Immiscible silicate liquids in the system SiO (sub 2) -TiO (sub 2) -Al (sub 2) O (sub 3)

Coexisting liquids in this system have been synthesized at 2000-2500 K with a laser-heated air levitation apparatus to determine the SiO (sub 2) -TiO (sub 2) miscibility gap and its extrapolation to the Al (sub 2) O (sub 3) -bearing system. Compositions of quenched coexisting liquids in the SiO (sub 2) -TiO (sub 2) and SiO (sub 2) -TiO (sub 2) -Al (sub 2) O (sub 3) system demonstrate the dramatic decline of the miscibility gap when as little as 3 mol.% Al (sub 2) O (sub 3) is added. With bulk compositions with 10 mol.% Al (sub 2) O (sub 3) no evidence was found for a stable liquid-liquid phase separation above 2073 K. Although experimental liquidus data can be modelled with Margules-type excess polynomials for the melt, the calculated ternary miscibility gap is in disagreement with experiments when conventional extrapolation are applied; however, the application of a generalized extrapolation method based on weighting methods reproduces the experimentally determined miscibility gap in detail. Extrapolation outside the immiscibility region yields a first-order approximation of the excess Gibbs free energy of the melt.