10:45 AM - 11:00 AM
[SMP33-07] High-pressure phase transitions of minerals in the system MgO-TiO2
Keywords:high pressure, titanate mineral, phase transition
Mg2TiO4, MgTiO3 and MgTi2O5 form spinel(Sp)-, ilmenite(Ilm)-, and pseudobrookite(Pbr)-type solid solutions, respectively, with other endmembers such as Fe2TiO4, FeTiO3 and FeTi2O5. These phases occur as minor minerals in various igneous and metamorphic rocks. Previous studies on stability relations of Mg2TiO4, MgTiO3 and MgTi2O5 at high pressure were limited below about 3 GPa, except for that the transitions in MgTiO3 were examined up to about 18 GPa. In this study, we have examined the phase transitions of these magnesium titanates up to 28 GPa and 1800 °C.
We have determined the high-pressure phase relations in Mg2TiO4, MgTiO3 and MgTi2O5 at 4-28 GPa and 1000-1800 °C using multianvil apparatus with the quench method. MgTiO3 ilm transformed to the LiNbO3(Ln)-type high-pressure phase at 16-20 GPa and 1200-1600 °C. The Ln phase was interpreted as the retrograde transformation product from perovskite(Pv)-type MgTiO3 stable at high pressure and high temperature (Linton et al., 1999). Above 21-25 GPa, the recovered phases were MgO and a-PbO2-type TiO2, the latter of which was interpreted to be converted from baddeleyite(Bd)-type TiO2. The transition boundary from Pv to MgO + TiO2(Bd) has a positive Clapeyron slope. Mg2TiO4 Sp dissociates to MgO + MgTiO3 Ilm at about 1 GPa, and at higher pressure they changes to MgO + MgTiO3 Pv and subsequently to 2MgO + TiO2(Bd). MgTi2O5 Pbr decomposes at 1-2 GPa to MgTiO3 Ilm + TiO2 rutile(Ru), the latter of which transforms to a-PbO2-type TiO2. At higher pressure, the assemblage changes to MgTiO3 Pv + a-PbO2-type TiO2, and subsequently to MgO + 2TiO2(Bd). These results show that the mixtures of MgO and TiO2(Bd) are stable above 20-25 GPa in the three magnesium titanates. We also performed Rietveld structure refinement of Ln-type MgTiO3 using synchrotron powder X-ray diffraction data, and confirmed that Ti4+ and Mg2+ ions are slightly deviated from the centrosymmetric positions in the octahedra which gives polarity of the MgTiO3 Ln-phase.
The high-pressure transition behaviors of Mg2TiO4, MgTiO3 and MgTi2O5 are similar to those of Fe2TiO4, FeTiO3 and FeTi2O5, respectively, up to moderate pressures, where the phases transform to the assemblages involving MTiO3 Ilm and Pv (M = Mg, Fe). However, the transition behaviors are different at higher pressures: Fe2TiO4 transforms to CaTi2O4(CT)-type phase, and FeTiO3 dissociates to Fe2TiO4 CT + a dense orthorhombic FeTi2O5 phase (Akaogi et al., 2017).
We have determined the high-pressure phase relations in Mg2TiO4, MgTiO3 and MgTi2O5 at 4-28 GPa and 1000-1800 °C using multianvil apparatus with the quench method. MgTiO3 ilm transformed to the LiNbO3(Ln)-type high-pressure phase at 16-20 GPa and 1200-1600 °C. The Ln phase was interpreted as the retrograde transformation product from perovskite(Pv)-type MgTiO3 stable at high pressure and high temperature (Linton et al., 1999). Above 21-25 GPa, the recovered phases were MgO and a-PbO2-type TiO2, the latter of which was interpreted to be converted from baddeleyite(Bd)-type TiO2. The transition boundary from Pv to MgO + TiO2(Bd) has a positive Clapeyron slope. Mg2TiO4 Sp dissociates to MgO + MgTiO3 Ilm at about 1 GPa, and at higher pressure they changes to MgO + MgTiO3 Pv and subsequently to 2MgO + TiO2(Bd). MgTi2O5 Pbr decomposes at 1-2 GPa to MgTiO3 Ilm + TiO2 rutile(Ru), the latter of which transforms to a-PbO2-type TiO2. At higher pressure, the assemblage changes to MgTiO3 Pv + a-PbO2-type TiO2, and subsequently to MgO + 2TiO2(Bd). These results show that the mixtures of MgO and TiO2(Bd) are stable above 20-25 GPa in the three magnesium titanates. We also performed Rietveld structure refinement of Ln-type MgTiO3 using synchrotron powder X-ray diffraction data, and confirmed that Ti4+ and Mg2+ ions are slightly deviated from the centrosymmetric positions in the octahedra which gives polarity of the MgTiO3 Ln-phase.
The high-pressure transition behaviors of Mg2TiO4, MgTiO3 and MgTi2O5 are similar to those of Fe2TiO4, FeTiO3 and FeTi2O5, respectively, up to moderate pressures, where the phases transform to the assemblages involving MTiO3 Ilm and Pv (M = Mg, Fe). However, the transition behaviors are different at higher pressures: Fe2TiO4 transforms to CaTi2O4(CT)-type phase, and FeTiO3 dissociates to Fe2TiO4 CT + a dense orthorhombic FeTi2O5 phase (Akaogi et al., 2017).