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 tectosilicate stoichiometry in this system are fully polymerised. The maximum in melt viscosity produced by the substitution of Al for Na does not generally occur at molar Na/Al = 1; rather, it deviates systematically into the peraluminous field for SiO₂ > 50 mol%. This deviation is greatest at 67 mol% SiO₂ and increases with decreasing temperature. These features may be most simply explained by the presence of "triclusters" consisting of a threefold coordinated oxygen to which one aluminate and two silicate tetrahedra are attached. The presence of such triclusters has important implications for phase equilibria, mechanisms of viscous flow, and the solubilities of elements in natural and synthetic highly polymerised aluminosilicate melts.