Thermodynamics of the R3̅ to R3̅c phase transition in the ilmenite-hematite solid solution
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Richard J. Harrison
Abstract
The thermodynamics of cation ordering in the ilmenite-hematite solid solution (FeTiO3)x(Fe2O3)1-x were re-examined in view of a recent in-situ neutron diffraction study of the R3̅ to R3̅ phase transition. A macroscopic thermodynamic model is developed, in which the excess enthalpy of ordering is described by a fourth-order polynomial function of the long-range order parameter, Q, and the excess entropy is described by the configurational point entropy. The enthalpy coefficients are determined as a function of composition by fitting to the neutron diffraction data. The fourth-order energy coefficient increases smoothly with increasing Ti-content, indicating a change in the character of the transition from second-order (x < 0.87) to first-order (x > 0.87). Monte Carlo simulations confirm that the excess enthalpy is well described by a fourth-order polynomial, and demonstrate that the reduction in configurational entropy due to short-range ordering can be described by simply scaling the point entropy. This allows the enthalpy coefficients in the macroscopic model to be corrected for the effects of short-range ordering, yielding a revised estimate of x = 0.92 for the position of the tricritical point.
Monte Carlo simulations are used to study both ordering and exsolution in the solid solution, allowing the paramagnetic portion of the equilibrium phase diagram to be derived. The changing character of the phase transition is reproduced successfully by treating Fe2+-Ti and Fe3+-Ti interactions separately in the simulations. Intralayer Fe2+-Ti interactions are negative (i.e., ordering of Fe2+ and Ti within the 001 layers is favored) whereas Fe3+-Ti intralayer interactions are positive (i.e., separation of Fe3+ and Ti is favored). The λ-line for the R3̅ - to R3̅ - c phase transition intersects the lowtemperature miscibility gap at a tricritical point near x = 0.6 and T = 800 °C, in reasonable agreement with previous thermodynamic models of the solid solution
© 2015 by Walter de Gruyter Berlin/Boston
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Articles in the same Issue
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- A double magmatic heat pump at the core-mantle boundary
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- Multiple pyroxene and amphibole assemblages in the amphibolite facies: Bulk compositional controls
- Margarite-corundum phyllites from the Appalachian orogen of South Carolina: Mineralogy and metamorphic history
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- Kyanite eclogite thermobarometry and evidence for thrusting of UHP over HP metamorphic rocks, Nordøyane, Western Gneiss Region, Norway
- Monazite geochronology of UHP and HP metamorphism, deformation, and exhumation, Nordøyane, Western Gneiss Region, Norway
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