Crystal structure and indicatopy significance of odikhinchaite from the Khibiny alkaline complex

Odikhinchaite, a Sr-Mn-Nb-CO3-dominant eudialyte-group mineral from the Khibiny alkaline complex has been investigated using the methods of electron probe microanalysis, X-ray diffraction, infrared and Mössbauer spectroscopy. The crystal structure was refined to R = 3.4 % in the anisotropic approximation of atomic displacements using 3815 independent reflections with F > 3s( F ). The unit-cell parameters are: a = 14.2709(1), c = 30.023(1) Å, V = 5295.33(7) Å3; the space group is R 3 m . The simplified formula of the mineral is ( Z = 3): Na11Sr3(Mn2+,Fe2+,Fe3+)Ca6Zr3Nb[Si25O72(OH)](CO3)Cl(OH,O)4. The studied sample is the second find of this rare mineral worldwide, represented by a variety with high Fe content in the M 2 micro-region, belonging to the taseqite - odikhinchaite solid-solution series. The significance of odikhinchaite as an indicator of a stage corresponding to maximum Sr activity in specific ultraagpaitic pegmatites is discussed.