PERALUMINOUS VISCOSITY MAXIMA IN NA2O-AL2O3-SIO2 LIQUIDS - THE ROLE OF TRICLUSTERS IN TECTOSILICATE MELTS

We present enhanced-precision viscosity determinations for Na2O-Al2O3- SiO2 liquids which challenge the commonly held view that melts of tect osilicate stoichiometry in this system are fully polymerised. The maxi mum in melt viscosity produced by the substitution of Al for Na does n ot generally occur at molar Na/Al = 1; rather, it deviates systematica lly into the peraluminous field for SiO2 > 50 mol%. This deviation is greatest at 67 mol% SiO2 and increases with decreasing temperature. Th ese features may be most simply explained by the presence of ''triclus ters'' consisting of a threefold coordinated oxygen to which one alumi nate and two silicate tetrahedra are attached. The presence of such tr iclusters has important implications for phase equilibria, mechanisms of viscous flow, and the solubilities of elements in natural and synth etic highly polymerised aluminosilicate melts. Copyright (C) 1997 Else vier Science Ltd.