High-Pressure and High-Temperature Phase Transitions in Fe2TiO4 and Mg2TiO4 with Implications for Titanomagnetite Inclusions in Superdeep Diamonds
High-Pressure and High-Temperature Phase Transitions in Fe2TiO4 and Mg2TiO4 with Implications for Titanomagnetite Inclusions in Superdeep Diamonds
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DOI：
10.3390/min9100614
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发表时间：
2019-10-01
期刊：
影响因子：
2.5
通讯作者：
Kojitani, Hiroshi
中科院分区：
文献类型：
--
作者：
Akaogi, Masaki;Tajima, Taisuke;Kojitani, Hiroshi
使用多砧装置在高达 28 GPa 和 1600 摄氏度的条件下检查 Mg2TiO4 和 Fe2TiO4 的相变。通过粉末X射线衍射检查淬火的样品。随着高温压力的增加，尖晶石型Mg2TiO4分解为MgO和钛铁矿型MgTiO3，钛铁矿型MgTiO3进一步转变为钙钛矿型MgTiO3。在接近 21 GPa 时，MgTiO3 钙钛矿 + MgO 的组合转变为具有斜锆石（或斜方 I）型结构的 2MgO + TiO2。 Fe2TiO4 在压力下会经历与 Mg2TiO4 类似的转变：尖晶石型 Fe2TiO4 解离成 FeO 和钛铁矿型 FeTiO3，后者转变成钙钛矿型 FeTiO3。释放压力后，MgTiO3 和 FeTiO3 钙钛矿都会转变为 LiNbO3 型相。然而，在 Fe2TiO4 中，钙钛矿型 FeTiO3 和 FeO 在接近 15 GPa 的压力下结合成钛酸钙型 Fe2TiO4。高压下钛酸钙型 Fe2TiO4 的形成可能是通过钛酸钙型 Fe2TiO4 八面体位点中 Fe2+ 的晶体场稳定和高自旋-低自旋转变的影响来解释的。从确定的相关系推断，最近在巴西朱伊纳圣路易斯发现的一些富含Fe2TiO4的钛磁铁矿包裹体，在过渡带或下地幔压力高于15 GPa时可能最初是钛酸钙型Fe2TiO4，在上地幔条件下转变为尖晶石型。
Phase transitions of Mg2TiO4 and Fe2TiO4 were examined up to 28 GPa and 1600 degrees 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 similar to 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 similar to 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 Sao Luiz, Juina, Brazil, may be originally calcium titanate-type Fe2TiO4 at pressure above similar to 15 GPa in the transition zone or lower mantle and transformed to spinel-type in the upper mantle conditions.