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PbO-SiO2 melts: structure and thermodynamics of mixing |
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| Authors: | Paul C Hess |
| Institution: | Department Geological Sciences, Brown University, Providence, R.I. 02912, U.S.A. |
| Abstract: | Thermodynamic properties of PbO-SiO2 melts, obtained from published data and calculated from freezing point depressions, reflect the gradual polymerization of silicate anions in the melt as the ratio is increased. The free energy of mixing curve at 1000°C has a minimum at 40 mole % SiO2 and is convex-upward between 72 and 98 mole % SiO2. The latter is an indication of metastable liquid immiscibility. The free energy minimum is correlated with the maximum in the distribution of nonbridging oxygens in the melt. In SiO2-poor melts, the activities of PbO and SiO2 (pure liquid standard states) show sharp negative deviations from ideality. The PbO activity reflects the paucity of free oxygen species in the melt whereas the SiO2 activity reflects the depolymerized state of the silicate anions. In more SiO2-rich melts, the activity of SiO2 shows a positive deviation from ideality which is qualitatively correlated to a polymerization parameter. The heat of mixing term has a minimum of ?2000 cal at 35 mole % SiO2 and a maximum of +200 cal at 90 mole % SiO2. The minimum is associated with the exothermic heat effect obtained during the reaction (O0) + (O2?) = 2(O?), whereas the maximum corresponds to the endothermic heat effect obtained when coordination polyhedra of oxygens form around the Pb cation. The entropy of mixing curve has the same form but is systematically smaller than a theoretical curve calculated on the assumption of random mixing of oxygen species. The discrepancy is due to the entropy loss obtained by the clustering of oxygen species to form complex silicate species. |
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