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Carnegie Institution of Washington, Geophysical Laboratory and Center for High Pressure Research, Washington, DC, United States
Solution mechanisms of P in peraluminous glasses and melts in the system CaO-Na 2 O-K 2 O-Al 2 O 3 -SiO 2 -P 2 O 5 have been examined with in-situ microRaman spectroscopy from ambient temperature to near 1200 degrees C. The principal aim was to examine the relative stabilities of phosphate complexes as functions of P content, peraluminosity, and temperature. Increasing peraluminosity was accomplished by increasing the proportions of Al (super 3+) and Ca (super 2+) of constant SiO 2 content. The molar ratio Al 2 O 3 /(CaO+Na 2 O+K 2 O) (A/CNK) ranged from approximately 1 to approximately 1.3. In all compositions, P (super 5+) is bonded to Al (super 3+) to form AlPO 4 complexes. In addition, there is evidence for pyrophosphate complexing (P 2 O 7 ). In melts with the highest (Ca+Na+K)/P, there is probably also a small fraction of orthophosphate complexes present. The relative importance of AlPO 4 -like complexes is correlated positively with peraluminosity (A/CNK), P 2 O 5 content, and increasing temperature at temperatures above that of the glass transition. These structural relationships among phosphate complexes are coupled with decreasing polymerization of the aluminosilicate melts.
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