Showing a high ability to various isomorphous substitutions, eudialyte can be used as an indica tor of the chemistry of magmatic and postmagmatic processes of the alkaline rock formation. We have systematically studied eudialyte from rischorrites (poikilitic nepheline syenites) of the Khibina Massif. The chemical composition of eudialyte is highly variable and heterogeneous. Its grains are partly replaced by later minerals, which indicates that metasomatism played an important role in the feldspathic rock formation. Principal eudialyte alterations are as follows: (a) in the feldspar-rich rocks (juvites and rischorrites), the initial Na-eudialyte is replaced by its alkali-rich K-Na variety, which is subsequently subjected to hypogene leaching to form K/Na variety; and (b) in contact zones between massive urtites and host rischorrites, initial eudialyte is enriched in Ti and Na or is replaced by zirsinalite (Na-rich Zr-silicate). The increasing K/Na ratio and total alkalinity of the eudialyte indicate the alkalinity increase and the development of acid-base interaction in the mineral-forming medium, as well as the metasomatic formation of rischorrites after melteigites and urtites.

Bomemanite is a rare alkali titanosilicate occurring in the natrolite zone of the Yubileynaya hyperagpaitic pegmatite. on Karnasurt Mountain, in the Lovozero massif, Kola Peninsula, Russia. The mineral is light yellow, lamellar (001) and elongate [010]. No single crystals suitable for X-ray crystallography are available. New electron-microprobe chemical analyses, selected-area electron diffraction (SAED) and X-ray powder diffraction show that bornemanite, BaNa3[(Na,Ti)(4)[(Ti,Nb)(2)O2Si4O14] (F,OH)(2)]PO4, is monoclinic I11b, a 5.498(4), b 7.120(6), c 47.95(4) Angstrom, gamma 88.4(1)degrees; Z = 4. By comparison with structural and chemical data for titanosilicates based on a bafertisite-like layer (heterophyllosilicates), a model of the structure of bornemanite has been obtained. This model has been refined by the distance least-squares technique (DLS program) and tested against calculated powder-diffraction and SAED patterns. The structure of bornemanite can be described as a [001] stacking of heterophyllosilicate layers, where lomonosovite and seidozerite contents alternate in the interlayer spaces. Thus this structure is the first documented case of a heterophyllosilicate based on modules of two other structures belonging to the same modular series, i.e., the mero-plesiotype bafertisite series. The lomonosovite-seidozerite polysomatic series is defined. In contrast to the original description, bornemanite is considered monoclinic and not orthorhombic, and lacks one cation per formula unit (mainly Na). Possible leaching of alkalis and the solid-state oriented transformation lomonosovite - bornemanite are discussed.