Genetic crystal chemistry of natural fluorite-related Ca,REE fluorides

Pekov I.V.*^,**, Yakubovich O.V.*

*Lomonosov Moscow State University, Moscow, Russia;

**Vernadsky Institute of Geochemistry and Analytical Chemistry RAS, Moscow, Russia

A 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 (Ca_1-xREE_x)F_2+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 Ca_0.607Ce_0.393F_2.393 (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 Norway and one in Texas. The first crystal structure determination of tveitite-(Y) (Bevan e.a., 1982) gave the approximative structural model (without details for the atom distribution over structural positions) and an ambiguous mineral formula. Á

Recently, we found tveitite-(Y) in an amazonitic pegmatite related to alkaline-granite intrusion at Rovgora (W. Keivy, Kola, Russia) and studied it in detail. Samples from Rovgora consist of two phases corresponding to varieties of tveitite-(Y) with different Ca:REE ratios. The REE-rich variety (it is the REE-richest known tveitite) forms regular 3D system of lamellae in a matrix of the Ca-enriched variety. The structure of tveitite-(Y) was studied for the REE-rich single-crystal from Rovgora and a refined formula was proposed. The mineral is trigonal, R-3, with a = 17.020, c = 9.679 Å (Yakubovich e.a., 2007). Tveitite-(Y) is considered to represent a fluorite-based interstitial solid solution. 11 independent F sites have been found in its crystal structure. Cation positions are practically the same as for Ca in fluorite. Symmetry lowering is caused by subdivision of the single A site of fluorite in four independent sites in tveitite-(Y): A1, A2, A3 and A4. Eight types of polyhedra AF_n with potentially different cation occupancies may occur in tveitite-(Y). The crystallochemical formula of tveitite-(Y) from Rovgora (Z=18) is: (Y_0.883Na_0.106)_A1(Ca_0.841LREE_0.159)_A2(Ca_0.716Na_0.204HREE_0.080)_A3(Ca_0.092Na_0.074)_A4F_6.952. The idealized sitructural formula is: (Y,Na)₆(Ca,LREE)₆(Ca,Na,HREE)₆(Ca,Na)F₄₂, or, in its simplified form can be presented as: (Ca,REE,Na)₁₃(Y,Na)₆F₄₂. In this mineral, LREE and HREE fractionate between two types of Ca-dominant sites different in both size and form of polyhedra. We also solved the crystal structure of natural yttrofluorite, using a sample with (Ca_0.80Na_0.02Y_0.12Ln_0.06)F_2.20 composition from an alkaline-granite-related amazonitic pegmatite at Mt. Ploskaya (W. Keivy). This mineral represents the fluorite ss structural type (Fm3m, a = 5.4926 Å) with an additional anion site F(2), which is typical for the synthetic analogues of RE-fluorite.

The (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 Høydalen, Norway (Bergstøl e.a., 1977), and Rovgora, we may certainly conclude that this mineral is a product of the solid-state transformation of cation-disordered REE-rich yttrofluorite. In Høydalen sample, differently oriented sub-individuals of polysynthetic twins of tveitite-(Y) are chemically identical. It is probably caused by chemical identity (~30 at. % REE of sum of cations) of the transformation product (rhombohedral tveitite) and the initial phase (cubic REE-fluorite). Rovgora crystals could be formed as a result of a phase transition of the cubic proto-phase containing more REE (36-37 at. %) with further breakdown to the system of isotypic but chemically different tveitite phases.

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 õsuccessfully overcomes the time examinationæ 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 ã (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. õExcessiveæ 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 õdactyloscopicæ breakdown structure were reported in Katugin (Siberia, Russia) (Arkhangelôskaya, 1970). If CO₂ or P present in the mineral-forming system then surplus of LREE released from fluorite to Y-poor bastnaesite-(Ce) or monazite-(Ce) giving the õemulsionæ impregnation in a fluoride matrix. A probability to find REE-rich fluorite with LREE > Y in atomic proportions (õcerfluoriteæ) seems to be low, in spite of an existence of synthetic (Ca_1-xLREE_x)F_2+x with x > 0.3 which are metastable quenched phases.

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 õusualæ 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 õfluoride + fluorocarbonateæ, typical for the derivatives of alkaline granites, change to the parageneses õcarbonate + fluorocarbonateæ that characterizesÁ 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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