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 P2(1)/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. The genesis of this unusual mineral assemblage is related to late metasomatic stages at hypabyssal conditions of the regional volcanic activity.