rare earth elements in Wyoming

diorites, and their associated pegmatites. The
mineral usually occurs as black to dark brown and
brownish-violet tabular grains. It has a conchoidal
fracture and is often metamict (i.e., its crystal lattice
is disrupted while the mineral grain retains its
original morphology) due to radioactive decay of
thorium, which can weaken the crystal structure. A
microscopic dark halo or ring seen inside of allanite
mineral grains is also an effect caused by radioactivity
(Long, Van Gosen, Foley, and Cordier, 2010).
The specific gravity of allanite is 3.4 to 4.2 and its
hardness is between 5 to 6.5 Moh’s scale (Deer and
others, 1967); these characteristics suggest a possibility
for placer concentrations. However, metamict
varieties may weather rapidly. Weathering of
allanite produces a reddish-brown, iron-rich, earthy
crust deficient in REE compared to unweathered
allanite (Deer and others, 1967).
Bastnasite (Ce,La,Y)CO 3 F
Bastnasite, also spelled bastnaesite or bastnäsite, is
an uncommon REE-bearing carbonate mineral (cerium-lanthanum-yttrium
carbonate fluoride) that
occurs in a mineral series with hydroxyl-bastnasite
[(Ce,La)CO 3 (OH,F)]. It is also closely related to
parisite [Ca(Ce,La) 2 (CO 3 ) 3 F 2 ], and may form replacement
crystals after allanite (Long, Van Gosen,
Foley, and Cordier, 2010). Although bastnasite is
not common, it is the most abundant REE-bearing
mineral (MinDat.org, 2011).
Bastnasite is primarily mined from rare, carbonaterich
igneous rocks called carbonatites. Carbonatites
contain greater than 50 percent carbonate minerals
and are usually associated with alkaline igneous
complexes. Bastnasite is one of the world’s primary
sources for REE, and is dominated by LREE (Hedrick,
2004). Bastnasite theoretically can contain
about 75 percent REO, but averages about 60
percent. Its yttrium content is generally very low at
about 0.05 percent (Hedrick, 1985). In addition
to being found in carbonatites, bastnasite is known
as an accessory mineral in some alkaline igneous
rocks and in some hydrothermal deposits (Hedrick,
2004).
Bastnasite may be massive or granular and typically
forms small, rounded hexagonal or short prismatic
crystals; it may also occur as spheres or rosettes
13
(Long, Van Gosen, Foley, and Cordier, 2010). It is
transparent to translucent, with a vitreous to greasy
or pearly luster. Its color varies from colorless to
shades of yellow, reddish-brown, and pink. Bastnasite
is relatively heavy with a specific gravity of 4.7
to 5.0. However, its low Moh’s hardness of 4.0 to
4.5 (MinDat.org, 2011), combined with its brittleness
and a prominent basal cleavage, suggest little
possibility for placer concentrations.
Euxenite (Y,Er,Ce,U,Pb,Ca)
(Nb,Ta,Ti) 2 (O,OH) 6
Euxenite is a slightly radioactive oxide mineral
in a series with polycrase [(Y,Ca,Ce,U,Th)
(Ti,Nb,Ta) 2 O 6 ]. Euxenite has similar properties to
other REE-bearing oxide minerals such as fergusonite
and samarskite, both uranium minerals,
and to aeschynite, a minor REE-bearing mineral.
Euxenite is typically found in granite pegmatites in
association with quartz, feldspars, columbite (ferrocolumbite),
tantalite (ferrotantalite or manganocolumbite),
and monazite (Long, Van Gosen, Foley,
and Cordier, 2010; MinDat.org, 2011).
Euxenite generally forms black to yellow- or greentinged,
greasy to submetallic, tabular to prismatic
crystals. Metamict crystals are often partially
amorphous. The mineral may also be granular to
massive, or may form radial aggregates. It is brittle,
exhibits a conchoidal fracture, and has no cleavage.
Its Moh’s hardness of 5.5 to 6.5 and specific gravity
of 5.3 to 5.9 facilitate its concentration in placer
deposits such as in detrital black sands (Long, Van
Gosen, Foley, and Cordier, 2010; MinDat.org,
2011).
Monazite (Ce,La,Y,Th)PO 4
Monazite is a REE-thorium phosphate mineral that
is similar to bastnasite as a LREE ore. However, it
hosts slightly more of the HREE, especially yttrium,
dysprosium, and gadolinium (Hedrick, 2004).
Monazite can theoretically contain 70 percent
REO, including about 2 percent yttrium oxide. In
reality, most concentrates contain about 55 to 65
percent REO (Hedrick, 1985), with typical values
in the 60 to 62 percent range (Long, Van Gosen,
Foley, and Cordier, 2010). Monazite’s high thorium
content (4.5 to10 percent ThO 2 equivalent)
and thorium’s naturally occurring radioactivity is