Cerium anomaly
Encyclopedia
The Cerium anomaly, in geochemistry
, is the phenomenon whereby Cerium
(Ce) concentration is either depleted or enriched in a rock relative to the other rare earth elements (REEs).
A Ce anomaly is said to be "negative" if Ce is depleted relative to the other REEs and is said to be "positive" if Ce is enriched relative to the other REEs.
Cerium is a rare earth element
(lanthanide
) characterized by two different redox states: III and IV. Contrary to other lanthanide elements which are only trivalent (with the notable exception of Eu2+
), Ce3+ can be oxidized by atmospheric oxygen (O2) to Ce4+ under alkaline conditions.
The cerium anomaly relates to the decrease in solubility which accompanies the oxidation of Ce(III) to Ce(IV). Under reducing conditions, Ce3+ is relatively soluble while under oxidizing conditions CeO2 precipitates. Sediments deposited under oxic or anoxic conditions can preserve on the long term the geochemical signature of Ce3+ or Ce4+ upon reserve that no early diagenetic transformation altered it.
Cerium can occur in nature as a 3+ or 4+ ion, and is a compatible element (at 4+ valency) in zircon and less commonly in silicon. Thomas et al., (2003) state that “terrestrial zircons commonly show a positive Ce anomaly due to the incorporation of Ce4+ into zircon, which is because Ce4+ has the same charge and a similar ionic radius than Zr4+ (Ce4+ = 0.97 Å; Zr4+ = 0.84 Å)”. As such, Ce4+ is incorporated into zircon much easier than the larger Ce3+ (ionic radius = 1.143 Å).
This shows that both Ce3+ and Ce4+ are present and that the Ce4+ being compatible in zircon is causing the anomaly.
Geochemistry
The field of geochemistry involves study of the chemical composition of the Earth and other planets, chemical processes and reactions that govern the composition of rocks, water, and soils, and the cycles of matter and energy that transport the Earth's chemical components in time and space, and...
, is the phenomenon whereby Cerium
Cerium
Cerium is a chemical element with the symbol Ce and atomic number 58. It is a soft, silvery, ductile metal which easily oxidizes in air. Cerium was named after the dwarf planet . Cerium is the most abundant of the rare earth elements, making up about 0.0046% of the Earth's crust by weight...
(Ce) concentration is either depleted or enriched in a rock relative to the other rare earth elements (REEs).
A Ce anomaly is said to be "negative" if Ce is depleted relative to the other REEs and is said to be "positive" if Ce is enriched relative to the other REEs.
Cerium is a rare earth element
Rare earth element
As defined by IUPAC, rare earth elements or rare earth metals are a set of seventeen chemical elements in the periodic table, specifically the fifteen lanthanides plus scandium and yttrium...
(lanthanide
Lanthanide
The lanthanide or lanthanoid series comprises the fifteen metallic chemical elements with atomic numbers 57 through 71, from lanthanum through lutetium...
) characterized by two different redox states: III and IV. Contrary to other lanthanide elements which are only trivalent (with the notable exception of Eu2+
Europium Anomaly
The Europium anomaly, in geochemistry, is the phenomenon whereby Europium concentration is either depleted or enriched in a rock relative to the other rare earth elements ....
), Ce3+ can be oxidized by atmospheric oxygen (O2) to Ce4+ under alkaline conditions.
The cerium anomaly relates to the decrease in solubility which accompanies the oxidation of Ce(III) to Ce(IV). Under reducing conditions, Ce3+ is relatively soluble while under oxidizing conditions CeO2 precipitates. Sediments deposited under oxic or anoxic conditions can preserve on the long term the geochemical signature of Ce3+ or Ce4+ upon reserve that no early diagenetic transformation altered it.
Cerium can occur in nature as a 3+ or 4+ ion, and is a compatible element (at 4+ valency) in zircon and less commonly in silicon. Thomas et al., (2003) state that “terrestrial zircons commonly show a positive Ce anomaly due to the incorporation of Ce4+ into zircon, which is because Ce4+ has the same charge and a similar ionic radius than Zr4+ (Ce4+ = 0.97 Å; Zr4+ = 0.84 Å)”. As such, Ce4+ is incorporated into zircon much easier than the larger Ce3+ (ionic radius = 1.143 Å).
This shows that both Ce3+ and Ce4+ are present and that the Ce4+ being compatible in zircon is causing the anomaly.
See also
- CeriumCeriumCerium is a chemical element with the symbol Ce and atomic number 58. It is a soft, silvery, ductile metal which easily oxidizes in air. Cerium was named after the dwarf planet . Cerium is the most abundant of the rare earth elements, making up about 0.0046% of the Earth's crust by weight...
- EuropiumEuropiumEuropium is a chemical element with the symbol Eu and atomic number 63. It is named after the continent of Europe. It is a moderately hard silvery metal which readily oxidizes in air and water...
- Europium anomalyEuropium AnomalyThe Europium anomaly, in geochemistry, is the phenomenon whereby Europium concentration is either depleted or enriched in a rock relative to the other rare earth elements ....
- LanthanideLanthanideThe lanthanide or lanthanoid series comprises the fifteen metallic chemical elements with atomic numbers 57 through 71, from lanthanum through lutetium...
s - Rare earth elementRare earth elementAs defined by IUPAC, rare earth elements or rare earth metals are a set of seventeen chemical elements in the periodic table, specifically the fifteen lanthanides plus scandium and yttrium...
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