Nioboixiolite-(Fe 2+ ), (Nb 2/3 Fe 2+ 1/3 )O 2 , a new mineral of the ixiolite group from Jordanów Śląski, Lower Silesia, Poland

Le résumé fourni par la source

Abstract Nioboixiolite-(Fe 2+ ), with ideal formula (Nb 2/3 Fe 2+ 1/3 )O 2 , occurs in granitic pegmatite adjacent to rodingitized calc-silicate rocks and serpentinite in a Jordanów Śląski quarry, Lower Silesia, Poland. It forms euhedral prismatic crystals up to 50 μm, scattered in lenticular aggregates of Ti,Nb-bearing minerals (Nb-rutile, ilmenite, titanite, Ca,Pb-hydropyrochlore) between quartz and albite. The mineral is opaque black, non-fluorescent, with a black streak, submetallic lustre, ∼5 Mohs hardness, brittle uneven-to-conchoidal fracture, and no cleavage. Density is 5.374 g·cm –3 (empirical formula, refined unit-cell volume). In reflected light, it appears grey (slightly darker than Nb,Ta-rutile), without pleochroism; reflectance 16.8(5)–18.4(5)%, very weak bireflectance, weak anisotropy, no internal reflections. The empirical structural formula, normalized to 2 oxygen atoms per formula unit (apfu) and expressed as groups of cations of the same valence, is as follows: [(Nb 0.509 Ta 0.048 ) Σ0.557 (Fe 2+ 0.144 Mn 0.061 Mg 0.006 ) Σ0.211 (Fe 3+ 0.064 Sc 0.058 ) Σ0.121 (Ti 0.102 Zr 0.004 Sn 0.002 ) Σ0.108 □ 0.003 ] Σ1.000 O 2.000 where □ = vacancy. It is dominated by pentavalent and divalent cations, among which Nb 5+ and Fe 2+ predominate. The crystal structure of nioboixiolite-(Fe 2+ ) was refined in the orthorhombic system (space group Pbcn ) to R 1 = 2.22%. Nioboixiolite-(Fe 2+ ) crystallises in an α-PbO 2 structure, which is typical for all minerals from the ixiolite group, in which all cations, surrounded octahedrally by oxygen atoms, are disordered at one M site. The holotype crystal has the following unit-cell parameters: a = 4.7368(2) Å, b = 5.7155(2) Å, c = 5.0831(2) Å, V = 137.616(9) Å 3 ( a: b: c = 0.8288: 1: 0.8894). Raman spectra show four groups of peaks attributed to lattice modes, bending modes, and symmetric and asymmetric stretching modes, with the strongest asymmetric 872(1) cm –1 band originating from the stretching vibrations of the shortest Ti–O, Nb–O and Ta–O bonds, modified by vibrations of larger Fe 3+ –O, Fe 2+ –O and Mn 2+ –O bonds.

Ce résumé expose les affirmations des auteurs. BNTIC ne l’interprète pas comme une validation indépendante des résultats.