The variable influence of P2O5 on the viscosity of melts of differing alkali/aluminium ratio: Implications for the structural role of phosphorus in silicate melts

Abstract

The shear viscosities of forty melts in the system Na2O‐Al2O3‐SiO2‐P2O5 have been determined in the temperature range 1652−1052°C using the concentric cylinder method. Six P-free compositions containing ∼67 mol% SiO2 varying in molar Na(Na + Al) from 0.70 (peralkaline) to 0.44 (peraluminous) were studied, to each of which successive additions of up to 7 mol% (13 wt%) P2O5 were made. At a fixed temperature, viscosities in the P-free system show a maximum, not at the ‘charge-balanced’ metaluminous composition (Na(Na + Al) = 0.50), but at Na(Na + Al) = 0.47. Addition of P to peralkaline melts results in an increase in viscosity. With progressive additions of P to mildly peralkaline melts (Na(Na + Al) < 0.60), there is a maximum in melt viscosity that occurs at lower P content as the peralkalinity of the melt decreases. In contrast, the addition of P to the metaluminous and peraluminous melts causes a decrease in melt viscosity. The magnitude of this decrease is identical for the metaluminous, and mildly peraluminous (Na(Na + Al) = 0.47) compositions, but smaller for the most peraluminous melt (Na(Na + Al) = 0.44).The following inferences are made from the present viscosity data, together with spectroscopic data from the literature: (1) At the metaluminous join in the P-free system, not all the Al is present as a charge-balanced network-former. Between the metaluminous join and the viscosity maximum the incorporation of a small proportion of Al (3% relative) in a charge-balancing role (for AlIV) could explain the observations. (2) The addition of P to peralkaline melts results in the formation of Na phosphate complexes which, upon exhaustion of excess Na, have the stoichiometry of extended metaphosphate chains with NaP ratios that tend to 1 as the metaluminous ioin is approached. (3) Estimates of the relative effects of Na and Al phosphate melt complexes on viscosity are consistent with the formation of both NaPO3 and AlPO4 melt complexes upon addition of P to metaluminous melts. (4) In the most peraluminous melts studied, P is inferred to interact with both excess Al and network-forming aluminates, suggesting that these two species have similar energetic stabilities. Given that many granites lie close to the metaluminous join in composition, the results of this study have implications for the physical and chemical evolution of such natural systems.