Aluminum coordination in metaaluminous and peralkaline silicate melts |
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| Authors: | Paul C Hess Marcus I Wood |
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| Institution: | (1) Department of Geological Sciences, Brown University, 02912 Providence, Rhode Island, USA;(2) Rockwell Hanford Operations, 99352 Richland, WA |
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| Abstract: | Incremental amounts of Na2O and K2O added to immiscible melts in the MgO-CaO-TiO2-Al2O3 SiO2 system cause a decrease in critical temperature, phase separation and change in the pattern of Al2O3 partitioning. Al2O3, which is concentrated in the low SiO2 immiscible melts in the alkali-free system, is increasingly partitioned into the high-SiO2 immiscible melt as the alkali/aluminium ratio is increased. However, K2O is more effective than Na2O in stabilizing Al2O2 in the SiO2-rich melt. The coordination changes occurring in the aluminosilicate melts upon the addition of the alkali oxides are described by CaAl2O4+2SiOK=2KAlO2+SiOCaOSi where K (or Na) displaces Ca as the charge-balancing cation for the networkforming AlO4 tetrahedra. The increased stability of the AlO4 species in the highly polymerized SiO2-rich melt and the consequent shrinkage of the miscibility gap is ascribed to positive configurational entropy and negative enthalpy changes associated with the formation of K, Na-AlO4 species. Element partition systematics indicate that (Na, K)AlO2 species favor the more polymerized, CaAl2O4 and TiO2 species, the less polymerized silicate structure in the melt. |
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