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Thermodynamics and stability of pseudobrookite-type MgTi₂O₅ (karrooite)

¹ Office of Astromaterials Research and Exploration Science, NASA JSC, Mail Code SA13, Houston, Texas 77058, U.S.A.
² Department of Chemistry, Texas Southern University, 3100 Cleburne Avenue, Houston, Texas 77004, U.S.A.
³ Department of Geosciences and Center for High Pressure Research, State University of New York at Stony Brook, Stony Brook, New York 11794-2100, U.S.A.
⁴ MDProductivity, 12710 Research Blvd., Ste. 205, Austin, Texas 78759, U.S.A.

Correspondence: ^* E-mail: dxirouch{at}ems.jsc.nasa.gov

Pseudobrookite-type MgTi₂O₅ (karrooite) is a synthetic crystalline phase with the Bbmm structure and a component in orthorhombic oxide solid solutions, R²Ti₂O₅-R³⁺₂TiO₅, which are present as accessory minerals in lunar and terrestrial rocks. In this study, we present a model for the molar Gibbs free energy of MgTi₂O₅ as a function of T, P, and the order parameter s = X^M1_Mg – 2X^M2_Mg, (–1 s 1). We describe the molar Gibbs free energy, (), with the equation: = ₀ + ₁· (1 – s) + 3/2 – ₂·(1 – s²) – T· _config, and take parameter g_o to represent the molar Gibbs free energy of ordered MgTi₂O₅ (s = 1), whereas parameters g₁ and g₂ may represent cation-disorder contributions. We used powder and single-crystal X-ray diffraction, and high-temperature relative enthalpy data, to calibrate the disorder contribution to the volume (b_o = 7.3822·10^–3 J/bar), and the model parameters g₁ (7370.8 J/mol) and g₂ (3576.1 J/mol), and heat capacity and volume equation coefficients. We also optimized standard state thermodynamic data from the elements for ordered MgTi₂O₅, (H⁰ = –2 498 515.28 J/mol, S⁰ = 149.55 J/(mol·K), G⁰ = –2 362 181.72 J/mol, V⁰ = 5.445 J/bar) consistent with the model parameters and equations, the thermodynamic data in QUILF, and phase-equilibrium experiments involving MgTi₂O₅, geikelite, rutile, orthoenstatite, and forsterite in the range 973 to 1673 K and 0.0001 to 2.0 GPa. Finally, we investigate theoretically the stability of MgTi₂O₅ (karrooite) with respect to geikelite, rutile, diopside, enstatite, and forsterite in the CaO-MgO-TiO₂-SiO₂ system. We find that diopside- and titanite-bearing reactions require extremely high temperatures, and are thus not stable with respect to liquid. The inferred phase relations can be of help in understanding the stability of MgTi₂O₅ with respect to rutile, geikelite, forsterite, and orthoenstatite, and by extrapolation that of armalcolite relative to rutile, ilmenite, olivine, and orthopyroxene in terrestrial mantle rocks and high-Ti lunar basalts.

This article has been cited by other articles:

				A. R. Lennie, K. S. Knight, and C. M. B. Henderson Cation ordering in MgTi2O5 (karrooite): Probing temperature dependent effects with neutrons American Mineralogist, July 1, 2007; 92(7): 1165 - 1180. [Abstract] [Full Text] [PDF]