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Thermodynamic modelling of crystal-liquid equilibria in the system Ab-Or-Qtz-H2O

Title data

Kirschen, Marcus ; Pichavant, Michel:
Thermodynamic modelling of crystal-liquid equilibria in the system Ab-Or-Qtz-H2O.
1999
Event: 6th Silicate Melt Workshop , 13.-17. April 1999 , Arc-et-Senans.
(Conference item: Workshop , Speech )

Abstract in another language

Computation of crystal-liquid phase equilibria requires a model of the free energy of the melt that defines the solution endmember activity at given temperature, pressure and composition. Liquidus and solubility data constrain non-ideal solution properties of the melt through identical chemical potentials µ = µ0 + RTln(a) of solution components in coexisting phases. Reported melting data in the system NaAlSi3O8 (ab) - KAlSi3O8 (or) - Si4O8 (qtz) - H2O and all binary and ternary subsystems were used to derive an internally consistent set of solution parameters for hydrous and dry melts by linear programming techniques. Thermodynamic data from Berman & Aranovich (1996) were used for the solid endmembers and the Elkins & Grove (1990) model for feldspar solid solution. SiO2 and H2O solubility in the feldspars was neglected. We assumed pure silica polymorphs. Reported enthalpies of fusion and partial molar heat capacities from Richet & Bottinga (1986) were used to further constrain the free energy of the liquid endmembers. Liquid volumes were approximated to 15 kbar by a Taylor series in p and T as a first approach. Margules-type excess polynomials were used to model non-ideality of the melt. We adopted the symmetric excess term from Hervig & Navrotsky (1984) for liquid ab-or mixing. While or-qtz mixing is close to ideal, a slightly positive excess function is required to model reported ab-qtz liquidus phase relations. Qtz-H2O, ab-H2O and or-H2O mixing behaviour is more complex. To constrain the free energy of the hypothetic H2O melt component we used solubility data from Paillat et al. (1992) and Holtz et al. (subm.) as additional constraints assuming the vapour phase to be pure H2O (Haar, 1984) below 1100C and 3kbar. Using Cp(T) from Richet (1987) and pressure independent V0 = 1.2 J/bar (Richet & Polian, 1998) we obtained H0 and S0 close to -293 kJ/mol and 58 J/Kmol as first estimates for H2O melt. Further refinements of the H2O standard state and improvements of ab, or, qtz liquid volume terms are on the way. Calculated phase relations of this study are compared to predictions of existing solution models.

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 07:32
Last Modified: 03 Jul 2019 07:32
URI: https://eref.uni-bayreuth.de/id/eprint/49791