Raite, manganonordite-(Ce), and ferronordite-(Ce) from the Lovozero Massif: Crystal structures and mineralogical geochemistry

The structures of three minerals discovered recently in alkaline rocks of L ovozero pegmatites have been determined. The structure and composition of r aite Na3Mn3Ti0.25[Si2O5](4)(OH)(2).10H(2)O are refined using a crystal with an effective size of 6 mu m (synchrotron radiation). It is found that the unit cell of the mineral is monoclinic and not orthorhombic. Comparison of raite with structurally related minerals makes it possible to describe its structure on the basis of the modular theory of crystal structures. The str uctures of two new representatives of the nordite mineralogical family, nam ely, ferronordite-(Ce) and manganonordite-(Ce), are refined. These minerals are described by the general formula Na3SrTRM2+Si6O17 With TR being predom inantly Ce-cations, and M2+ being predominantly Fe2+ and Mn2+ in ferronordi te-(Ce) and manganonordite-(Ce), respectively. The silicate ribbons in nord ite are topologically comparable with these in vlasovite and haiweeite. New Mn-silicates found in high-alkaline rocks from different areas and the res ults of structural studies make it possible to reveal crystal-chemical feat ures associated with their crystallogenesis.