Perrierite-(Ce) and zirconolite from a syenitic ejectum of the Roccamonfina volcano (Latium, Italy): implications for the mobility of Zr, Ti and REE in volcanic environments

This paper describes a new occurrence of the rare minerals perrierite-(Ce) and zirconolite in the tephra of the Roccamonfina volcano. The host rock is a foid-bearing alkali-syenite and is composed predominantly of sanidine with minor clinopyroxene, magnetite and sodalite. Accessory phases include titanite, zircon and baddeleyite. Intense phyllic and argillic alteration is observed at alkali-feldspars grain boundaries or along fractures within crystals. Perrierite-(Ce) occurs as brownish-red, transparent, well-developed, euhedral, prismatic crystals inside the miarolitic cavities of the ejectum. Its powder pattern can be indexed in the monoclinic P21/m space group, with (in Angstrom) a = 13.676(11), b = 5.655(2), c = 11.763(8), beta = 113.75 (4). It is Zr-rich and the entry of Zr in the C site of the structure can be described by the charge balancing substitution: Ca-[A]([C])(Ti4+,Zr4+) = (REE3+[C])-R-[A](M-3+,M2+). Zirconolite occurs as very small acicular or stubby prismatic crystals typically associated with baddeleyite in the altered parts of the rock. In the ternary REE-Act-Me5+ its composition plots well within the field of zirconolites from alkali syenitic environments.