Aluminosilicate melts: structure, composition and temperature |
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| Authors: | B Mysen |
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| Institution: | (1) Geophysical Laboratory, 5251 Broad Branch Rd., NW, Washington DC 20015, USA, US |
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| Abstract: | The anionic structure of aluminosilicate melts of intermediate degree of polymerization (NBO/T = 0.5) and with along the composition join (LS4-LA4) has been examined in-situ to ˜1480 °C, and compared with recent data for melts along the analog composition join
and with less polymerized melts along the join and O_5. With , the anionic equilibrium, (1) , adequately describes the structure. With , a second expression, (2) , is required because an additional structural unit, Q1, is stabilized in the melts. The enthalpy, , of reaction (1) increases from − 36 ±4 kJ/mol in the absence of aluminum to 34± 5 kJ/mol at and 64 ± 4 kJ/mol at Al/(Al + Si) = 0.45. Similar trends are reported for other alkali aluminosilicate melts. Least-squares
fitting of abundance of structural units as a function of temperature and bulk composition has been conducted. The unit abundance
is dominantly a function of temperature, Al/(Al +Si), and bulk melt polymerization. Configurational entropy and heat capacity
of mixing of melts above their glass transition temperatures have been calculated with the aid of the least-squares fitted
equations. The values of these parameters indicate that as the ionization potential of the metal cations increases, configurational
heat capacity of alkali aluminosilicate melts becomes temperature dependent. As a result, transport properties (viscosity,
diffusivity, and conductivity) of such melts will not show Arrhenian behavior even in the high-temperature range. Further,
discontinuous changes in entropy and heat capacity of mixing results from temperature-induced changes in types of structural
units in the melts. Such discontinuous changes would also be reflected in discontinuous changes of temperature-dependent transport
properties.
Received: 26 September 1996 / Accepted: 18 October 1996 |
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