Silicate groupings in glassy and crystalline 2PbO . SiO2

Abstract

The type and the amount of silicate groupings existing in glassy and crystalline 2PbO·SiO2 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·SiO2 and each of the three main crystalline polymorphs are characterized by its own specific silicate anion distribution: the distribution in vitreous 2PbO·SiO2 is of a polyanionic nature; in T---Pb2SiO4 dimetic groups [Si2O7]6- prevail; M1---Pb2SiO4 contains predominantly [Si4O12]8- rings and H---Pb2SiO4 is a typical polysilicate with chain anions [SiO32-]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.