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)F _3 natural system

摘要

Relationships between the chemical compositions and structures of the mineral tveitite from the southern Norway pegmatites (with the idealized formula Ca _(14)Y _5F _(43)) and Na-tveitite from the Rov mountain (Keivy, Kola Peninsula) Na _(2.5)Ca _(10)Ln _(1.5)Y _5F _(42) are considered. According to the structural mechanism of its formation, Na-tveitite is a nanocomposite crystal based on the crystalline matrix Ca _(14)Y _5F _(43) with the ordered arrangement of {Ca _8[CaY _5]F _(69)} clusters which contain anionic {F _(13)} cuboctahedra with F ~(1-) at the center. When Na-tveitite is formed, 29% of these clusters are statistically replaced by Na-"Y" clusters {[Na _(0.5)(Y,Ln) _(0.5)] _(14)F _(64)} with {F8} cubes at the center (analogs of matrix fluorite groups {Ca _(14)F _(64)}). This replacement gives rise to composition-imperfect (Na, Ca, "Y") cationic positions and occupancy-deficient F positions, which correspond to {F _(13)} cuboctahedra and the {F _8} cubes that replace them. The difference between Na-tveitite and fluorite phases M _(1-x)R _xF _(2+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).