Structural mechanism of the formation of mineral Na-tveitite-a new type of phase with a fluorite-derivative structure-in the NaF-CaF-(Y, Ln)F natural system.

Golubev, A.; Otroshchenko, L.; Sobolev, B.

Relationships between the chemical compositions and structures of the mineral tveitite from the southern Norway pegmatites (with the idealized formula CaYF) and Na-tveitite from the Rov mountain (Keivy, Kola Peninsula) NaCa LnYF are considered. According to the structural mechanism of its formation, Na-tveitite is a nanocomposite crystal based on the crystalline matrix CaYF with the ordered arrangement of {Ca[CaY]F} clusters which contain anionic {F} cuboctahedra with F at the center. When Na-tveitite is formed, 29% of these clusters are statistically replaced by Na-'Y' clusters {[Na(Y, Ln)]F} with {F} cubes at the center (analogs of matrix fluorite groups {CaF}). This replacement gives rise to composition-imperfect (Na, Ca, 'Y') cationic positions and occupancy-deficient F positions, which correspond to {F} cuboctahedra and the {F} cubes that replace them. The difference between Na-tveitite and fluorite phases M RF is as follows: its matrix is the structure of the ordered phase (tveitite) into which Na-containing rare earth fragments of fluorite-type structure are incorporated instead of ordered-phase structural blocks (clusters).

KOLA Peninsula (Russia); RUSSIA; MINERALS; CRYSTAL structure; FLUORITE; POLYHEDRA; PEGMATITES

Crystallography Reports, 2012, Vol 57, Issue 2, p308

1063-7745

Academic Journal

10.1134/S1063774512020083