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Immiscible Liquids in CaO-SiO2-TiO2

Title data

Kirschen, Marcus ; De Capitani, Christian:
Immiscible Liquids in CaO-SiO2-TiO2.
Event: MinPet96 - Gemeinschaftstagung der Schweizerischen Mineralogischen und Petrographischen Gesellschaft (SMPG) und der Österreichischen Mineralogischen Gesellschaft (ÖMG) , 23.-29. September 1996 , Schwaz, Österreich.
(Conference item: Conference , Speech )

Abstract in another language

The computation of equilibrium phase diagrams of silicate systems requires a model of the Gibbs free energy of the melt. While in binary silicate systems non-ideal contributions to the free energy are often modelled by equations with few interaction parameters (e.g. Berman & Brown, 1984), extrapolation in higher order systems remains unclear.
The method of extrapolation strongly affects the shape of the excess free energy surface. Since compositions of coexisting liquids depend only on the topology of the free energy surface of the melt, they represent an excellent tool to test possible methods of extrapolation.
In order to determine miscibility gaps in silicate melts, a high temperature furnace has been constructed. Coexisting liquids in the silicate system CaO-SiO2-TiO2 at T=1600C, p=1bar have been quenched to glasses and their compositions have been determined by electron microprobe analysis.
Measured compositions of the quenched liquid phases correspond more or less to the miscibility gap proposed by DeVries et al. (1955) at T=1600C. In contrast to their observations, the SiO2-rich phase contains less than 1wt% CaO.
Some extrapolation models from binaries into the ternary system (see Hillert, 1980 and Chou, 1987 for a review) have been tested with the observed miscibility gap using the Gibbs free energy minimizing algorithm theriak (DeCapitani, 1994). Formulations proposed by Kohler, Muggianu, and Colinet permit extrapolation without fitting a specific ternary parameter. While the composition of the SiO2-rich phase is correctly predicted, the calculated SiO2 content of the TiO2-rich liquid phase is in general too low. To reproduce the experimentally determined miscibility gap, we propose a generalization of an extrapolation formula of Kohler (1960). In Kohler's original formula, the exponent k is fixed to the degree m of the binary Margules polynomial: k=m-2. k used as a variable parameter controls the expansion of non-ideal binary contributions to the Gibbs free energy of the ternary. To calculate the miscibility gap in CaO-SiO2-TiO2 we used Kohler's formula for SiO2-TiO2 and CaO-SiO2 binaries and k=1.62 for CaO-TiO2.
The proposed method represents a flexible extrapolation scheme for computing equilibrium phase diagrams of higher order silicate systems from the corresponding subsystems.

Further data

Item Type: Conference item (Speech)
Refereed: Yes
Institutions of the University: Faculties > Faculty of Engineering Science
Research Institutions > Affiliated Institutes > Fraunhofer Center for High Temperature Materials and Design (HTL)
Result of work at the UBT: No
DDC Subjects: 500 Science > 530 Physics
500 Science > 540 Chemistry
500 Science > 550 Earth sciences, geology
Date Deposited: 03 Jul 2019 08:37
Last Modified: 03 Jul 2019 08:37
URI: https://eref.uni-bayreuth.de/id/eprint/49799