Tveitite-(Y) and REE-enriched fluorite from amazonite pegmatites of the Western Keivy,Kola Peninsula,Russia: Genetic crystal chemistry of natural Ca,REE-fluorides

Tveitite-(Y) as pods up to 10 cm across has been found at Mt. Rovgora, the Western Keivy, Kola Peninsula, Russia, in an albite-quartz-amazonite pegmatite vein related to alkaline granite. Tveitite-(Y) grains (up to 4 cm) are parallel microintergrowths of two isostructural varieties, Ca_9.5Na_1.7Y_5.2Ln_2.0F_42.6 and Ca_11.4Na_1.9Y_4.4Ln_1.4F_42.0. The idealized structural formula (Z = 3) is (Y, Na)₆(Ca, LREE)₆(Ca, Na, HREE)₆(Ca, Na)F₄₂; the simplified formula is (Ca, REE, Na)₁₃(Y, Na)₆F₄₂; space group R \(\bar 3\) a = 17.020, c = 9.679 Å. Lanthanoides are abbreviated in this paper as Ln, whereas Ln + Y as REE]. Nine fluorite samples containing from 0 to 18 mol % (REE)F₃ were examined by electron microprobe, X-ray powder diffraction, and IR spectroscopy. The crystal structure of natural yttrofluorite has been determined for the first time (R _aniso = 1.47 %): Fm3m, a = 5.493 Å; the structural formula is (Ca_0.82Y_0.12Ln_0.06)F_2.15. Earlier published and new data show that yttrofluorite containing (REE)F₃ > 20 mol % and REE-enriched fluorite with LREE > Y (HREE) are metastable under room conditions. In nature, tveitite-(Y) is a product of solid-state transformation of metastable yttrofluorite with (REE)F₃ > 20 mol %. Inferred protophases could have been exsolved into tveitite-(Y) variable in composition or tveitite-(Y) + yttrofluorite stable under normal conditions. The formation of tveitite-(Y) requires the erichment of a protophase not only in Y but also in LREE and HREE as stabilizing admixtures regularly distributed by different types of Ca-dominant structural sites. Tveitite-(Y) and yttrofluorite are geochemical indicators of a medium that is not only enriched in Y, Ln, and F, but also depleted in Na, Ca, CO₂ and P.