Chemistry:Cerium anomaly
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).[1] 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.[1]
Cerium oxidation states
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.[2]
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.[1] 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.[1]
Cerium anomalies in zircon
Zircon (ZrSiO4) is commonly found in felsic igneous rock.[3] Because both Ce3+ and Ce4+ can substitute for zirconium, Zircon often has a positive Ce anomaly.[3] Ce4+ substitutes with Zr much more easily than Ce3+ because Ce4+ (ionic radius 0.97Å) has the same charge and a similar ionic radius as Zr4+ (ionic radius 0.84Å).[1] Therefore, the oxidation state of the magma is what determines the Ce anomaly in Zircon.[3] If the oxygen fugacity is high, more Ce3+ will oxidize to Ce4+ and create a larger positive Ce anomaly in the zircon structure. At lower levels of oxygen fugacity, the level of Ce anomaly will also be lower.[3]
Cerium anomalies in coal
Negative cerium anomalies
Cerium in coal is typically weakly negative, meaning that it is present at slightly lower concentrations than the other rare-earth elements.[2] Cerium anomalies in coal are influenced by the sediment source region.[2] Coal mined from mafic regions dominated by basalts, such as the location of the Xinde Mine in China, does not have a Ce-anomaly.[2] In contrast, coal mined in felsic rock regions, such as Guxu Coalfield in China, does have weakly negative Ce-anomalies.[2] Negative Ce-anomalies can also be attributed to the weathering and oxidation of the coal-mining region.[2] During oxidation, Ce3+ precipitates out as CeO2, leaving less Ce in the coal.[1]
Positive cerium anomalies
While cerium anomalies in coal are usually negative, they can rarely be positive as well.[4] This can occur during volcanic eruptions when volcanic ash is weathered into mafic tuffs with positive Ce-anomalies.[1] The Pavlovka deposit in Far East Russia has large positive Ce-anomalies in its Fe-Mn oxyhydroxide ores.[4] Because cerium is one of only two REEs that can obtain an oxidation number of +4, Ce4+ is absorbed into Mn(IV) oxides instead of other REEs and this results in a positive Ce-anomaly.[4]
See also
External links
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Thomas, J. B.; Bodnar, R. J.; Shimizu, N.; Chesner, C. A. (2003). "Melt Inclusions in Zircon". Reviews in Mineralogy and Geochemistry 53 (1): 63–87. doi:10.2113/0530063. ISSN 1529-6466. Bibcode: 2003RvMG...53...63T. https://doi.org/10.2113/0530063.
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 Dai, Shifeng; Graham, Ian T.; Ward, Colin R. (2016). "A review of anomalous rare earth elements and yttrium in coal". International Journal of Coal Geology 159: 82–95. doi:10.1016/j.coal.2016.04.005. ISSN 0166-5162. http://dx.doi.org/10.1016/j.coal.2016.04.005.
- ↑ 3.0 3.1 3.2 3.3 Zhong, Shihua; Seltmann, Reimar; Qu, Hongying; Song, Yingxin (2019). "Characterization of the zircon Ce anomaly for estimation of oxidation state of magmas: a revised Ce/Ce* method" (in en). Mineralogy and Petrology 113 (6): 755–763. doi:10.1007/s00710-019-00682-y. ISSN 0930-0708.
- ↑ 4.0 4.1 4.2 Laveuf, C.; Cornu, S. (2009). "A review on the potentiality of Rare Earth Elements to trace pedogenetic processes" (in en). Geoderma 154 (1–2): 1–12. doi:10.1016/j.geoderma.2009.10.002. https://linkinghub.elsevier.com/retrieve/pii/S0016706109003206.
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