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-K alpha radiation) can be indexed in space group P2(1)/n with a = 6.816(4); b = 6.976(4); c = 6.471(3) Angstrom; beta = 103.63(3)degrees; V = 299.0(6) Angstrom(3). 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.
DELLA VENTURA, G., Mottana, A., PARODI G., C., Raudsepp, M., Bellatreccia, F., Caprilli, E., et al. (1996). Monazite-huttonite solid-solutions from the Vico Volcanic Complex, Latium, Italy. MINERALOGICAL MAGAZINE, 60, 751-758 [10.1180/minmag.1996.060.402.04].
Monazite-huttonite solid-solutions from the Vico Volcanic Complex, Latium, Italy
DELLA VENTURA, Giancarlo;BELLATRECCIA, FABIO;
1996-01-01
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-K alpha radiation) can be indexed in space group P2(1)/n with a = 6.816(4); b = 6.976(4); c = 6.471(3) Angstrom; beta = 103.63(3)degrees; V = 299.0(6) Angstrom(3). 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.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
