Structural mechanism of the formation of mineral Na-tveitite—a new type of phase with a fluorite-derivative structure—in the NaF-CaF2-(Y,Ln)F3 natural system

Relationships between the chemical compositions and structures of the mineral tveitite from the southern Norway pegmatites (with the idealized formula Ca14Y5F43) and Na-tveitite from the Rov mountain (Keivy, Kola Peninsula) Na2.5Ca10Ln1.5Y5F42 are considered. According to the structural mechanism of its formation, Na-tveitite is a nanocomposite crystal based on the crystalline matrix Ca14Y5F43 with the ordered arrangement of {Ca8[CaY5]F69} clusters which contain anionic {F13} cuboctahedra with F1− at the center. When Na-tveitite is formed, 29% of these clusters are statistically replaced by Na-"Y" clusters {[Na0.5(Y,Ln)0.5]14F64} with {F8} cubes at the center (analogs of matrix fluorite groups {Ca14F64}). This replacement gives rise to composition-imperfect (Na, Ca, "Y") cationic positions and occupancy-deficient F positions, which correspond to {F13} cuboctahedra and the {F8} cubes that replace them. The difference between Na-tveitite and fluorite phases M1−xRxF2+x 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).