Summary
The highest (Nb, Ta) content ever encountered in titanite is reported from the Maršíkov 11 pegmatite in northern Moravia, Czech Republic. This dike is a member of a pegmatite swarm of the beryl-columbite subtype, metamorphosed under conditions of the amphibolite facies. The pegmatite carries, i.a., rare tantalian rutile intergrown with titanian ixiolite, titanian columbite-tantalite, fersmite and microlite. Fissures generated in the Nb, Ta oxide minerals during deformation are filled with titanite, formed by reaction of the oxide minerals with metamorphic pore fluids. The titanite displays limited degrees of substitutions Na(Ta > Nb)(CaTi)−1, (Ta > Nb)4□Ti−4Si−1 and AI(OH, F)(TiO)−1, but an extensive (and occasionally the sole significant) substitution (Al > Fe3+)(Ta > Nb)Ti−2, responsible for widespread oscillatory zoning. This substitution reduces the proportion of the titanite componentsensu stricto, CaTiSiO4,O, to less than 50 mole % in many analyzed spots. The extreme composition corresponds to (Ca0.994Na0.011)(Ti0.436Sn0.007Al0.280Fe3+ 0.006Ta0.199Nb0.079)Si0.988O4(O0.974F0.026). However, so far this substitution fails to generate compositions that would define a new species.
Zusammenfassung
Die AI(Nb, Ta)Ti−2 Substitution im Titanit: Auftauchen einer neuen Mineralspecies? Die höchsten (Nb, Ta) Gehalte, die jemals für Titanit gefunden wurden, werden für den Maršíkov II Pegmatit in Nordmähren, Tschechei, berichtet. Der Intrusivgang ist Teil eines Amphibolit-faziell überprägten Pegmatitschwarms vom Beryll-Columbit Subtypus Der Pegmatit führt u.a. seltene tantalbetonte Rutile verwachsen mit titanbetontem Ixiolith, titanbetontem Columbit-Tantalit, Fersmit and Mikrolith. Deformationsbedingte Frakturen in den (Nb, Ta) Oxiden sind mit Titanit, als Folge der Reaktion der metamorphen Porenlösungen mit den Oxidmineralen, verkittet. Titanit zeigt begrenzte Substitutionen Na(Ta > Nb)(CaTi)−1,(Ta > Nb)4□Ti−4Si−1 and Al(OH, F)(TiO)−1, aber extensive (und gelegentlich einzig bedeutsame) Substitution (Al >> Fe3+)(Ta > Nb)Ti−2, die eine weitverbreitete, oszillierende Zonierung hervorruft. Diese Substitution verringert den Anteil der Titanit-Komponentesensu stricto, CaTiSiO,O, auf weniger als 50 Mol% in vielen Analysen. Die Extremzusammensetzung entspricht Ca0.994Na0.11) (T10.436Sn0.007Al0.280Fe3+ 0.006Ta0.199Nb0.079)Si0.988O4(O0.974F0.026). Das AusmaB dieser Substitution ist unzureichend, um eine neue Mineralspecies zu definieren.
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Černý, P., Novák, M. & Chapman, R. The Al (Nb, Ta) Ti(in−2) substitution in titanite: the emergence of a new species?. Mineralogy and Petrology 52, 61–73 (1995). https://doi.org/10.1007/BF01163126
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DOI: https://doi.org/10.1007/BF01163126