Pekov I.V.*,**, Yakubovich O.V.*
**Vernadsky Institute of Geochemistry and Analytical Chemistry RAS,
remarkable crystal chemical feature of fluorite is its capacity for concentration
of REE3+ (Y and
lanthanides, Ln) with formation of
the interstitial solid solutions with general formula
(Ca1-xREEx)F2+x. These crystals keep the space group Fm3m,
the fluorite unit cell and a unique cation site fully occupied jointly by Ca and
REE, whereas additional one or
several partially occupied F sites appear. The upper limit of REE content possible for the fluorite
structural type is still unknown. The solid solution with 66 mol. % (REE)F3 in CaF2 was
reported by Besse & Capestan (1967); among structurally studied synthetic RE-bearing fluorites, the sample Ca0.607Ce0.393F2.393
(Aleksandrov & Garashina, 1969) is the richest in REE. Yttrofluorite with (REE)F3
content up to 10, rarely to 20 mol. % is uncommon mineral known in the derivatives
of alkaline granites and in rare-earth granitic pegmatites. Tveitite-(Y), a
cation-ordered rhombohedral mineral structurally related to yttrofluorite with
higher REE content is significantly
rarer. Before our work it has been described only in three pegmatites related
to alkaline granites: two in
we found tveitite-(Y) in an amazonitic pegmatite related to alkaline-granite
intrusion at Rovgora (W. Keivy,
(REE)F3 content in natural
yttrofluorite is not higher than 20 mol. % whereas it is not lower than 30 mol.
% in tveitite-(Y). Basing on the patterns of micro-heterogeneity of
tveitite-(Y) samples from
Many synthetic cation-disordered cubic (Fm3m) defect fluorite-type compounds occur in the compositional (REE:Ca ratio) field corresponding to tveitite or its hypothetical proto-phases. Note that they were grown using the Stockbarger method i.e. under high temperature with F-rich fluid, similar to the natural postmagmatic processes. However, unlike quenched synthetic crystals, minerals were annealed for geologigal time favoring phase transitions and breakdowns. Thus, yttrofluorite with (REE)F3 less than ~20 mol. % only could §successfully overcomes the time examinationŠ whereas its more REE-rich varieties are metastable and have been transformed.
A lot of synthetic compounds with structures different from the fluorite type are known in the CaF2 Ń (REE)F3 system. Several of them lie in the same compositional field as yttrofluorite and tveitite. At the same time, (1) no synthetic analogues of tveitite and (2) no other natural Ca,REE fluorides, except tveitite-(Y) and yttrofluorite. This fact can be explained by a significantly simpler composition of synthesis systems using only Y or one of Ln together with Ca and F, while a full series of REE is present in natural systems. The stabilization of tveitite-(Y) is probably possible if threshold concentrations not only of Y, but also of both LREE and HREE would be overcome. From this point of view, a transformation of one cationic site in fluorite structure in four different positions in tveitite looks reasonable.
An affinity of fluorite to Y and HREE
compared to its affinity to LREE is
significantly stronger. §ExcessiveŠ increase of LREE in RE-fluorite
results in its breakdown to yttrofluorite and fluocerite, (LREE)F3, a hexagonal phase having different crystal structure.
E.g., aggregates of yttrofluorite and
fluocerite-(Ce) with a characteristic §dactyloscopicŠ breakdown structure were
reported in Katugin (
The rare occurrence of yttrofluorite and, especially, tveitite-(Y) can be explained by specific chemical conditions of their mineral-forming media. Thus, they could be considered as sensitive geochemical indicators. Except an enrichment of the medium by Y, Ln and F, a deficiency of Ca, Na, CO2 and P looks necessary for their formation. An increase of the Ca concentration, if the F activity is high, results in the increase of a fluorite amount and its depletion by REE. An increase of the CO2, P or Na activities results in a re-distribution of REE into phases with different structures i.e. the REE-depleted fluorite parageneses with rare-earth carbonates, or phosphates, or gagarinite-(Y), NaCaYF6, should be expected. As a first step, yttrofluorite releases LREE (forming bastnaesite or monazite); the further increase of CO2 or P activity initiates the depletion of yttrofluorite also by Y and HREE, with formation of the §usualŠ fluorite. The decrease of F activity in the mineral-forming medium, when the REE, CO2 and Ca concentrations are high shift the equilibrium < fluorite + bastnaesite/synchysite ↔ calcite + bastnaesite/synchysite > to the right, i.e. the parageneses §fluoride + fluorocarbonateŠ, typical for the derivatives of alkaline granites, change to the parageneses §carbonate + fluorocarbonateŠ that characterizes┴ miascitic and, surely, carbonatitic systems.
This study was supported by grant of President of Russain Federation No. 863.2008.5 and grant of Russian Science Support Foundation (I.V.P.).
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