Major Minerals

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 Cu (68 ppm), and Ce is even more abundant than common metals such as Pb (13 ppm) and Sn (2.1 ppm). Cerium can be found in various minerals, but the most significant commercial sources are the minerals of the monazite (Fig. 7.23) and bastna¨site groups(Fig. 7.19), where it makes up about 50% of the total lanthanide content.

FIGURE 7.23 Monazite-(Ce), Ce(PO4) (always contains major to minor amounts of other REEs (Nd, La, Sm, etc.) replacing Ce. Also often contains trace amounts of U and Th (coupled with Ca).), good twinned crystals to 8 mm in aggregate.

FIGURE 7.19 Bastna¨site-(Ce), Ce(CO3)F, a well-crystallized rosette 1 cm across.

There are around 150 minerals containing cerium as part of their chemical composition. Five halides are known to contain Ce, for example, chukhrovite-(Ce) (Ca3(Ce,Y)[F|SO4|(AlF6)2]·10H2O) and fluocerite-(Ce) ((Ce,La)F3). The oxide class contains 16 minerals with Ce, such as aeschynite-(Ce) ((Ce,Ca,Fe,Th)(Ti,Nb)2(O,OH)6), cerianite-(Ce) ((Ce41,Th)O2), davidite-(Ce) ((Ce,La)(Y,U,Fe)(Ti,Fe)20(O,OH)38), and fergusonite-(Ce) ((Ce,La,Y)NbO4). Some 30 minerals are present in the carbonate class, for example, ancylite-(Ce) (CeSr(CO3)2(OH)1 H2O) (Fig. 7.18), bastna¨site- (Ce) (Ce(CO3)F) (Fig. 7.19), hydroxylbastna¨site-(Ce) (Ce(CO3)(OH)), parasite-(Ce) (CaCe2(CO3)3F2) (Fig. 7.20), and synchysite-(Ce) (CaCe(CO3)2F) (Fig. 7.21). Two borates, braitschite-(Ce) ((Ca,Na2)7(Ce,La)2B22O43· 7H2O) and peprossiite-(Ce) ((Ce,La)(Al3O)2/3B4O10), are known with Ce in their crystal structure.

Tancaite-(Ce) (FeCe(MoO4)3· 3H2O) is the only molybdate in the sulfate class with Ce. About 20 different minerals are known in the phosphate class to contain Ce, for example, agardite-(Ce) (CeCu6(AsO4)3(OH)6· 3H2O), florencite-(Ce) (CeAl3(PO4)2(OH)6) (Fig. 7.22), gasparite- (Ce) (Ce(AsO4)), monazite-(Ce) (Ce(PO4)) (Fig. 7.23), and rhabdophane-(Ce) (Ce(PO4)· H2O). The largest group of around 70 minerals is found in the silicates class, for example, allanite-(Ce) ({CaCe}{Al2Fe2+}(Si2O7)(SiO4)O(OH)) (Fig. 7.24), britholite-(Ce) ((Ce,Ca)5(SiO4)3OH) (Fig. 7.25), cerite-(Ce) ((Ce,Ca)9(Mg,Fe)(SiO4)3(HSiO4)4(OH)3), gadolinite-(Ce) ((Ce,La,Nd,Y)2Fe21Be2Si2O10), joaquinite-(Ce) (NaBa2Ce2FeTi2[Si4O12]2O2(OH,F)· H2O) (Fig. 7.26), and steenstrupine-(Ce) (Na14Mn2+ 2 (Fe3+,Mn3+)2Ce6(Zr,Th) (Si6O18)2(PO4)6(HPO4)(OH)2· 2H2O).

Chemistry Properties

Cerium is a soft, ductile, and silvery-white metal that it is soft enough to be cut with a knife. Cerium is the second element in the lanthanide series, and though it frequently shows the 13 oxidation state typical for the series, it also exceptionally has a stable 1 4 state that does not oxidize water (Table 7.4). It is also considered one of the REEs. In the periodic table, it appears between the lanthanides lanthanum to its left and praseodymium to its right, and above the actinide thorium. It is a ductile metal with a hardness similar to that of silver. Its 58 electrons are arranged in the configuration [Xe]4f1 5d1 6s2 , of which the four outer electrons are valence electrons. Directly after lanthanum, the 4f orbitals abruptly contract and are lowered in energy to the point that they participate readily in chemical reactions; yet, this effect is not yet strong enough at cerium and thus the 5d subshell is still occupied. Most lanthanides can utilize just three electrons as valence electrons, as afterward the remaining 4f electrons are too strongly bound. Cerium is an exception due to the stability of the empty f shell in Ce41 and the fact that it comes very early in the lanthanide series, where the nuclear charge is still low enough until neodymium to allow the removal of the fourth valence electron by chemical means.

Reactions

Cerium tarnishes in air, forming a spalling oxide layer like iron rust; a centimeter-sized piece of cerium metal corrodes completely in about a year. It burns readily at 150℃ to form the pale-yellow cerium(IV) oxide, also known as ceria.

This may be reduced to cerium(III) oxide with hydrogen gas. Cerium metal is highly pyrophoric, that is, when it is ground or scratched, the resulting shavings catch fire. This reactivity conforms to periodic trends, as cerium is one of the first and therefore one of the largest lanthanides. Cerium(IV) oxide has the fluorite (CaF2) structure, comparable to the dioxides of praseodymium and terbium. Cerium oxide is utilized as a catalytic converter for the minimization of CO emissions in the exhaust gases from cars. When there is a shortage of oxygen, cerium(IV) oxide is reduced by carbon monoxide to cerium(III) oxide:

When there is an oxygen surplus, the process is reversed and cerium(III) oxide is oxidized to cerium(IV) oxide:

Cerium is a highly electropositive metal and reacts with water. The reaction is slow with cold water but speeds up with increasing temperature, producing cerium(III) hydroxide and hydrogen gas.

Cerium metal reacts with all the halogens to produce trihalides

Cerium dissolves readily in dilute sulfuric acid to form solutions containing the colorless Ce31 ions, which exist as [Ce (H2O)9] 31 complexes.