Genetic
Pekov I.V.*,**,
Yakubovich O.V.*
*
**Vernadsky Institute of Geochemistry and Analytical Chemistry RAS,
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
(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
Recently,
we found tveitite-(Y) in an amazonitic pegmatite related to alkaline-granite
intrusion at Rovgora (W. Keivy,
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
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 (
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.).
References:
Aleksandrov
V.B., Garashina L.S. New data on structures of CaF2 ã TRF3
solid solutions // Doklady AN SSSR. 1969. Vol. 189. No. 2. P. 307-310 (in
Russian).
Arkhangelôskaya
V.V. On solid-solution breakdown structure in natural rare-earth fluorite //
Doklady AN SSSR. 1970. Vol. 195. No. 6. P. 1411-1414 (in Russian).
Bergstøl S., Jensen B.B., Neumann H. Tveitite,
a new calcium yttrium fluoride // Lithos. 1977. Vol. 10. P. 81-87.
Besse
J.P., Capestan M. Solutions solides de CeF3 dans quelques fluorides
divalents type fluorine // Bull. Soc. Chim. de France. 1967. P. 1341-1344.
Bevan D.J.M., Strähle J., Greis O. The
crystal structure of tveitite, an ordered yttrofluorite mineral // J. Solid
State Chem. 1982. Vol. 44. P. 75-81.
Yakubovich O.V.,