Na-dominant (at the M2 site) eudialyte analogue: crystal structure and indicatory significance

Minerals of the eudialyte group from ultra-agpaitic associations are often characterized by high contents (up to the dominance) of sodium at the M 2 site, which is populated with iron in eudialyte. The features of blocky isomorphism with the replacement of IVFe2+ by IVNa and VNa at the M 2 micro-region are discussed. Using the methods of electron probe microanalysis, X-ray diffraction and IR spectroscopy, a potentially new mineral, M2Na-dominant analogue of eudialyte from the Ilimaussaq alkaline massif (Greenland), was investigated. Its crystal structure was refined to R = 5.6 % in the anisotropic approximation of atomic displacements using 1095 independent reflections with F > 3s( F ). The unit-cell parameters are: a = 14.208(1), c = 30.438(1) Å, V = 5321(1) Å3; the space group is R -3 m . The idealized formula of the mineral is ( Z = 3): (Na,H3O)15Ca6Zr3[Na2Fe][Si26O72](OH)2Cl∙2H2O.