SOME GEOLOGICALLY IMPORTANT ELEMENTS - Chemical Fundamentals of Geology and Environmental Geoscience - page 175

Rare earth elements

continuously in behaviour from incompatible to
selectively compatible.
For reasons discussed in Chapter 11, an element of
even atomic number tends to be about ten times
more abundant in the cosmos than its neighbours with
odd Z-values, giving the rare earths in particular
(Figure 11.2 inset) a characteristic zigzag abundance
pattern that is found in all Solar System matter. When
examining the rare-earth pattern of a terrestrial or
lunar rock, therefore, geochemists eliminate this
sawtooth effect by dividing each REE concentration in
the rock (ppm) by the average concentration of the
same element in chondrite meteorites (also in ppm).
Chondrites (Box 11.1) serve as a sensible reference
value here because they provide a reasonable estimate
of the primordial composition of the Earth's mantle,
the 'starting point' from which all igneous rocks have
ultimately been derived. The result is a smooth 'chon-
drite-normalized' REE pattern showing how much
each rare earth has been 'enriched' in the sample of
interest relative to a model mantle source.
Figure 9.10 shows a chondrite-normalized REE pat-
tern for two lunar basalts. The lower pattern represents
lava formed directly by melting of the lunar mantle;
the pattern is relatively flat, suggesting that the lunar

Following the element lanthanum, La (the first member
of the third transition series - see Figure 9.7), electrons
begin to occupy the seven 4f orbitals, forming the
14 metals from cerium (Ce) to lutetium (Lu) known
as the lanthanides or rare earth elements (REEs ). The
distinction between individual rare earths lies in
the number of 4f electrons. These are mostly not
involved in bonding, and the chemical properties of
all 14 elements, together with lanthanum, are there-
fore remarkably similar. All have stable trivalent
states (Figure 9.9).
Owing to the increase in nuclear charge, there is a
steady decrease of ionic radius in progressing from
lanthanum La 3+ to lutetium Lu 3+ (the lanthanide con-
traction -Figure 9.9). The 'light rare earth elements'
('LREEs', La-Sm) are incompatible elements. Owing
to their smaller ionic radii, however, the 'heavy rare
earths' ('HREE', Gd-Lu) are more easily accomm-
odated in the crystal structures of a few rock-forming
minerals, particularly garnet and amphibole. The
rare earth elements therefore provide the geochemist
with an array of trace elements which, though virt-
ually identical in other chemical properties, range

The element
promethium
(Pm, Z = 61) has no
stable isotope

Europium has two
stable valence states:
II and III. Eu 2+ is
known to occur in
igneous melts under
low f O 2 conditions, and
substitutes for Ca 2+ in
plagioclase.

Eu 2+

0.13

La

Ce 3+

Eu

Pr

0.12

'LANTHANIDE
CONTRACTION'

Nd

Sm

Ce

Eu 3+

Gd

Y 3+

0.11

Tb

Dy

Ho

Er

Ce 4+

Tm

Yb

Lu

0.10 Cerium is the only REE

to form a IV valence
state (in oxidising sed-
imentary environments)

0.09

57 60 65

Atomic number Z

70

Figure 9.9 Ionic radii of the rare earth elements. Y 3+ represents the related element yttrium (Figure 9.7).