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The MgTiO 3 -FeTiO 3 join at high pressure and temperature

1999; Mineralogical Society of America; Volume: 84; Issue: 10 Linguagem: Inglês

10.2138/am-1999-1013

1945-3027

Jennifer Linton, Yingwei Fei, Alexandra Navrotsky,

Thermal and Kinetic Analysis

The phase relations at high pressure and high temperature for the FeTiO 3 -MgTiO 3 join were determined using several different experimental methods.Through a series of multi-anvil experiments, a phase boundary with a negative slope was observed between MgTiO 3 I (ilmenite structure) and a high pressure phase with the MgTiO 3 II (lithium niobate structure) after quenching.The enthalpy of transformation of MgTiO 3 I to MgTiO 3 II was determined through transposed-temperature-drop calorimetry to be 28.78 ± 1.45 kJ/mol.The enthalpy of transformation from ilmenite to lithium niobate structure was also determined for three intermediate compositions on the FeTiO 3 -MgTiO 3 join, Fe 0.2 Mg 0.8 TiO 3 , Fe 0.5 Mg 0.5 TiO 3 and Fe 0.8 Mg 0.2 TiO 3 , and confirmed for FeTiO 3 , and was found to be a linear function of composition.These experiments represent one of the first successful calorimetric measurements on small samples (1 to 3 mg) synthesized at high pressures (15 to 21 GPa).X-ray analysis during compression of Fe 0.5 Mg 0.5 TiO 3 II in a diamond cell confirmed a room temperature transition at 28 GPa to Fe 0.5 Mg 0.5 TiO 3 III (a GdFeO 3 -type perovskite structure), similar to the transitions previously observed in FeTiO 3 and MnTiO 3 .The Fe 0.5 Mg 0.5 TiO 3 sample was heated to 802 °C at 21 GPa, and it was observed that the stable high temperature, high pressure phase is perovskite, Fe 0.5 Mg 0.5 TiO 3 III.The above data combined confirm the stability of a continuous perovskite solid solution at high pressure and temperature for the FeTiO 3 -MgTiO 3 join.21 GPa and 1200 °C, and phases with the lithium niobate structure were also synthesized for intermediate compositions between FeTiO 3 and MgTiO 3 (Linton et al. 1997).These magnesium-iron titanates with lithium niobate structures may be metastable quench phases from perovskites.This study shows through calorimetry and X-ray diffraction during compression of MgTiO 3 -FeTiO 3 with the lithium niobate structure in a diamond anvil cell (DAC) that these compounds are stable in perovskite-type structures at high pressure.This is the first example of a complete solid solution between iron and magnesium in the perovskite structure. EXPERIMENTAL METHODS SynthesisThe geikielite (MgTiO 3 I) starting material was synthesized from stoichiometric quantities of MgO and TiO 2 ground together under ethanol for one hour.Both MgO and TiO 2 were dried prior to weighing in Pt crucibles at 1400 and 1150 °C, respectively, for ~24 hours.The mixture was first heated at 1200 °C for 48 hours and then reground and heated for 24 hours at 1450 °C.The temperature was reduced to 800 °C and the sample removed and cooled in a desiccator.X-ray diffraction patterns of the product showed only geikielite peaks.MgTiO 3 -FeTiO 3 ilmenite solid solutions were synthesized from mixtures of predried Fe 2 O 3 and TiO 2 plus MgTiO 3 .The mixture was ground in an agate mortar for 1 hour, then heated