Silica Transformations in the System PbO-SiO2

Mixtures of PbO and silicic acid were fired at 710°, 820°, and 910°C for periods up to 1000 hr in air. At each temperature cristobalite and tridymite crystallized rapidly, with the greatest development in samples with compositions of 1 mole PbO to 50 and 60 moles SO2. At 820° and 91O°C tridymite continued to crystallize at the expense of cristobalite. At 710°C quartz formed, after a nucleation time of approximately 200 hr, at the expense of cristobalite and tridymite. The equilibrium assemblages were cristobalite and tridymite at 820° and 910°C and quartz and PbSiO2 at 710°C. The results are interpreted in terms of the tentative phase relations outlined by Holmquist for the high‐silica regions of binary silicate systems. Phase development curves and lattice parameter measurements characterize the initially formed cristobalite as a metastable solid solution containing approximately 2 mole % PbO. Equilibrium was approached by exsolution of PbO from the cristobalite structure.