Monazite-huttonite solid-solutions from the Vico Volcanic Complex, Latium, Italy
- 1 October 1996
- journal article
- Published by Mineralogical Society in Mineralogical Magazine
- Vol. 60 (402) , 751-758
- https://doi.org/10.1180/minmag.1996.060.402.04
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+ → REE 3+ + 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.Keywords
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