Nioboixiolite-(Fe ³⁺ ), (Nb _0.5 Fe ³⁺ _0.5 )O ₂ , a new ixiolite-group mineral from the Eifel palaeovolcanic region, Germany

Abstract The new ixiolite-group mineral nioboixiolite-(Fe 3+ ), ideally (Nb 0.5 Fe 3+ 0.5 )O 2 , was discovered in nosean sanidinite of the Laach Lake (Laacher See) volcano, Eifel region, Rhineland-Palatinate, Germany. The associated minerals are sanidine, K-bearing albite, nosean, biotite, Nb-rich ilmenite, Ti-rich magnetite, hercynite, corundum, samarskite-(Y), ekebergite and columbite-(Fe). Nioboixiolite-(Fe 3+ ) forms long prismatic to acicular crystals up to 0.03 × 0.06 × 1 mm and epitaxial intergrowths with ilmenite and intermediate members of the samarskite–ekebergite series. Both colour and streak are black and the lustre is submetallic. The new mineral is brittle, with the Vickers’ micro-indentation hardness of 499 kg mm –2 which corresponds to the Mohs’ hardness of 5. No cleavage is observed. The fracture is conchoidal. The calculated density is 5.033 g·cm –3 . In reflected light, nioboixiolite-(Fe 3+ ) is grey, no pleochroism is observed. The reflectance values ( R min , %/ R max , %/λ, nm) are: 14.7/16.4/470, 14.3/15.9/546, 14.1/15.7/589 and 14.0/15.8/650. The Raman spectrum shows bands corresponding to stretching vibrations of (Nb,Ti)–O–(Nb,Ti) and (Nb,Ti)–O–Mn 2+ and the absence of bands of OH groups. The chemical composition is (electron microprobe data, wt.%): MgO 0.41, MnO 3.52, Al 2 O 3 0.42, Cr 2 O 3 0.75, Fe 2 O 3 20.23, TiO 2 22.26, ZrO 2 0.76, Nb 2 O 5 51.82, total 100.17. The empirical formula is (Mg 0.04 Mn 2+ 0.20 ) Σ0.24 (Al 0.03 Cr 0.04 Fe 3+ 1.01 ) Σ1.08 (Ti 1.11 Zr 0.02 ) Σ1.13 Nb 1.55 O 8 ( Z = 1). The strongest lines of the powder X-ray diffraction pattern [ d , Å ( I , %) ( hkl )] are: 3.586 (29) (110), 2.917 (100) (111), 2.503 (18) (002), 2.170 (18) (121), 1.738 (22) (130), 1.689 (26) (221). The crystal structure was determined using single-crystal X-ray diffraction data and refined to R = 0.0447. Nioboixiolite-(Fe 3+ ) is orthorhombic with space group Pbcn, a = 4.6578(6), b = 5.6230(7), c = 5.0182(5) Å and V = 131.43(3) Å 3 . The new mineral is isostructural with other members of the ixiolite group.