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(1%), and the remaining rare earth oxides REO, which amount to less than 1%.
Xenotime, as with monazite, occurs in minor quantities and is found in the same
types of rock. Weathering processes transport and comminute the mineral, widely
dispersing it over the entire crust.
In previous chapters the reader has seen that the energy needed for obtaining a
substance from its mineral is a function of its chemical composition, concentration
and comminution energy. In turn, these properties depend on the given mineral's
ore. So this begs the question, why are rare earths scattered and expensive to
obtain?
To answer this question, the authors look to, as an example, the giant
REE NbFe deposit of Bayan Obo, Inner Mongolia. Yang et al. (2003, 2009),
Lai et al. (2012) and Cuchi, J.A. 2012 (private communication) state that it was
generated by the reactive interaction of mantle fluids (containing REE, Nb and Fe
at temperatures in the range of 1000-1500
C) with previously formed sedimentary
carbonate rocks, resulting in a carbonatite melt confined in a crustal chamber. The
subsequent cooling process undertook fractional crystallisation with an upgrade in
REE concentration (metasomatism). The rich REE fluids were then channelled
upwards along a rift penetrating in the rock or diffusing and filling the veins of a
lenticular shape. Later on, weathering processes converted them into fine-grained
dolomite marbles and dolomite/calcite carbonatites. These in turn have variable
amounts of magnetite, monazite, bastnaesite and fluorite with a total REE content
ranging from 15,370 ppm to 85,192 ppm, a steep contrast to the continental crust
which holds an average 117 ppm (Lai et al., 2012). The metasomatic process as
described above, falls into the common case of formation of pegmatite rocks. They
consist of a structure of interlocking crystals of intrusive igneous origin, in which the
rare earth elements, in addition to niobium, tantalum and lithium (as spodumene
or lepidolite) are commonly found.
Therefore the answer as to why rare earths are scattered, can now be tentatively
formed. They come from the magma and as they are chemically similar they appear
together with only fractional crystallisation permitting a partial separation. Magma
is composed of ten principal elements: Si;Al;Fe;Mg;Ca;Na;K;H; and O, forming
various silicates and oxides, with their exact nature being depth and pressure de-
pendent (Helffrich and Wood, 2001). All other elements collectively, including rare
earths, set up the remaining 1% (see for example Brophy (2012)). Therefore, the
fractional crystallisation of magma plays a key role in concentrating rare earths. To
understand this process, one first needs to be able to distinguish between compat-
ible and incompatible elements. Incompatible elements are those whose ionic radii
are so large that they do not fit in the crystal structure of the mantle silicates like
olivine, pyroxene and garnet and oxides of the spinel group. This is the case of rare
earths and a selection of other elements including K;Rb;Cs;Ta;Nb;U;Th;Y;Hf;
and Zr. When the mantle starts melting these large ions they are primarily dis-
placed from their crystallographic sites and enter the liquid (fractional melting).
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