Abstract :
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∞1}[3Si3O9]H2O and K4Pb2{uB, 1r}[Si6O18]2H2O. The thermal behavior of these compounds was determined by dynamic and isothermal thermogravimetry combined with powder X-ray diffraction between room temperature and 550 °C. 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.
