THE VARIABLE INFLUENCE OF P2O5 ON THE VISCOSITY OF MELTS OF DIFFERINGALKALI ALUMINIUM RATIO - IMPLICATIONS FOR THE STRUCTURAL ROLE OF PHOSPHORUS IN SILICATE MELTS/

The shear viscosities of forty melts in the system Na2O-Al2O3-SiO2-P2O 5 have been determined in the temperature range 1652-1052 degrees C us ing the concentric cylinder method. Six P-free compositions containing similar to 67 mol% SiO2 varying in molar Na/(Na + Al) from 0.70 (pera lkaline) to 0.44 (peraluminous) were studied, to each of which success ive additions of up to 7 mol% (13 wt%) P2O5 were made. At st fixed tem perature, 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 pe ralkaline melts (Na/(Na + Al) < 0.60), there is a maximum in melt visc osity that occurs at lower P content as the peralkalinity of the melt decreases. In contrast, the addition of P to the metaluminous and pera luminous melts causes a decrease in melt viscosity. The magnitude of t his decrease is identical for the metaluminous, and mildly peraluminou s (Na/(Na + Al) = 0.47) compositions, but smaller for the most peralum inous melt (Na/(Na + Al) = 0.44). The following inferences are made fr om the present viscosity data, together with spectroscopic data from t he 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 sm all proportion of Al (3% relative) in a charge-balancing role (for Al- IV) could explain the observations. (2) The addition of P to peralkali ne melts results in the formation of Na phosphate complexes which, upo n exhaustion of excess Na, have the stoichiometry of extended metaphos phate chains with Na/P ratios that tend to 1 as the metaluminous join is approached. (3) Estimates of the relative effects of Na and Al phos phate melt complexes on viscosity are consistent with the formation of both NaPO3 and AlPO4 melt complexes upon addition of P to metaluminou s melts. (4) In the most peraluminous melts studied, P is inferred to interact with both excess Al and network-forming aluminetes, suggestin g that these two species have similar energetic stabilities. Given tha t 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.