page_title: Cerium
section_title: Occurrence and production
hierarchical_section_title: Cerium / Occurrence and production
caption_attribution_description: English: Crystal structure of bastnaesite-(Ce) - CeCO3F. Donnay G, Donnay J D H Colour code: &#x2002; Carbon, C: grey &#x2002; Florine, F: green &#x2002; Cerium, Ce: yellow &#x2002; Oxygen, O: red &#x2002; Cell: silver
context_page_description: Cerium is a chemical element with the symbol Ce and atomic number 58. Cerium is a soft, ductile and silvery-white metal that tarnishes when exposed to air, and it is soft enough to be cut with a knife. Cerium is the second element in the lanthanide series, and while it often shows the +3 oxidation state characteristic of the series, it also has a stable +4 state that does not oxidize water. It is also considered one of the rare-earth elements. Cerium has no biological role in humans and is not very toxic.
  Despite always occurring in combination with the other rare-earth elements in minerals such as those of the monazite and bastnäsite groups, cerium is easy to extract from its ores, as it can be distinguished among the lanthanides by its unique ability to be oxidized to the +4 state. It is the most common of the lanthanides, followed by neodymium, lanthanum, and praseodymium. It is the 26th-most abundant element, making up 66 ppm of the Earth's crust, half as much as chlorine and five times as much as lead.
context_section_description: Cerium is the most abundant of all the lanthanides, making up 66 ppm of the Earth's crust; this value is just behind that of copper (68 ppm), and cerium is even more abundant than common metals such as lead (13 ppm) and tin (2.1 ppm). Thus, despite its position as one of the so-called rare-earth metals, cerium is actually not rare at all. Cerium content in the soil varies between 2 and 150 ppm, with an average of 50 ppm; seawater contains 1.5 parts per trillion of cerium. Cerium occurs in various minerals, but the most important commercial sources are the minerals of the monazite and bastnäsite groups, where it makes up about half of the lanthanide content. Monazite-(Ce) is the most common representative of the monazites, with "-Ce" being the Levinson suffix informing on the dominance of the particular REE element representative. Also the cerium-dominant bastnäsite-(Ce) is the most important of the bastnäsites. Cerium is the easiest lanthanide to extract from its minerals because it is the only one that can reach a stable +4 oxidation state in aqueous solution. Because of the decreased solubility of cerium in the +4 oxidation state, cerium is sometimes depleted from rocks relative to the other rare-earth elements and is incorporated into zircon, since Ce⁴⁺ and Zr⁴⁺ have the same charge and similar ionic radii. In extreme cases, cerium(IV) can form its own minerals separated from the other rare-earth elements, such as cerianite (correctly named cerianite-(Ce)), (Ce,Th)O
  ₂.
  Bastnäsite, LnᴵᴵᴵCO₃F, is usually lacking in thorium and the heavy lanthanides beyond samarium and europium, and hence the extraction of cerium from it is quite direct. First, the bastnäsite is purified, using dilute hydrochloric acid to remove calcium carbonate impurities. The ore is then roasted in the air to oxidize it to the lanthanide oxides: while most of the lanthanides will be oxidized to the sesquioxides Ln₂O₃, cerium will be oxidized to the dioxide CeO₂. This is insoluble in water and can be leached out with 0.5 M hydrochloric acid, leaving the other lanthanides behind.
  The procedure for monazite, (Ln,Th)PO
  ₄, which usually contains all the rare earths, as well as thorium, is more involved. Monazite, because of its magnetic properties, can be separated by repeated electromagnetic separation. After separation, it is treated with hot concentrated sulfuric acid to produce water-soluble sulfates of rare earths. The acidic filtrates are partially neutralized with sodium hydroxide to pH 3–4. Thorium precipitates out of solution as hydroxide and is removed. After that, the solution is treated with ammonium oxalate to convert rare earths to their insoluble oxalates. The oxalates are converted to oxides by annealing. The oxides are dissolved in nitric acid, but cerium oxide is insoluble in HNO₃ and hence precipitates out. Care must be taken when handling some of the residues as they contain ²²⁸Ra, the daughter of ²³²Th, which is a strong gamma emitter.
caption: Crystal structure of bastnäsite-(Ce). Color code: carbon, C, blue-gray; fluorine, F, green; cerium, Ce, white; oxygen, O, red.
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