Dissymmetrization in eudialyte-group minerals. VI. Features of P3-model structure of the high calcium eudialyte analogue from the Tamazert massif (Marocco)

Crystal structure of a high calcium eudialyte group member from the Tamazert massif, Morocco, with the following stoichiometry (calculated on 26 atoms Si + Nb, Z = 3): Na8.1K0.45Ln0.6Ca7.2Mn2.2Fe1.5Zr3.1Nb0.8—0.9Si25.1—25.2Cl0.3(CO3)0.4 by X-ray has been studied. The studied mineral is a fragment of a crystal of potentially new high calcium mineral with a complex growth zoning studied earlier. It differs from the latter by higher contents of Ca, Na and Nb. The unit-cell parameters are: a = 14.1530(1) Å, c = 30.0801(2) Å, V = 5218.1(5) Å3, space group P3. The structural model characterized by 163 crystallographic sites is refined to R = 5.48 % using 4644 independent reflections with с F > 3s(F). The cation distribution over the key sites in the frame of the low symmetry (space group P3) is more detailed as compared to the classical model with rhombohedral R-symmetry.