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Monazite-huttonite solid-solutions from the Vico Volcanic Complex, Latium, Italy

Published online by Cambridge University Press:  05 July 2018

Giancarlo Della Ventura
Affiliation:
Dipartimento di Scienze Geologiche, Università di Roma Tre, Via Ostiense 169, I-00154 Roma, Italy
Annibale Mottana
Affiliation:
Dipartimento di Scienze Geologiche, Università di Roma Tre, Via Ostiense 169, I-00154 Roma, Italy
Gian Carlo Parodi
Affiliation:
Museo di Mineralogia, Università di Roma “La Sapienza”, Piazzale Aldo Moro 5, I-00185 Roma, Italy
Mati Raudsepp
Affiliation:
Department of Geological Sciences, University of British Columbia, 6339 Stores Road, Vancouver, B.C. V6T 1Z4, Canada
Fabio Bellatreccia
Affiliation:
Dipartimento di Scienze della Terra, Università di Roma “La Sapienza”, Piazzale Aldo Moro 5, I-00185 Roma, Italy
Enrico Caprilli
Affiliation:
Dipartimento di Scienze della Terra, Università di Roma “La Sapienza”, Piazzale Aldo Moro 5, I-00185 Roma, Italy
Paolo Rossi
Affiliation:
Dipartimento di Scienze della Terra, Università di Roma “La Sapienza”, Piazzale Aldo Moro 5, I-00185 Roma, Italy
Salvatore Fiori
Affiliation:
Dipartimento di Scienze della Terra, Università di Roma “La Sapienza”, Piazzale Aldo Moro 5, I-00185 Roma, Italy

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.

Type
Mineralogy
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1996

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Footnotes

*

Present address: Museum National d'Histoire Naturelle, Laboratoire de Mineralogie, 61, rue Buffon, 75005 Paris, France.

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