| Autor: |
Mysen, Bjorn O., Lucier, Amie, Cody, George D. |
| Předmět: |
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| Zdroj: |
American Mineralogist; Nov/Dec2003 Part 1, Vol. 88 Issue 11/12, p1668-1678, 11p, 2 Charts, 12 Graphs |
| Abstrakt: |
The structural behavior of Al³+; in peralkaline glasses and melts along the Na[sub2]Si[sub3]O[sub7-]Na[sub2](NaAl)[sub3]O[sub7] join has been examined to 1200 °C at ambient pressure with [sup29]Si MAS NMR and Raman spectroscopy. The distribution of Al³+; among coexisting Q[sup4], Q³, and Q² structural units in the glasses and melts was determined as a function of bulk Al/(Al + Si) and temperature. The Al³+; resides principally in Q[sup4] structural units, which contain more than 70% of the total amount of Al³+;. The Q² units contain the smallest amount of Al³+; among the Q[sup4], Q³, and Q² structural units. There is no evidence for temperature-dependent distribution of Al³+; among the coexisting structural units at least to 11001200 ° C. The equilibrium constant for the speciation reaction, 2Q³ ↔ Q[sup4] + Q², passes through a maximum with increasing bulk melt Al/(Al + Si) at constant temperature. Its temperature-dependence also depends on bulk melt Al/(Al + Si) so that the enthalpy of the speciation reaction, ΔH, passes through a maximum with increasing bulk Al/(Al + Si). This ΔH ranges between 13 and 23 kJ/mol for bulk melt Al/(Al + Si) between 0 and 0.33. These composition-dependent maxima in the equilibrium constant (at constant temperature) and ΔH of the speciation reaction reflect the changes in Al/(Al + Si) of Q², Q³, and Q[sup4] structural units with Al/(Al + Si) of the melt. [ABSTRACT FROM AUTHOR] |
| Databáze: |
Complementary Index |
| Externí odkaz: |
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