · 0.11 H2O, SiO2 · 0.04 H2O, SiO2 prepared by dehydration of H2Si14O29 · 5.4 H2O, H2Si8O17 · xH2O and H2Si4O9. The “water content” can amount up to 3 weight percent and results from uncondensed inter-layer silanol groups or from water molecules which are trapped between the layers. In some cases dehydration in the presence of organic interlayer molecules leads to different forms of silica.