rare earth elements in Wyoming

neodymium-145 through electron capture, a type
of radioactive decay.
Two other elements not within this series, scandium
(atomic number 21) and yttrium (atomic
number 39) are generally included within the
group of REE because of their shared occurrence in
nature with the lanthanides. Including scandium
and yttrium, there are a total of 16 naturally occurring
REE.
Rare earth elements received their name from
perceived scarcity during the 18 th and 19 th centuries
and from ‘earth’ as an obsolete term for
oxide (Hedrick, 2004; Long, Van Gosen, Foley,
and Cordier, 2010). REE are now known to be
relatively common within the earth’s crust. Even
the two least abundant REE, thulium (Tm) and
lutetium (Lu) are in the range of 140 to 180 times
more common than gold. However, economically
mineable concentrations of REE are not common.
REE range in crustal abundances from cerium
(25th most abundant of the 78 common elements
in the earth’s crust at 60 parts per million, or ppm,
to thulium and lutetium (least abundant REE at
about 0.5 ppm). Estimates of crustal abundances
vary depending on the reference source.
REE are subdivided into Light rare Earth Elements
(LREE) and Heavy rare Earth Elements
(HREE) based on their atomic weight (table 2)
(Hedrick, 2004). LREE include the elements
lanthanum (atomic number 57, atomic weight
138.91) through europium (atomic number 63,
atomic weight 151.96) while HREE are gadolinium
(atomic number 64, atomic weight 157.25)
through lutetium (atomic number 71, atomic
weight 174.97).
REE in their elemental form are iron-gray to silver,
malleable and ductile, soft metals that are unstable
in the presence of oxygen, and readily react to form
metal oxides. Chemically, REE are generally trivalent,
but exceptions occur with cerium, europium,
and ytterbium. Cerium forms compounds with
a valence of +4. Europium can be divalent and
oxidizes much more readily than the trivalent REE,
forming a yellow surface coating of Eu (OH) within
minutes after exposure to moist air. Ytterbium
5
has a closed-shell electron configuration (reluctant
to share electrons with oxygen or other elements)
and is thus relatively inert; it can be handled in air
and remains stable (Borzone and others, 1999).
The reactivity of most REE is problematic for
shipping and long-term storage in elemental form.
Because of this, REE are often reported in metric
tons (1 tonne = 1000 kilograms [kg]) of rare Earth
oxide (rEo) with all REE lumped together. The
designation total rare Earth oxides (trEo) is
used in mining and exploration to describe the
total REE content in a deposit or samples in an
equivalent oxide form even though their mineral
occurrence may not be in the form of an oxide.
By convention, TREO addresses only 14 lanthanide
series elements plus yttrium; scandium is
not included in TREO (Rare Element Resources,
2011). A mixture of REE in metallic form (usually
admixed with iron) is called misch metal, or
mischmetal (Hedrick, 1985).
REE with even atomic numbers, such as Ce and
Nd, have greater cosmic and terrestrial abundances
than adjacent REE with odd atomic numbers (La
and Pr; table 1). Cerium is the most abundant with
a continental crustal abundance of about 60 ppm,
followed by lanthanum at 34 ppm and neodymium
at 33 ppm. The cumulative total abundance of
REE in the earth’s crust, including scandium and
yttrium, ranges from about 20 ppm to 219 ppm,
depending on the reference cited (Long, Van
Gosen, Foley, and Cordier, 2010; WebElements.
com, 2012).
To depict low REE concentrations and compare
slight deviations between samples, this report addresses
elemental concentrations in ppm. However,
commercial REE assay values are, by convention,
reported in terms of TREO. The difference between
reported ppm in assay values and reported
TREO will vary with the specific geochemistry
of individual deposits. TREO equivalents found
in Rare Element Resources’ Bear Lodge project
are approximately 15.6 percent higher than the
combined raw metal assay values (Rare Element
Resources, 2011). However, using the Bear Lodge
deposit as a general indication, 3,000 ppm in raw
metal assay would give an estimated TREO of