Columbit-tantalit z pegmatitu pri Ráztočne v pohorí Žiar (stredné Slovensko)

Pages 109-112
Bulletin mineralogicko-petrologického oddělení Národního muzea v Praze | 2008/16/1

Accessory columbite-tantalite forms black tabular crystals (0.2 to 2 mm in size) in fine-grained albite-quartz-muscovite parts of granitic pegmatite dike, associated with Hercynian S-type granite, near Ráztočno village in the Žiar Mountains (Tatric Superunit, Central Western Carpathians, Central Slovakia). Columbite-tantalite shows regular oscillatory zoning, generally with increasing Ta/Nb from central to rim parts of crystals. Electron-microprobe analyses reveal dominancy of columbite-(Fe) to tantalite-(Fe) with molar ratios Mn/(Mn+Fe) = 0.17 to 0.25 and Ta/(Ta+Nb) = 0.07 to 0.50. Rarely were also detected columbite-(Mn) compositions with Mn/(Mn+Fe) = 0.66 to 0.79 and Ta/(Ta+Nb) = 0.08 to 0.17. Contents of Ti are low (up to 0.03 apfu, 0.2 - 0.5 wt. % TiO2), concentrations of W, Sn, Zr, Sc, Mg and Zn are negligible. Secondary microlite replaces the columbite-tantalite along fissures and rims. Studied columbite-tantalite represents a typical example of late-magmatic mineral of less fractionated LCT-suite and beryl-columbite subtype of rare-element granitic pegmatites.

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