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

Tuite, -Ca-3(PO4)(2), is a potential host for rare-earth elements in the deep mantle. Eu-bearing tuite single crystals with chemical formulas of Ca2.93Eu0.04(PO4)(2) and Ca2.88Eu0.07Na0.06(PO4)(2) were synthesized at 16 GPa and 1600 degrees 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 2Eu(3+)+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.