Geology Reference
In-Depth Information
8.13.2 Geochemistry ofREE
Rare earths, despite their name, are not in fact rare but dispersed. It would be there-
fore more appropriate to refer to them as “disperse earths”. Cerium, for example, is
as abundant as copper or nickel. All REE have very similar chemical properties due
to their electron structure
26
. Scandium, yttrium, lanthanum, cerium and lutetium
have one electron each in their external subshell d and each of the lanthanides fills
one more electron in their subshell 4f. Due to this progressive internal sub-shielding
they present the so-called lanthanide contraction, which consists of a gradual de-
crease in ionic size, M
+3
, as atomic number increases. This explains their chemical
similarities, their di
culty of separation, their own geological particularities - found
all together-, and even their intricate discovery history.
REE can be grouped into two sets relative to their natural occurrence: those
composed of the light lanthanide elements La;Ce;Pr;Nd;Pm;Sm and Eu, called
the cerium subgroup; and the heavier ones composed of Gd;Tb;Dy;Ho;Er;Tm;Y b
and Lu together with Y , called the yttrium subgroup or heavy lanthanides. All rare
earth metals are stable, except for the radioactive promethium.
Although around 200 additional rare earth minerals are known with presence
>0.01% of rare earths, the predominately minable minerals for rare earths extraction
are bastnaesite, monazite and xenotime which collectively contain more than 95%
of the crustal rare earths.
Bastnaesite is a fluorocarbonate of the cerium subgroup REFCO
3
with variable
quantities of Ce (27-32% of cerium oxide), La (49-50%), Nd (13-15%), Pr (4-5%),
Sm (0.5-1%) and other REE amounting to less than 1%. The mineral appears in
small crystals (phenocrysts) interlocked with grains of carbonatites of an igneous
origin and also in carbonatite magmatic dykes and fluorite bearing veins of epither-
mal origin.
Monazite is an orthophosphate of the cerium subgroup, REPO
4
, with variable
quantities of La (44-47%), Ce (18-24% of cerium oxide), Nd (17-20%), Pr (4-6%),
Sm (2.5-5%), Gd (1-6%) and Y (1-3%) with the remaining rare earths, together
with thorium, contributing to a figure of less than 1% overall. It occurs in minor
quantities in igneous rocks, typically granite and metamorphosed igneous rocks such
as gneiss, which when exposed to erosion appear in coarse grained rocks in sands
or in other detrital placer deposits.
Finally, xenotime is a thorium orthophosphate with a rare earth content be-
longing to the yttrium subgroup, Y PO
4
. The Malaysian xenotime contains 60%
of yttrium oxide, with the remaining 40% constituting oxides of Ce (5%), Dy
(8%), Er (5%), Y b (6%), Gd (4%), Nd (2%), Ho (2%), Sm (2%), Tb
26
Scandium (atomic number 21, external subshell 3d
1
), yttrium (39, 4d
1
), the lanthanides: lan-
thanum (57, 5d
1
), cerium (58, 4f
15
d
1
), praseodymium (59, 4f
3
), neodymium (60, 4f
4
), prome-
thium (61, 4f
5
), samarium (62, 4f
6
), europium (63, 4f
7
), gadolinium (64, 4f
8
), terbium (65, 4f
9
),
dysprosium (66. 4f
10
), holmium (67, 4f
11
), erbium(68, 4f
12
), thulium (69, 4f
13
), ytterbium (70,
4f
13
), and lutetium (71, 4f
14
5d
1
).
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