Zirconolite with significant REEZrNb(Mn,Fe)O7, from a xenolith of the Laacher See eruptive center, Eifel volcanic region, Germany

We report a new occurrence of the mineral zirconolite, ideally CaZrTi2O7, from xenoliths in the pyroclastic formations outcropping near Niedermendig, Laacher See eruptive center, Eifel volcanic region, Germany. This example of zirconolite is crystalline, a rare feature for a mineral of this group; it gives an X-ray-diffraction pattern corresponding to the orthorhombic polytype (zirconolite-3O), space group Acam, with a 10.145(8), b 14.18(8), c 7.284(5) Å. It contains the highest concentration of Mn reported from natural zirconolite (6% MnO); it is also rich in Nb (~18% Nb2O5) and REE (up to 19.4% Y2O3 + REE2O3), with Ce the dominant REE. The mineral formula, expressed in terms of three of the five end-member components identified for the compositional space of zirconolite, is: 32% CaZrTi2O7, 53% REEZrMe5+Me2+O7, and 6% ACTZrTiMe2+O7 (with 9% unassigned). The compositional variation can best be described by the coupled substitution: REE3+ + (Fe2+,Mn2+) + Nb5+ ↔ Ca + 2Ti. The host rock is an alkali syenite xenolith almost exclusively composed of alkali feldspar (Or45Ab55), with minor biotite. Zirconolite crystallizes as an accessory mineral in voids between feldspar crystals, together with baddeleyite, manganoan magnetite, and monazite-(La), a rare species of monazite. This suite of minerals, with their unusual chemical compositions, crystallized from late-stage metasomatic fluids enriched in Nb and Mn, and having an exceptional La-enriched REE signature.