Abstract

Accessory Nb, Ta-oxide minerals occur in two narrow veins of relatively poorly fractionated granitic pegmatite that crosscut the parent Hercynian pluton, the Prasiva biotite granodiorite-granite, in central Slovakia. High Ti contents are typical of all members of this magmatic suite, and of their relevant minerals. In the pegmatites, niobian to tantalian rutile (< or =24 wt.% Nb 2 O 5 , < or =30 wt.% Ta 2 O 5 ) and ferrocolumbite are most widespread. Ferrocolumbite (to manganocolumbite) contains 2.5 to 5 wt.% TiO 2 , and titanian ixiolite has 11 to 21 wt.% TiO 2 . Tiny lamellae of niobian-tantalian armalcolite to pseudobrookite (6 to 10 wt.% Nb 2 O 5 , 0.5 to 5.5 wt.% Ta 2 O 5 ) exsolved from the rutile matrix; complex exsolution-induced domains of niobian ilmenite rimming intergrowths of ilmenite and titanian hematite are rare. Exsolution of armalcolite-pseudobrookite probably took place at magmatic temperatures, and its subsolidus survival is possibly metastable.

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