57
Lanthanides
57.1 INTRODUCTION
Lanthanides (LAs), often called rare earth elements (REEs), comprise 15 elements,
of which only one, promethium [Pm, 61], does not occur naturally in the Earth’s
crust. The term REE is related to LA series, distinguished in the periodic table of
elements. However, the International Union of Pure and Applied Chemistry proposed to add to the REEs other elements, such as Sc and Y. To avoid any confusion,
the term LAs will be used for the elements from La to Lu. Two subgroups of LAs
are distinguished: the first one (light, LLAs) obtains more basic and more mobile
light elements, from La to Gd, than the elements of the second group (heavy, HLAs),
from Tb to Lu.
LAs are relatively abundant in rocks and soils (Tables 57.1 and 57.2). Their terrestrial distribution is peculiar; their contents decrease with an increase of atomic
weights and atomic number. According to the Oddo–Harkins rule, the element with
the even atomic number is more frequent than the next element with an odd atomic
number. This rule governs the distribution of all elements in the universe, but is
much less pronounced than in the case of LAs.
During weathering processes, LAs are fractionated and are likely to accumulate
in weathered materials. Almost all LAs occur preferable as +3 cations, exhibit an
affinity for oxygen, and are likely to be concentrated in acidic igneous rocks and
argillaceous sediments (Table 57.1). Cerium is the most frequent element in both
the Earth’s crust (60 mg/kg) and rocks (>250 mg/kg), whereas lutetium is the less
abundant element (0.4 and >1.2 mg/kg, respectively).
The global mobility of LAs, calculated as the ratio of water-dissolved concentrations to those of the upper continental crust is relatively low, at about 0.01 ratio value
(Gaillardet et al. 2003). However, during longer period, the mobility of LAs affects
their distribution.
LAs are constituents of several minerals, such as monazite [(La,Ce,Th)PO 4 ], bastnasite [(Ce,F)CO 3 ], and cheralite [(Ce,La,Y,Th)PO 4 ]. Lanthanum is also associated
with xenotime (YPO 4 ). All these minerals are likely to be concentrated in phosphorites, which resulted in elevated amounts of some LAs in P fertilizers. In addition,
some calcite rocks may contain higher amounts of LAs.
Global production of LAs in 2003 was estimated to be about 75 Mt (WMSY
2004). Two of their major sources, and especially of Ce, are monazite and bastnasite.
Monazite is associated with granites and gneisses, and due to its great resistance
to weathering, it is concentrated in weathered materials and often accumulated in
beach sands.
Elevated amounts of LAs are reported for fly ash and fire clay. LLAs in fly ash are
within the range of 424–493 mg/kg, and HLAs are within the range of 55–61 mg/kg
(Hower et al. 2013). Therefore, there are suggestions to extract them from coal ash.
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