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

Abstract

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 varietiesspace group 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)F 3 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 %): Fm 3 m , a = 5.493 Å; the structural formula is (Ca 0.82 Y 0.12 Ln 0.06 )F 2.15 . Earlier published and new data show that yttrofluorite containing (REE)F 3 > 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)F 3 > 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, CO 2 and P.

REE-rich Fluorite in Granite from Zinnwald/East Erzgebirge/Germany

“…Figure 7 shows Y + Ln versus Ln (values in at%) with the zero point for pure water-clear synthetic fluorite (not shown in the table). The circles are the data from Table 1 plus unpublished data [1], and the triangles represent the data for REE-rich fluorites and tveitite-(Y) from Pekov et al, 2009 [12]. There are two trends: the dashed blue line [12] corresponds to the tveitite-(Y) trend, and the dashed black line corresponds to the fluocerite trend for the REE-rich Zinnwald fluorites.…”

“…The simultaneous crystallization of xenotime-(Y) and monazite-(Y) together with the REE-rich fluorite prevents the crystallization of the fluocerite [12] standing at the end of the trend (fluocerite trend of Figure 7). That is also true for other REE-rich fluorite in the region and around Ehrenfriedersdorf.…”

“…Compared with this, the REE-rich Zinnwald fluorite contains up to 5.3 % (g/g) Y and 33.1 % (g/g) REE's. The high REE content of the fluorite here is the result of the supercritical fluids in the Zinnwald region being deleted in phosphorus [12]. Figure 8 shows the chondrite standardized REE distribution of the two different fluorites from Zinnwald (ZW) and the Sachsenhöhe near Zinnwald [13].…”