Crystal chemistry of Eu-bearing tuite synthesized at high-pressure and high-temperature conditions

Tuite, γ-Ca ₃ (PO ₄ ) ₂ , is a potential host for rare-earth elements in the deep mantle. Eu-bearing tuite single crystals with chemical formulas of Ca _2.93 Eu _0.04 (PO ₄ ) ₂ and Ca _2.88 Eu _0.07 Na _0.06 (PO ₄ ) 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 Eu ³⁺ for the smaller 6-coordinated Ca1 site with a coupled vacancy due to the substitution of 2Eu ³⁺ + □ → 3 Ca ²⁺ . In NaEu-bearing tuite, both Eu ³⁺ and Na ⁺ occupy the smaller 6-coordinated Ca1 site according to the coupled substitution of Eu ³⁺ + Na ⁺ → 2 Ca ²⁺ . The presence of Na results in more Eu ₂ O ₃ 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.