Reaction of mixtures of cobalt nitrate, colloidal silica and a metal hydroxide (MOH) under hydrothermal conditions produced a range of Co hydroxysilicates, the components of which depended on the identity of M, T and reactant ratios. At 250oC, for M = Na, a smectite Na0.06Co3.07Si3.95O10(OH)2 was produced; M = K gave either a mica, KCo2.5Si4O10(OH)2, intermediate between di- and trioctahedral, or a Si-deficient mica, KCo3Si3.75O10(OH)2, depending on reactant ratios. For M = Cs, either a vermiculite or a 2:1 layer silicate was obtained; M = Li gave the non-clay Li2CoSiO4; tetra-alkylammonium hydroxides yielded chrysotile. All phases were characterized by chemical analysis, TEM and XRD; further characterization of the smectite was undertaken by diffuse reflectance, IR and X-ray photoelectron spectroscopy. The layer charge in these clays appears to arise from cation vacancies within an almost trioctahedral sheet and possibly within the tetrahedral sheets.-R.A.H.