Abstract: Hydrous magnesium silicates having a crystal structure similar to that of natural hectorite may be prepared utilising as starting material a novel intermediate which may be prepared from talc. The novel intermediate, "mesotalc" is the product obtained by heating a mixture of talc and sodium carbonate so as to modify the talc structure while controlling the heating to avoid destroying it.The "mesotalc" may be utilised in the production of hydrous magnesium silicates by forming an aqueous suspension containing the constituents of the hydrous magnesium silicate at least a part of the magnesium and silicon content being provided by "mesotalc" and hydrothermally treating the suspension until crystal growth occurs. The resulting crystalline product generally has rheological properties better than those of natural hectorite and may be used as a basis for organophilic derivatives.