UNIVERZITA KOMENSKÉHO V BRATISLAVE VZÁCNOPRVKOVÁ ...

of Nb-Ta minerals (manganotantalite III, manganocolumbite III, ferrotapiolite III and
ferrowodginite III) most likely formed during the final stage of the pegmatite
consolidation and it is characterized by increase of Mn/(Mn+Fe) ratio. Microlite as well as
secondary phenakite and bertrandite, represent the youngest phases which formed
probably during a post-magmatic, low-temperature, hydrotermal overprint of the Jezuitské
lesy granitic pegmatite.
Accessory Nb-Ta oxide minerals were identified in pegmatitic leucogranite near
Duchonka, at the ridge between Lipová and Soľnisko hill, Považský Inovec Mts.. Studied
granite is a medium to coarse-grained leucocratic rock, which consists of quartz, Kfeldspar,
plagioclase (An04–28), biotite and muscovite, with accessory garnet (almandine –
spessartine) and fibrolitic sillimanite, rarely zircon, apatite, monazite and identified Nb-Ta
minerals. It can be geochemically characterized as relatively highly fractionated granites
with S-type characteristics.
Ferrocolumbite to ferrotantalite forms discrete tabular crystals 30-350 μm in size,
in association with quartz, alkali-feldspar, muscovite and sillimanite. The mineral
commonly shows progressive zoning with central parts enriched in Nb (ferrocolumbite)
and rims enriched in Ta (ferrotantalite). However, irregular convoluted zoning of border
parts, as a result of the late-magmatic to subsolidus dissolution-reprecipitation processes
are also present, or a reversal trend of zoning with decrease of Ta towards to border parts
of the crystals was detected. The composition of columbite-(Fe) to tantalite-(Fe) shows a
relatively constant Mn/(Mn+Fe) = 0.20-0.27 (locally 0.35-0.40), but an extreme range of
Nb-Ta fractionation: Ta/(Ta+Nb) = 0.18-0.72. Moreover, group of analyses in the rim
zones of the tantalite crystals attain Ta/(Ta+Nb) ratio over 0.63 value and pass to the field
of the miscibility gap between tantalite and tapiolite. Contents of Ti, W, Sn, Mg and other
elements in columbite-group minerals are low. Titanium attains mostly about 0.5 wt.%
TiO2 and a slightly decrease of Ti with increasing values of Ta/(Ta+Nb) was recorded.
Ferrotapiolite forms discrete irregular or shortly prismatic crystals, ca. 15-75 μm in size,
in association with quartz and sillimanite. Ferrotapiolite displays slightly irregular
compositional zonality in BSE with Mn/(Mn+Fe) = 0.03-0.04 and Ta/(Ta+Nb) = 0.88-
0.97, Ti and Sn contents are slightly higher than in columbite group minerals; ≤0.8 wt.%
SnO2 and ≤1.3 wt.% TiO2.
The Duchonka pegmatitic leucogranite revealed a presence of accessory
columbite-tantalite and tapiolite with unusually wide and up to extremely high Ta/Nb
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