Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original Paper

Petr Černý, Milan Novák, Ron Chapman, Karen J. Ferreira

Subsolidus behavior of niobian rutile from the Písek region, Czech Republic: a model for exsolution in W- and Fe²⁺>>Fe³⁺-rich phases

Journal of Geosciences, volume 52 (2007), issue 1-2, 143 - 159

DOI: http://doi.org/10.3190/jgeosci.008

Niobian rutile with substantial W content is associated with black tourmaline, beryl, zircon, xenotime-(Y), monazite-(Ce), "písekite", and possibly pseudorutile or pseudobrookite in two cogenetic, beryl-columbite-subtype, granitic pegmatites at Písek and Údraž, southern Bohemia, Czech Republic. Crystals, up to 3×1 cm in size, consist of relics of a primary, phase-homogeneous but oscillatorilly zoned niobian rutile phase and of exsolution intergrowths of depleted niobian rutile with granular titanian-tungstenian ixiolite. Primary niobian rutile has the monorutile structure and shows extensive substitution (Fe, Mn, Mg)²⁺₊₁ (Nb, Ta)⁵⁺₊₂ Ti⁴⁺_-3 (up to 26.34 wt. % Nb₂O₅ and 8.60 wt. % Ta₂O₅; 26 to 37 mol. % columbite component), significant (Fe, Mn, Mg)²⁺₊₁ W⁶⁺₊₁ Ti⁴⁺_-2 (up to 3.12 wt. % WO3) and minor (Fe, Sc)³⁺₊₁ (Nb, Ta)⁵⁺₊₁ Ti⁴⁺_-2 substitutions. The values of Fe²⁺/(Fe²⁺+ Fe³⁺) are high and average at ~ 0.8. Oscillatory zoning is controlled by Nb, Ta and Fe; variations in W are independent and erratic. Depleted niobian rutile still contains 8 to (rarely) 32 mol. % ferrocolumbite component. Individual grains of the exsolved orthorhombic ixiolite are commonly heterogeneous in terms of the W content. They display highly variable degree of the substitution Fe²⁺₊₁W⁶⁺₊₃Nb⁵⁺_-4 (up to 49.00 wt. % WO₃), moderate but widespread (Ti, Zr, Hf)⁴⁺₊₃ (Fe, Mn, Mg, Ca)²⁺_-1 (Nb, Ta)⁵⁺_-2 (up to 9.83 wt. % TiO₂, 1.94 wt. % ZrO₂), and subordinate (Fe, Sc)³⁺₊₃ (Fe, Mn, Mg, Ca)²⁺_-2 (Nb, Ta)⁵⁺_-1 (up to 1.51 wt. % Sc₂O₃). Exsolution reveals the preference of Sn⁴⁺ and Fe³⁺ for the rutile structure, whereas Fe²⁺, Mn, Mg, Nb, Ta, Sc, Zr, Hf and W are concentrated in the ixiolite. The results confirm the extent of the W substitution in pegmatitic niobian rutile and its significance for the style of subsolidus breakdown, on par with the magnitude of effects exerted by (Nb > Ta + Fe²⁺), (Nb > Ta + Sc + Fe³⁺), (Nb > Ta + Fe²⁺/Fe³⁺~1), or (Fe/Nb > 0.5).