Monazite-Huttonite Solid-Solutions from the Vico Volcanic Complex, Latium, Italy

Giancarlo Della Ventura, Annibale Mottana, Gian Carlo Parodi^*, Mati Raudsepp, Fabio Bellatreccia, Enrico Caprilli, Paolo Rossi and Salvatore Fiori

Dipartimento di Scienze Geologiche, Università di Roma Tre, Via Ostiense 169, I-00154 Roma, Italy
Museo di Mineralogia, Università di Roma “La Sapienza”, Piazzale Aldo Moro 5, I-00185 Roma, Italy
Department of Geological Sciences, University of British Columbia, 6339 Stores Road, Vancouver, B.C. V6T 1Z4, Canada
Dipartimento di Scienze della Terra, Università di Roma “La Sapienza”, Piazzale Aldo Moro 5, I-00185 Roma, Italy

^*Present address: Museum National d'Histoire Naturelle, Laboratorie de Minéralogie, 61, rue Buffon, 75005 Paris, France.

Abstract: The crystal-chemical relationships occurring within a single grain of monazite-(Ce) from Vetralla, Vico Volcanic Complex, north of Rome, are outlined. The sample is from a miarolitic cavity in a holocrystalline ejectum consisting of K-feldspar plus minor plagioclase, mica and Fe-oxides, collected from a pyroclastic explosive level. The Gandolfi film (Cu-Kα radiation) can be indexed in space group P2₁/n with a = 6.816(4); b = 6.976(4); c = 6.471(3) Å; β = 103.63(3)°; V = 299.0(6) Å³. Electron-probe microanalyses plot within the field of monazite along the huttonite-monazite edge of the huttonite-monazite-brabantite triangle. Despite patchy and irregular zoning, the grain shows a clear enrichment towards pure monazite at the outer rim. A constant Th:Si ratio of 1:1 indicates the existence of a simple solid-solution between huttonite and monazite. The substitution can be written as Th⁴⁺ + Si⁴⁺ → REE³⁺ + P⁵⁺ without requiring any electrostatic compensation by divalent cations, or by anionic groups. The REE distribution pattern is compatible with that of monazites from syenitic rocks.