Cations ordering in minerals with complex composition: P3-sergevanite - the second representative of eudialyte group with primitive cell

Sergevanite with idealized formula (Na12KH3O)[Ca3(Mn2Ca)](Na2Fe)Zr3Ti0.5Si25.5 O72OH2.(H2O)2 (Z = 3) - a low calcium eudialyte group mineral from Lovozero alkaline complex was investigated repeatedly using obtained earlier the X-ray diffraction data. Its crystal structure with 174 sites was refined in the frame of trigonal primitive cell to R = 4.7 % on the base of 5613 averaged independent reflections with F > 3 (F). The unit-cell parameters were: a = 14. 2179(1), c = 30.3492(3) Å, V = 5313.1(1) Å3, space group P3. More detailed, close to real, cations distribution on sites was established. Studied sergevanite was the second mineral of eudialyte group with lowering symmetry from rhombohedral (R3) to trigonal (P3).