Non-metamict zirconolite polytypes from the feldspathoid-bearing alkali-syenitic ejecta of the Vico volcanic complex (Latium, Italy)

This paper presents the results of a combined electron microprobe and analytical TEM study that describes the chemical and structural characteristics of non-metamict zirconolite from a volcanic ejectum collected within a pyroclastic formation of the Vico volcanic complex, near Tre Croci Community, Latium (Italy). The host rock is a foid-bearing alkali-syenite and consists of potassium-feldspar and plagioclase, with minor biotite, titanite, clinopyroxene and magnetite. Sodalite occurs as well-faceted crystals inside miarolitic cavities between intersecting feldspars and it is variably altered to a kaolinite-like product. Accessory minerals include zircon, baddeleyite, stillwellite, hellandite, fluorite, apatite and monazite. Zirconolite occurs both as elongated acicular crystals of orthorhombic symmetry (zirconolite-3O), space group Acam, with (in Å) a = 10.13(1), b = 14.16(1) and c = 7.288(2), and as pseudo-hexagonal platy crystals of trigonal symmetry (zirconolite-3T), space group P312, with a = 7.279(2) and c = 16.89(2). TEM studies revealed additionally the presence of polytype zirconolite-2M in a subset of grains analyzed. Zirconolite displays pronounced compositional zoning, observed in backscattered electron images, resulting mainly from an antipathetic variation in the actinide elements (Th + U) and REE contents. Analytical TEM results indicate compositional differences exist between the zirconolite-3T and zirconolite-2M polytypes. Zirconolite, and associated accessory REE, Th, U minerals crystallized after the formation of the major rock-forming minerals of the ejectum, from metasomatic fluids rich in F and P.