Abstract
Phase transitions of Mg2TiO4 and Fe2TiO4 were examined up to 28 GPa and 1600 °C using a multianvil apparatus. The quenched samples were examined by powder X-ray diffraction. With increasing pressure at high temperature, spinel-type Mg2TiO4 decomposes into MgO and ilmenite-type MgTiO3 which further transforms to perovskite-type MgTiO3. At 21 GPa, the assemblage of MgTiO3 perovskite + MgO changes to 2MgO + TiO2 with baddeleyite (or orthorhombic I)-type structure. Fe2TiO4 undergoes transitions similar to Mg2TiO4 with pressure: spinel-type Fe2TiO4 dissociates into FeO and ilmenite-type FeTiO3 which transforms to perovskite-type FeTiO3. Both of MgTiO3 and FeTiO3 perovskites change to LiNbO3-type phases on release of pressure. In Fe2TiO4, however, perovskite-type FeTiO3 and FeO combine into calcium titanate-type Fe2TiO4 at 15 GPa. The formation of calcium titanate-type Fe2TiO4 at high pressure may be explained by effects of crystal field stabilization and high spin–low spin transition in Fe2+ in the octahedral sites of calcium titanate-type Fe2TiO4. It is inferred from the determined phase relations that some of Fe2TiO4-rich titanomagnetite inclusions in diamonds recently found in São Luiz, Juina, Brazil, may be originally calcium titanate-type Fe2TiO4 at pressure above 15 GPa in the transition zone or lower mantle and transformed to spinel-type in the upper mantle conditions.
Highlights
Titanomagnetite (Fe3–x Tix O4 ), one of the important magnetic minerals in igneous and metamorphic rocks, is a solid solution between Fe3 O4 magnetite (x = 0) and Fe2 TiO4 ulvöspinel (x = 1), formed by replacement of Fe3+ by Ti4+
The previous study showed that at pressure below 1–2 GPa spinel (Sp)-type Mg2 TiO4 first dissociates into ilmenite (Ilm)-type MgTiO3 + MgO periclase (Pc) [7]
Our results indicate that Fe2 TiO4 Sp undergoes phase transitions to FeTiO3 Ilm + FeO Wu and subsequently to FeTiO3 Pv + FeO Wu at high pressure and high temperature (Figure 4)
Summary
Titanomagnetite (Fe3–x Tix O4 ), one of the important magnetic minerals in igneous and metamorphic rocks, is a solid solution between Fe3 O4 magnetite (x = 0) and Fe2 TiO4 ulvöspinel (x = 1), formed by replacement of Fe3+ by Ti4+. Fe2 TiO4 ulvöspinel has an inverse spinel structure where tetrahedral sites are occupied by Fe2+ and octahedral sites are occupied randomly by Fe2+. Mg2 TiO4 qandilite is one of endmembers of spinel solid solution in the system. Mg2 TiO4 qandilite has the inverse spinel structure [3]. Phase transitions of Fe2 TiO4 spinel at room temperature and high pressure have been studied in detail [2,4,5,6]. At 12 GPa, the tetragonal spinel transforms to CaTi2 O4 -type structure (Cmcm), in which octahedral
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