The new mineral stefanweissite, IMA2018-020, was discovered in sanidinite volcanic ejecta from the Laach Lake (Laacher See) paleovolcano, Eifel region, Rhineland-Palatinate, Germany. Associated minerals are sanidine, nosean, biotite, augite, titanite, ferriallanite-(La), magnetite, baddeleyite and a pyrochlore-group mineral. Stefanweissite is brown and reddish-brown, with adamantine lustre; the streak is light brown to yellow. It forms long-prismatic crystals up to 0.03 mm × 0.07 mm × 1.0 mm and acicular crystals up to 2 mm long and 0.02 mm thick typically combined in radiated aggregates in cavities in sanidinite. D calc. = 5.254 g/cm 3 . The mean refractive index calculated from the Gladstone–Dale equation is 2.260. The Raman spectrum shows the absence of hydrogen-bearing groups. The chemical composition is (electron microprobe, wt.%): CaO 7.63, MnO 2.51, FeO 7.86, Al 2 O 3 0.25, La 2 O 3 2.28, Ce 2 O 3 6.54, Pr 2 O 3 1.01, Nd 2 O 3 1.59, ThO 2 3.71, UO 2 1.09, TiO 2 17.32, ZrO 2 28.03, HfO 2 0.91, Nb 2 O 5 19.96, total 99.69. The empirical formula based on 14 O atoms per formula unit is Ca 1.13 (Ce 0.33 La 0.12 Nd 0.08 Pr 0.05 ) Σ0.58 Th 0.12 U 0.03 Mn 0.29 Fe 0.91 Al 0.04 Zr 1.89 Hf 0.04 Ti 1.80 Nb 1.19 O 14 . The simplified formula is (Ca, REE ) 2 Zr 2 (Nb,Ti)(Ti,Nb) 2 Fe 2+ O 14 . Stefanweissite is orthorhombic, with space group Cmca . The unit-cell parameters are: a = 7.2896(4) Å, b = 14.1435(5) Å, c = 10.1713(4) Å and V = 1048.68(7) Å 3 . The crystal structure was solved using single-crystal X-ray diffraction data. Stefanweissite is an analogue of zirconolite-3 O with Nb dominant over Ti in one of two octahedral sites. The strongest lines of the powder X-ray diffraction pattern [ d , Å ( I , %) ( hkl )] are: 2.983(100)(202), 2.897(71)(042), 1.828(38)(154, 400, 333), 1.793(25)(244), 1.767(16)(080), 1.517(10)(282), 1.187(19)(483, 1.11.3, 602). Type material is deposited in the collections of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia, with the registration number 5191/1.
