Abstract

Euhedral crystals of complexly zoned niobian titanite (up to 0.3 mm) are enclosed in hedenbergite (Hd53-81Di15-43Jh3-5) and quartz from a hedenbergite vein skarn at Kamenné doly near Písek, Czech Republic. They are associated with minor clinozoisite-epidote (Ps3-22), calcite, plagioclase (An95), scapolite (Me80-82), scheelite, pyrrhotite, fluorapatite, arsenopyrite, native bismuth and Bi,Te-minerals. The following textural and compositional subtypes were recognized: (I) Nb-rich titanite, (II) Nb-moderate titanite in the central zone, (III) Nb-poor, Sn-enriched titanite and (IV) Nb-poor, Al,F-rich titanite in the outer zone. The substitution Al(Nb,Ta)Ti-2 is dominant in subtypes I and II, the titanite subtype I being characterized by elevated contents of Al ≤0.257 atoms per formula unit (a.p.f.u.), Nb (≤0.161 a.p.f.u.) and Ta (≤0.037 a.p.f.u.). Amounts of Al, Nb and Ta in subtype II are smaller and more variable. The minor substitution SnTi-1 occurs chiefly in titanite subtype III with a content of Sn ≤0.039 a.p.f.u.. The substitution Al(F,OH)(TiO)-1 is typical for titanite subtype IV exhibiting elevated contents of Al (≤0.221 a.p.f.u.), F (≤0.196 a.p.f.u.) and Fe (≤0.039 a.p.f.u.).

The negative relationship of substitutions Al(F,OH)(TiO)-1vs. SnTi-1 and Al(Nb,Ta)Ti-2 is constrained chiefly by crystal structure rather than by the composition of parent medium alone. Textural relations suggest that the Nb-moderate titanite in the core zone and entire outer zone are products of fluids-induced dissolution-reprecipitation processes. The studied niobian titanite represents a new F-enriched type from a medium-grade, calc-silicate rock.

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