The well-crystallised polycrystalline sample of the potassium-doped waterless apatite Ca_8.40K_0.34 · [(PO ₄)_3.15(HPO₄)_1.30(CO ₃)]_1.55(OH)₂ (a = 9.4015(3), c = 6.8985(2) Å, sp.gr. P6₃/m, Z = 1) with very high carbonate content (7.7 wt% of CO₂) has been prepared by the reaction of calcite with KH ₂PO₄ solution under hydrothermal conditions. The Rietveld refinement (R_wp = 0.052) of its crystal structure allowed to find the new orientation of a carbonate triangle. It crosses a PO₄ tetrahedron just in its centre so that the positions of a carbon atom coincides with that of a phosphorous atom, and the triangle plane is parallel to the z-axis. The charge disbalance is compensated mainly by the vacancies in the cation sites. The absence of lattice water in the channels is the consequence of the extremely high concentrations of the divalent anions replacing PO ₄ tetrahedra in the structure.