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Thermodynamics and stability of pseudobrookite-type MgTi2O5 (karrooite)

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Published/Copyright: March 28, 2015
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American Mineralogist
From the journal American Mineralogist Volume 87 Issue 5-6

Abstract

Pseudobrookite-type MgTi2O5 (karrooite) is a synthetic crystalline phase with the Bbmm structure and a component in orthorhombic oxide solid solutions, R2+Ti2O5-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 = XM1Mg - 2XM2Mg, (-1 ≤ s ≤ 1). We describe the molar Gibbs free energy, (G̅), with the equation: G̅ = g̅0 + g̅1·(1 - s) + 3/2g̅2·(1 - s2) - T·S̅config, and take parameter go 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 (bo = 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, (ΔH0 = - 2 498 515.28 J/mol, S0 = 149.55 J/(mol·K), ΔG0 = -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.

Received: 2001-6-4
Accepted: 2002-1-22
Published Online: 2015-3-28
Published in Print: 2002-5-1

© 2015 by Walter de Gruyter Berlin/Boston

Articles in the same Issue

  1. Formation of secondary pyrite and carbonate minerals in the Lower Williams Lake tailings basin, Elliot Lake, Ontario, Canada
  2. Hydroxyl in MgSiO3 akimotoite: A polarized and high-pressure IR study
  3. High-pressure IR-spectra and the thermodynamic properties of chloritoid
  4. Infrared and Raman study of interlayer anions CO32–, NO3–, SO42– and ClO4– in Mg/Al-hydrotalcite
  5. Effect of Fe oxidation state on the IR spectra of Garfield nontronite
  6. High-pressure single-crystal X-ray diffraction study of katoite hydrogarnet: Evidence for a phase transition from Ia3d →I4̅3d symmetry at 5 GPa
  7. The high-temperature P21/c-C2/c phase transition in Fe-free pyroxene (Ca0.15Mg1.85Si2O6): Structural and thermodynamic behavior
  8. Thermodynamics and stability of pseudobrookite-type MgTi2O5 (karrooite)
  9. Molecular orbital calculations on aluminosilicate tricluster molecules: Implications for the structure of aluminosilicate glasses
  10. Surface structures, stabilities, and growth of magnesian calcites: A computational investigation from the perspective of dolomite formation
  11. Spinels and other oxides in Mn-rich rocks from the Hutter Mine, Pittsylvania County, Virginia, U.S.A.: Implications for miscibility and solvus relations among jacobsite, galaxite, and magnetite
  12. An occurrence of igneous orthorhombic amphibole, Eriksberg gabbro, southern Sweden
  13. Crystal chemistry of Cr3+-V3+-rich clinopyroxenes
  14. The crystal structures of cesanite and its synthetic analogue—A comparison
  15. Disordered distribution of Cu in the crystal structure of leightonite, K2Ca2Cu(SO4)4 · 2H2O
  16. Neustädtelite and cobaltneustädtelite, the Fe3+ - and Co2+ -analogues of medenbachite
  17. Ciprianiite and mottanaite-(Ce), two new minerals of the hellandite group from Latium (Italy)
  18. Re-definition, nomenclature and crystal-chemistry of the hellandite group
  19. The new mineral baumstarkite and a structural reinvestigation of aramayoite and miargyrite
  20. Letters. Isotopic and elemental partitioning of boron between hydrous fluid and silicate melt
  21. Experimental evidence of three coexisting immiscible fluids in synthetic granitic pegmatite
  22. Quantifying surface areas of clays by atomic force microscopy
  23. A new high-pressure phase of FeSi
https://doi.org/10.2138/am-2002-5-608

Articles in the same Issue

  1. Formation of secondary pyrite and carbonate minerals in the Lower Williams Lake tailings basin, Elliot Lake, Ontario, Canada
  2. Hydroxyl in MgSiO3 akimotoite: A polarized and high-pressure IR study
  3. High-pressure IR-spectra and the thermodynamic properties of chloritoid
  4. Infrared and Raman study of interlayer anions CO32–, NO3–, SO42– and ClO4– in Mg/Al-hydrotalcite
  5. Effect of Fe oxidation state on the IR spectra of Garfield nontronite
  6. High-pressure single-crystal X-ray diffraction study of katoite hydrogarnet: Evidence for a phase transition from Ia3d →I4̅3d symmetry at 5 GPa
  7. The high-temperature P21/c-C2/c phase transition in Fe-free pyroxene (Ca0.15Mg1.85Si2O6): Structural and thermodynamic behavior
  8. Thermodynamics and stability of pseudobrookite-type MgTi2O5 (karrooite)
  9. Molecular orbital calculations on aluminosilicate tricluster molecules: Implications for the structure of aluminosilicate glasses
  10. Surface structures, stabilities, and growth of magnesian calcites: A computational investigation from the perspective of dolomite formation
  11. Spinels and other oxides in Mn-rich rocks from the Hutter Mine, Pittsylvania County, Virginia, U.S.A.: Implications for miscibility and solvus relations among jacobsite, galaxite, and magnetite
  12. An occurrence of igneous orthorhombic amphibole, Eriksberg gabbro, southern Sweden
  13. Crystal chemistry of Cr3+-V3+-rich clinopyroxenes
  14. The crystal structures of cesanite and its synthetic analogue—A comparison
  15. Disordered distribution of Cu in the crystal structure of leightonite, K2Ca2Cu(SO4)4 · 2H2O
  16. Neustädtelite and cobaltneustädtelite, the Fe3+ - and Co2+ -analogues of medenbachite
  17. Ciprianiite and mottanaite-(Ce), two new minerals of the hellandite group from Latium (Italy)
  18. Re-definition, nomenclature and crystal-chemistry of the hellandite group
  19. The new mineral baumstarkite and a structural reinvestigation of aramayoite and miargyrite
  20. Letters. Isotopic and elemental partitioning of boron between hydrous fluid and silicate melt
  21. Experimental evidence of three coexisting immiscible fluids in synthetic granitic pegmatite
  22. Quantifying surface areas of clays by atomic force microscopy
  23. A new high-pressure phase of FeSi
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