Tuite, the high-pressure γ-form of the Ca3(PO4)2 system, has been synthesized from chlorapatite at 15 GPa and 1300 °C using a multi-anvil apparatus. Its crystal structure was determined with single-crystal X-ray diffraction. It is isostructural with palmierite, with space group R3̄m and unit-cell parameters a = 5.2522(9) and c = 18.690(3) Å. The structure of tuite is characterized by three distinct polyhedra, PO4, Ca1O12, and Ca2O10, that are translationally interconnected in the sequence of PO4-Ca2O10-Ca1O12-Ca2O10-PO4 along the c axis.
Comparison of the CaSiO3 and Ca3(PO4)2 polymorphic systems shows a striking resemblance in the evolution of atomic packing arrangements as the polymorphic density increases. In both cases, the Ca atoms are progressively incorporated into the (Ca+O) close-packed monolayers, consistent with the hypothesis that close packing is a consequence of volume decrease as density increases. Based on this observation, we predict a possible high-pressure post-tuite phase.