Structural features of potential new mineral of eudialyte group from the Lovozero massif, Kola peninsula

A potentially new representative of the eudialyte group from the Lovozero massif, which differs from all known minerals of the eudialyte group by a number of chemical and structural features has been investigated using electron microprobe analyses, single-crystal X-ray diffraction and IR spectroscopy. The mineral is trigonal, space group R 3 m ; the unit cell parameters are a = 14.198(1) , c = 30.380(1) , and V = 5303.9(1) (1) 3. The crystal structure is refined, to final reliability factor R = 4.2 % in the anisotropic approximation of atomic displacements using 3174 reflections with F > 3s( F ). The crystal-chemical formula of the potentially new mineral ( Z = 3) is Zr3[Ca3.9Mn1.2Fe0.9] [Na10.38(H3O,H2O)2.79Ce0.45Sr0.4K0.2] [NaVII2.13ZrV0.87] [Si24O72] [Si1.48Ti0.31Nb0.21] (OH)2.9(H2O)1.26(Cl,S)0.94. The idealized formula is (Na,H3O)13(Ca4Mn2)Zr3(Na2Zr)[Si26O72](OH)2Cl·H2O. Specific features of cation distribution over key sites including substitution of Fe2+ with Na at the М 2 site are discussed. The data obtained are in a good agreement with the IR spectrum of the potentially new mineral which shows characteristic bands corresponding to stretching vibrations of ZrV-O bonds, H2O molecules and H3O+ cations as well as strong band corresponding to SiO4 tetrahedra at the M 3 and M 4 sites. A short review of crystal structures of Na-dominant (at the M 2 site) eudialyte-group minerals is given. It is concluded that these minerals are specific accessory components of highly agpaitic magmatic rocks (hyperalcaline foyaites) and pegmatites.