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Equilibria among titanite, hedenbergite, fayalite, quartz, ilmenite, and magnetite; experiments and internally consistent thermodynamic data for titanite

Dimitrios Xirouchakis and Donald H. Lindsley
Equilibria among titanite, hedenbergite, fayalite, quartz, ilmenite, and magnetite; experiments and internally consistent thermodynamic data for titanite
American Mineralogist (August 1998) 83 (7-8): 712-725

Abstract

For iron-rich compositions, reactions among titanite, clinopyroxene, olivine, quartz, magnetite, and ilmenite can constrain the conditions under which common rocks form. Phase equilibrium experiments in the system CaO-FeO-Fe (sub 2) O (sub 3) -TiO (sub 2) -SiO (sub 2) constrain two of these reactions: (1) clinopyroxene + ilmenite = titanite + olivine and (2) clinopyroxene + ilmenite = titanite + magnetite+quartz, in the range 600-1100 degrees C and <1 bar -3.8 kbar, and under controlled f (sub O2) conditions. The experimental results indicate that the assemblage clinopyroxene-ilmenite represents reducing conditions relative to titanite-magnetite-quartz. However, the latter is not necessarily indicative of oxygen fugacity conditions higher than that of the FMQ buffer. The data also strongly suggest that the equilibrium positions of reaction 1 and 2 reflect changes in pressure, temperature, f (sub O2) a (sub SiO2) mu Fe (super 2+) Ti (super 4+) Fe (super 3+) (sub -2) and mu CaFe (sub -1) in the coexisting Fe-Ti oxides and silicates. Standard state thermodynamic data and solution models for Ca-Fe olivine and clinopyroxene solid solutions. Fe-Ti ilmenite and magnetite solid solutions, and quartz (QUILF database) allow refinement of standard state thermochemical data for end-member titanite (A2/a space group): Delta H (super 0) = -2607.41 kj/mol; S (super 0) = 106.00 J/(mol.K); Delta G (super 0) = -2458.95 kJ/mol and require modification of published heat capacity equations to: C (sub p) = 591460608/T (super 3) -5118324/T (super 2) -1038.4/T + 247.47. The derived data for titanite are internally consistent with an enthalpy of formation of -2610.13+ or -2.90 (2 std. error) kJ/mol. established by drop solution calorimetry, and are specific to the QUILF database. However, analysis of the experimental results using different internally consistent databases suggest that the revision proposed in this study of the currently accepted entropy value of 129.20 J/(mol-K) is largely independent of the databases used. The new entropy is required by the enthalpy of formation for titanite adopted here, which may also require revision of the heat capacity.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 83
Serial Issue: 7-8
Title: Equilibria among titanite, hedenbergite, fayalite, quartz, ilmenite, and magnetite; experiments and internally consistent thermodynamic data for titanite
Affiliation: State University of New York at Stony Brook, Department of Geosciences, Stony Brook, NY, United States
Pages: 712-725
Published: 199808
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 58
Accession Number: 1998-054260
Categories: General geochemistry
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 7 tables
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute.
Update Code: 199820
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