Stefanweissite, (Ca,REE) (sub 2) Zr (sub 2) (Nb,Ti)(Ti,Nb) (sub 2) Fe (super 2+) O (sub 14) , a new zirconolite-related mineral from the Eifel paleovolcanic region, Germany

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 mmX0.07 mmX1.0 mm and acicular crystals up to 2 mm long and 0.02 mm thick typically combined in radiated aggregates in cavities in sanidinite. D (sub calc.) =5.254 g/cm (super 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 (sub 2) O (sub 3) 0.25, La (sub 2) O (sub 3) 2.28, Ce (sub 2) O (sub 3) 6.54, Pr (sub 2) O (sub 3) 1.01, Nd (sub 2) O (sub 3) 1.59, ThO (sub 2) 3.71, UO (sub 2) 1.09, TiO (sub 2) 17.32, ZrO (sub 2) 28.03, HfO (sub 2) 0.91, Nb (sub 2) O (sub 5) 19.96, total 99.69. The empirical formula based on 14 O atoms per formula unit is Ca (sub 1.13) (Ce (sub 0.33) La (sub 0.12) Nd (sub 0.08) Pr (sub 0.05) ) (sub Sigma 0.58) Th (sub 0.12) U (sub 0.03) Mn (sub 0.29) Fe (sub 0.91) Al (sub 0.04) Zr (sub 1.89) Hf (sub 0.04) Ti (sub 1.80) Nb (sub 1.19) O (sub 14) . The simplified formula is (Ca,REE) (sub 2) Zr (sub 2) (Nb,Ti)(Ti,Nb) (sub 2) Fe (super 2+) O (sub 14) . Stefanweissite is orthorhombic, with space group Cmca. The unit-cell parameters are: a=7.2896(4) Aa, b=14.1435(5) Aa, c=10.1713(4) Aa and V=1048.68(7) Aa (super 3) . The crystal structure was solved using single-crystal X-ray diffraction data. Stefanweissite is an analogue of zirconolite-3O with Nb dominant over Ti in one of two octahedral sites. The strongest lines of the powder X-ray diffraction pattern [d, Aa (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.