K2MSi3O9.H2O (M = Sn, Pb) synthetic framework silicates are isotypical with umbite and kostylevite minerals, respectively. The silicate anion exhibits two structural types, one of which is an infinite chain polysilicate (umbite) and the other a cyclohexasilicate (kostylevite). The structural formulas may be written as K2Sn{uB, 1(infinity)(1)}[(Si3O9)-Si-3]H2O and K4Pb2{uB, 1r}[Si6O18]2H(2)O. The thermal behavior of these compounds was determined by dynamic and isothermal thermogravimetry combined with powder X-ray diffraction between room temperature and 550degreesC. The kostylevite-type compound presents a very rigid framework, the umbite-type structure being much more flexible. In both cases, the dehydration process takes place with a reduction in cell volume. Anhydrous tin silicate presents a monoclinic cell whose volume is almost invariant with temperature. K2PbSi3O9 is obtained as an amorphous material. The structural characteristics of these phases are related to their ion exchange properties. (C) 2004 Elsevier B.V. All rights reserved.