中国科学院地球化学研究所机构知识库

Tuite, γ-Ca₃(PO₄)₂, is a potential host for rare-earth elements in the deep mantle. Eu-bearing tuite single crystals with chemical formulas of Ca_2.93Eu_0.04(PO₄)₂ and Ca_2.88Eu_0.07Na_0.06(PO₄)₂ were synthesized at 16 GPa and 1600 °C, and their structures were investigated at room temperature with single-crystal X-ray diffraction. The structure refinements of Eu-bearing tuite indicate the preference of Eu³⁺ for the smaller 6-coordinated Ca1 site with a coupled vacancy due to the substitution of 2Eu³⁺ + □ → 3 Ca²⁺. In NaEu-bearing tuite, both Eu³⁺ and Na⁺ occupy the smaller 6-coordinated Ca1 site according to the coupled substitution of Eu³⁺ + Na⁺ → 2 Ca²⁺. The presence of Na results in more Eu₂O₃ in NaEu-bearing tuite, because the coupled substitution is energetically more favorable. REE-bearing tuite could be found on the Moon and the deep Earth’s mantle as a consequence of REE-bearing apatite or/and merrillite transformation under high-pressure and high-temperature conditions.