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The role of phosphorus in rhyolitic liquids as determined from the homogeneous iron redox equilibrium

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Abstract

The redox ratio of iron is used as an indicator of solution properties of silicate liquids in the system (SiO−Al2O3−K2O−FeO−Fe2O3−P2O5). Glasses containing 80–85 mol% SiO2 with 1 mol% Fe2O3 and compositions covering a range of K2O/Al2O3 were synthesized at 1400°C in air (fixed fO2). Variations in the ratio FeO/FeO1.5 resulting from the addition of P2O5 are used to determine the solution behavior of phosphorus and its interactions with other cations in the silicate melt. In 80 mol% SiO2 peralkaline melts the redox ratio, expressed as FeO/FeO1.5, is unchanged relative to the reference curve with the addition of 3 mol% P2O5. Yet, the iron redox ratio in the 85 mol% SiO2 potassium aluminosilicate melts is decreased relative to phosphorus-free liquids even for small amounts of P2O5 (0.5 mol%). The redox ratio in peraluminous melts is decreased relative to phosphorus- free liquids at P2O5 concentrations of 3 mol%. In peraluminous liquids, complexing of both Fe+3−O−P+5 and Al+3−O−P+5 occur. The activity coefficient of Fe+3 is decreased because more ferric iron can be accommodated than in phosphorus-free liquids. In peralkaline melts, there is no evidence that P+5 is removing K+ from either Al+3 or Fe+3 species. In chargebalanced melts with 3 mol% Fe2O3 and very high P2O5 concentrations, phosphorus removes K+ from K−O−Fe+3 complexes resulting in a redox increase. P2O5 should be accommodated easily in peraluminous rhyolitic liquids and phosphate saturation may be suppressed relative to metaluminous rhyolites. In peralkaline melts, phosphate solubility may increase as a result of phosphorus complexing with alkalis. The complexing stoichiometry may be variable, however, and the relative influence of peralkalinity versus temperature on phosphate solubility in rhyolitic melts deserves greater attention.

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  1. Rosa Gwinn

    Present address: Dames and Moore, Inc., 7101 Wisconsin Avenue, 20814, Bethesda, MD, USA

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  1. Department of Geological Sciences, Brown University, 02912, Providence, RI, USA

    Rosa Gwinn & Paul C. Hess

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Gwinn, R., Hess, P.C. The role of phosphorus in rhyolitic liquids as determined from the homogeneous iron redox equilibrium. Contr. Mineral. and Petrol. 113, 424–435 (1993). https://doi.org/10.1007/BF00286932

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