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Complexly zoned niobian titanite from hedenbergite skarn at Písek, Czech Republic, constrained by substitutions Al(Nb,Ta)Ti_-2, Al(F,OH)(TiO)_-1 and SnTi_-1

¹ Department of Mineralogy and Petrography, Moravian Museum, Zeln trh 6, 659 37 Brno, Czech Republic
² Department of Geological Sciences, Masaryk University, Kotláská 2, 611 37 Brno, Czech Republic

^* E-mail: jcempirek{at}mzm.cz

Euhedral crystals of complexly zoned niobian titanite (up to 0.3 mm) are enclosed in hedenbergite (Hd_53-81Di_15-43Jh_3-5) and quartz from a hedenbergite vein skarn at Kamenné doly near Písek, Czech Republic. They are associated with minor clinozoisite-epidote (Ps_3-22), calcite, plagioclase (An₉₅), scapolite (Me_80-82), 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)_-1 vs. 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.

KEYWORDS: titanite, niobium, aluminium, tin, fluorine, electron microprobe, substitutions, skarn, Moldanubian Zone, Czech Republic