IMMISCIBLE SILICATE LIQUIDS IN THE SYSTEM SIO2-TIO2-AL2O3

Abstract

Coexisting liquids in the SiO2-TiO2-Al2O3 system have been synthesized at 2000 K to 2500 K with a laser-heated air levitation set-up in order to determine the SiO2-TiO2 miscibility gap and its extrapolation to the Al2O3-bearing system. Compositions of quenched coexisting liquids in the SiO2-TiO2 and SiO2-TiO2-Al2O3 system demonstrate the dramatic decline of the miscibility gap when as little as 3 mol% Al2O3 is added. With bulk compositions containing 10 mol% Al2O3 we found no evidence of a stable liquid-liquid phase separation above 2073 K. While experimental liquidus data in the three binary subsystems can be modelled with Margules-type excess polynomials for the melt, the calculated ternary miscibility gap is strongly discrepant with experiments when conventional extrapolation methods are applied. To overcome this problem we propose a generalized extrapolation method based on weighting of the binary excess polynomials. Applying this multicomponent excess function an additional ternary excess term is not required and the experimentally determined liquid miscibility gap is reproduced in detail. Extrapolation outside the immiscibility region yields a valuable first-order approximation of the excess Gibbs free energy of the melt.