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 Angstrom) a = 10.13(1), b = 14.16(l) and c 7.288(2), and as pseudo-hexagonal platy crystals of trigonal symmetry (zirconolite-3T), space group P3(I)2, 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 divided by 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.