Modular structure of a potassium-rich analogue of eudialyte with a doubled parameter c

The structure of a new potassium-rich representative of the eudialyte group described by the idealized formula Na27K8Ca12Fe3Zr6[Si52O144](O,OH,H2O)6Cl2 was established by the methods of the X-ray diffraction analysis (R = 0.045, 3900 reflections). The unit-cell parameters are a = 14.249(1) Å, c = 60.969(7) Å, V = 10 720.3 Å3, sp. gr. R3m. The structure of the mineral is characterized by in-layer order in cation arrangement resulting in doubling of the parameter c and the formation of two different modules, of which one corresponds to composition and structure of eudialyte (with an admixture of kentbrooksite) and the other is the derivative of alluaivite.