Two cobalt layer silicates were synthesized by hydrothermal treatment at 275℃ and 54 atm from corresponding stoichiometric coprecipitate of cobalt hydroxide and silica. From IR adsorption spectra and X-ray diffraction patterns, one sample was proved to be cobalt montmorillonite[Co₃(Si₂O₅)₂(OH)₂], while the structure of the other was cobalt antigorite[Co₃Si₂
O₅(OH)₄] of one silica layer combined with an octahedral layer of the type Co₃²⁺(OH)₆.