Silicate groupings in glassy and crystalline 2PbO·SiO2

The type and the amount of silicate groupings existing in glassy and crystalline 2PbO·SiO 2 have been determined by direct chemical methods: paper chromatography, trimethylsilylation combined with gas-liquid partition chromatography and by the molybdate method. The results obtained by these three different methods are in good agreement and demonstrate, that glassy 2PbO·SiO 2 and each of the three main crystalline polymorphs are characterized by its own specific silicate anion distribution: the distribution in vitreous 2PbO·SiO 2 is of a polyanionic nature; in TPb 2 SiO 4 dimetic groups [Si 2 O 7 ] 6− prevail; M 1 Pb 2 SiO 4 contains predominantly [Si 4 O 12 ] 8− rings and HPb 2 SiO 4 is a typical polysilicate with chain anions [SiO 3 2− ] n . The results fit a structural model according to which glass is a random array of discrete polyatomic groupings; the gradual transition from the glassy state to the stablest crystalline structure is connected with degradation and polymerization of silicate anions.