Nioboheftetjernite, ScNbO4, from pegmatites of the Karkonosze granite, Southwestern Poland

Nioboheftetjernite, ScNbO4, is an extremely rare mineral found in granitic pegmatites of the NYF petrogenetic family. In addition to the type locally, the Befanamo pegmatite in Madagascar, this mineral is only known from the Kožichovice II pegmatite in the Třebíč Pluton, Czech Republic, and from the Krucze Skały pegmatite in Karpacz and small pegmatite miaroles in the aplogranite of Szklarska Poręba in Poland. We characterized nioboheftetjernite from the latter location in detail (using EPMA, SCXRD, and Raman spectroscopy). This nioboheftetjernite is opaque, black, with a submetallic luster. The two crystals examined showed the following compositions: M1(Sc0.60Fe3+0.22Mn2+0.07Nb0.04Sn0.04Mn3+0.02Zr0.01Ta<0.01)M2(Nb0.83Ti0.10Ta0.04W0.03Sn<0.01)O4.00 (crystal SP81 with the refined structure), M1(Sc0.47Fe3+0.30Nb0.07Mn2+0.06Sn0.06Fe2+0.03Zr0.01) M2(Nb0.69Ta0.16Ti0.13W0.02)O4.00 (crystal SP75, domain 1), and M1(Sc0.49Fe3+0.29Nb0.09Mn2+0.08Sn0.04Zr0.01)M2(Nb0.72Ti0.18Ta0.09W0.01)O4.00 (crystal SP75, domain 2). They have a wolframite-type structure (space group P2/c). The refined SP81 crystal has the following unit-cell parameters: a = 4.7650(4) Å, b = 5.6826(5) Å, c = 5.0894(5) Å, β = 90.826(9)°, and V = 137.80(2) Å3 (Z = 2), and a density of 4.989 g·cm–3, calculated from the empirical formula and the unit-cell volume. In the structure of both crystals, the M1 site is dominated by Sc, while the M2 site is mainly occupied by Nb. The Sc deficiency is compensated by Fe3+, Mn3+, Sn4+, Zr4+, and Mn2+, while the Nb deficiency is compensated by Ta5+, Ti4+, W6+, and Sn4+, with limited Nb, Ta, and Sn disorder between these sites. Nioboheftetjernite in the Szklarska Poręba pegmatite most probably formed as a result of dissolution–reprecipitation processes associated with the replacement of primary ilmenite with rutile in NYF-type pegmatite. The most probable source of Sc was a highly evolved aplogranite magma, enriched in SiO2 and volatile compounds. As temperature decreased, the mass crystallization of large amounts of minerals incompatible with Sc, such as feldspars and quartz, must have resulted in a rapid increase in Sc concentration, initiating its incorporation into the structures of ilmenite, rutile, columbite-group minerals, nioboixiolite-(Fe2+), and nioboixiolite-(Mn). High-temperature fluids associated with the residual magma released Sc from these precursors, which ultimately led to the hydrothermal crystallization of thortveitite and Ca,Sc-bearing silicates, such as scandiobabingtonite and kristiansenite-group minerals.