UNIVERZITA KOMENSKÉHO V BRATISLAVE VZÁCNOPRVKOVÁ ...

fractionation level. Ferrotantalite compositions with Ta/(Ta+Nb) = 0.63-0.72 and
Mn/(Mn+Fe) = 0.20-0.27, which penetrate the empirically defined natural tapiolitetapiolite
miscibility gap, belong to striking feature of the Duchonka leucogranite. Despite
lack of XRD determination, the phase very likely belongs to ferrotantalite, and not
ferrotapiolite. These anomalous compositions form external parts of columbite-tantalite
crystals and they represent a part of the same columbite to tantalite evolutional trend with
rapid Ta/Nb increasing and stable Mn/Fe ratio. On the contrary, ferrotapiolite occurs as
separate grains with different textural pattern and chemistry. Such tantalite or tapiolite
compositions are exceptional and they are considered as metastable. On the other hand, an
empirically determined field of the tantalite-tapiolite miscibility gap is based only on
known data from granitic pegmatites. However, the Duchonka pegmatitic leucogranite
had to crystallize at higher temperature compared to classical granitic pegmatites. For that
reason, the tantalite-tapiolite miscibility gap has most probably smaller range at such
conditions.
Nb-Ta-W mineral assemblage was found in the highly fractionated leucogranite at
Dlhá dolina, near Gemerská Poloma. The studied rock can be characterized as highly
evolved, specialized S-type topaz-albite granite enriched in Li, B and F with greisenized
and albitized parts. It consists mainly of albite, quartz, muscovite and zinnwaldite.
Common accessory minerals include topaz, cassiterite, turmaline, rutile, wolframite,
apatite and Nb-Ta-W phases. Besides the most widespread manganocolumbite, rarely
ferrocolumbite, W-rich ixiolite, pyrochlore, microlite and uranmicrolite were identified.
Ferrocolumbite forms discrete crystals up to 30 µm in size, usually enclosed in
albite or quartz, in association with cassiterite, rarely also microlite. The chemical
composition is relatively constant, with Ta/(Ta+Nb) = 0.12–0.16 and Mn/(Mn+Fe) =
0.32–0.46. Ferrocolumbite reveals elevated content of W (up to 7.2 wt.% WO3).
Manganocolumbite forms discrete irregular crystals up to 200 µm in size, in
association with cassiterite, rutile or apatite. The chemical composition is relatively
heterogeneous with Ta/(Ta+Nb) = 0.06–0.50 and Mn/(Mn+Fe) = 0.50–0.95.
Manganocolumbite is often replaced by microlite. They have developed progressive
zoning with central parts enriched in Nb and outer parts enriched in Ta. Simultaneously,
decrease in Mn/(Mn+Fe ratio was detected with increase of Ta/(Ta+Nb) ratio in outer
parts of manganocolumbite crystals. Manganocolumbite exhibit elevated content of W (up
to 10 wt.% WO3) and Ti (up to 3.8 wt.% TiO2).
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