URANIUM MINERALS IN CHATTANOOGA SHALE Amis Judzis and Arvids Judzis, Jr. Department of Chemical Engineering University of Michigan Ann Arbor, MI 48109 ABSTRACT A large quantity of low-grade uranium ore, Chattanooga shale, is present in much of east-central United States. Shale, an alternate source of uranium and oil, may one day be processed for its energy content. Chattanooga shale samples from DeKalb County, Tennessee, were studied with an elec tron microprobe. Preliminary results show that uranium in concentrations up to 130 ppm is not dispersed uniformly within the kerogen or inorganic matrix. Distinct uranium miner als, such as uraninite, apatite, and titanium bearing oxides, ranging in size from 3 to 180 ym, are evident on shale surfaces exposed by polishing. INTRODUCTION The Devonian and Mississippian shales of east-central United States contain an esti mated 2,000 to 3,000 billion barrels (319.6- 479.4 hM/m3) of oil equivalent (Yen 1974). In addition, Chattanooga shale is a low grade source of uranium; its content averaging 60 ppm for the Gassaway Member in regions of Kentucky, Tennessee, and Alabama. Future recovery of hydrocarbons or uranium from these shales may partially alleviate concerns of dwindling domestic oil and gas reserves. Extraction of uranium from Chattanooga shale has interested numerous researchers in the past. Investigations by Ewing (1949), Brown (1950), and Pollara (1958), for example, have shown that up to ninety percent of the shale's uranium may be removed by various dissolution techniques. The search as to how uranium is dispersed within Chattanooga shale, however, goes on. McKelvey and Nelson (1950) reported that "most of the uranium in the black shale is in an acid soluble form and seems to be in the fine grained fraction of the rock. Beyond that, nothing is known as to its mineral ogy." On the other hand, Frederickson (1948) postulated that U02 ions are ad sorbed between graphite layers of carbon aceous material. Until it is known how and where uranium resources are contained within Chattanooga shale, no accurate determination of uranium dissolution mechanisms is possible. With the advent of the electron microprobe, the ability to characterize rock matrices has vastly improved. Studies of very small surface areas, dif ficult during the 1950' s, are now possible. Hakkila and others (1977) demonstrated the utility of the electron microprobe in distinguishing differences between western and Devonian oil shales. They clearly identified the common mineral constitu ents of the shale matrix, such as pyrite, apatite, quartz, aluminosilicates, and others. In preliminary work, we have demonstrated the utility of the electron microprobe in the search of uranium and uranium containing minerals in Chattanooga shale. Finely-polished shale samples reveal the presence of at least three uranium-containing grains: uranium ox ides, uraniferous apatite, and titanium- bearing, multiple oxides. With the ex ception of uraniferous apatite, uranium- bearing grains are extremely small, 5 to 343
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URANIUM MINERALS IN CHATTANOOGA SHALE
Amis Judzis and Arvids Judzis, Jr.Department of Chemical Engineering
University of Michigan
Ann Arbor, MI 48109
ABSTRACT
A large quantity of low-grade uranium
ore, Chattanooga shale, is present in much
of east-central United States. Shale, an
alternate source of uranium and oil, may one
day be processed for its energy content.
Chattanooga shale samples from DeKalb
County, Tennessee, were studied with an elec
tron microprobe. Preliminary results show
that uranium in concentrations up to 130 ppm
is not dispersed uniformly within the kerogen
or inorganic matrix. Distinct uranium miner
als, such as uraninite, apatite, and titanium
bearing oxides, ranging in size from 3 to
180 ym, are evident on shale surfaces exposed
by polishing.
INTRODUCTION
The Devonian and Mississippian shales of
east-central United States contain an esti
mated 2,000 to 3,000 billion barrels (319.6-
479.4 hM/m3) of oil equivalent (Yen 1974).
In addition, Chattanooga shale is a low grade
source of uranium; its content averaging 60
ppm for the Gassaway Member in regions of
Kentucky, Tennessee, and Alabama. Future
recovery of hydrocarbons or uranium from
these shales may partially alleviate concerns
of dwindling domestic oil and gas reserves.
Extraction of uranium from Chattanooga
shale has interested numerous researchers in
the past. Investigations by Ewing (1949),
Brown (1950), and Pollara (1958), for example,
have shown that up to ninety percent of the
shale's uranium may be removed by various
dissolution techniques. The search as to
how uranium is dispersed within Chattanooga
shale, however, goes on. McKelvey and
Nelson (1950) reported that "most of the
uranium in the black shale is in an acid
soluble form and seems to be in the fine
grained fraction of the rock. Beyond
that, nothing is known as to its mineral
ogy."
On the other hand, Frederickson
(1948) postulated that U02 ions are ad
sorbed between graphite layers of carbon
aceous material. Until it is known how
and where uranium resources are contained
within Chattanooga shale, no accurate
determination of uranium dissolution
mechanisms is possible.
With the advent of the electron
microprobe, the ability to characterize
rock matrices has vastly improved.
Studies of very small surface areas, dif
ficult during the1950'
s, are now possible.
Hakkila and others (1977) demonstrated
the utility of the electron microprobe in
distinguishing differences between western
and Devonian oil shales. They clearly
identified the common mineral constitu
ents of the shale matrix, such as pyrite,
apatite, quartz, aluminosilicates, and
others. In preliminary work, we have
demonstrated the utility of the electron
microprobe in the search of uranium and
uranium containing minerals in Chattanooga
shale. Finely-polished shale samples
reveal the presence of at least three
uranium-containing grains: uranium ox
ides, uraniferous apatite, and titanium-
bearing, multiple oxides. With the ex
ception of uraniferous apatite, uranium-
bearing grains are extremely small, 5 to
343
10 ym in size being common.
The removal of oil or uranium from
Chattanooga shale by itself may always be
uneconomical. Based on uranium concentra
tions of 60 to 100 ppm, the mineral value is
$6-10 per ton. An oil content of 34 to 58
ym /kg (8 to 12 gal/ton) is worth $2-4 per
ton. Only a combined hydrocarbon-uranium
recovery scheme is likely to compete with
the costs of other forms of energy.
EXPERIMENTAL PROCEDURE
Shale Samples
Chattanooga shale samples from the
Gassaway Member were obtained from four out
crop locations in central Tennessee. Non-
weathered samples, from beneath the surface,
were collected. To ascertain oil and uranium
richness, the shale samples were assayed with
the modified Fischer retort, as described by
Stanfield and Frost (1949) and neutron acti
vation analysis, respectively. Assay re
sults, appearing on table 1, show that sam
ples rich in oil and uranium, typical for the
Gassaway Member, were obtained. Shale sam
ples from two locations in DeKalb County
were subsequently polished with Linde A 0.3
ym micropolish and coated with carbon to re
veal the surface microstructure.
Assays of Chattanooga shale samplesTable 1. Assays c
Sample
No. Location
2 DeKalb Co.
3 DeKalb Co.
4 Putnam Co.
7 Clay Co.
Fischer Assay
(gal/ton) (,am3/kg)
15.1 63
13.6 57
14.2 59
10.1 42
Uranium
(ppm)
74
132
53
67
Electron Microprobe Studies
An Applied Research Laboratories, Model
EMX-SM, electron microprobe was used for this
study. Uranium-bearing grains were located
in the following manner: First, an x-ray
spectrometer was set to 3.9098A, the charac
teristic wavelength of uranium (Ma) , using
a uranyl nitrate standard; second, the
polished Chattanooga shale surface was
exposed to an electron beam which was
swept over an area of 160*200 ym. The
x-rays characteristic to uranium were
meanwhile monitored. If no counts higher
than background were detected in the spe
cified area, the sample was moved to view
another 160*200 ym region.
Once located, the uranium-bearing
grain was exposed to a narrow beam (ap
proximately 1 ym in diameter) of electrons.
A multichannel analyzer counted the char
acteristic x-rays coming from the exposed
grain. Uranium and common elements, such
as aluminum, silicon, phosphorus, calcium,
and potassium, established intensity peaks
recorded on an oscilloscope screen. The
knowledge of grain constituents then al
lowed the identification of uranium-bear
ing minerals.
The problems of identifying single
grains of uranium compounds, however,
were great. Typical uranium-bearing min
erals are less than 10 ym in size! For
polished samples parallel to the bedding
plane, the depth of uranium grains is apt
to be 1 to 4 ym. At electron accelerating
voltages greater than 10 KV, electron
surface penetration may exceed 3 ym, thus
stray x-rays, characteristic of adjacent
grains, may be detected. As long as sin
gle minerals cannot be readily isolated,
the quantitative analyses of these com
pounds are estimates at best.
RESULTS AND DISCUSSION
Surface Microstructure
The fine-grained nature of Chatta
nooga shale is evident in figures 1 and
2. These photomicrographs of shale sam
ples collected in DeKalb County, Tennes
see, were taken at magnifications of 100X
and 1000X, respectively. The surface in
figure 1 is unpolished. In figure 2, the
large dark particles are pyrite, whereas
344
Figure 1. Unpolished Chattanooga shale sur
face (100X).
Figure 2. Polished Chattanooga shale surface
(1000X).
the small grains comprise the siliceous, car
bonate, and oxide compounds. The largest
pyrite grain, appearing in the upper left
corner of figure 2, is 30 ym in length.
Aluminosilicates and SiO~ make up most of
the siliceous matter.
Uranium-bearing Minerals
Uranium Oxides
McKelvey and others (1955) stated
that "no clear picture has emerged of the
exact nature of the uranium-bearing com
pounds [in blackshales]."
They suggested
that uranium-organic complexes, finely
disseminated uraninite, and adsorption by
some deposits (perhaps clays) account for
the shale's high concentration of uranium.
Electron microprobe studies showed the
presence of uraninite grains ranging in
size from 3 to 30 ym. Figures 3 and 4
show the shale's surface at a magnifica
tion of 1000X, and the x-ray counts (in
tensity) characteristic of uranium, re
spectively. The lightly colored grain
at the center of figure 3 is representa
tive of uraninite. X-ray counts outside
the uraninite grain are background. Fig
ure 5 shows characteristic x-ray counts
of elements in the uraninite grain. Only
three peaks are evident, corresponding to
the elements silicon, lead, and uranium.
Silicon is recorded from stray x-rays in
adjacent grains. Lead is, most likely,
the radioactive decay product of uranium.
Figure 3. Uraninite grain, Chattanooga
shale sample number 2 (1000X)
345
Uranium exhibits two characteristic x-ray
peaks, M and M,a
Figure 4. X-ray counts characteristic to
uranium (uraninite) .
Si Pb
Up
Figure 5. Elemental scan of uraninite grain
(intensity peaks) .
Uraniferous Apatite
Finely divided apatite (Ca5(P04)3(F,OH,
Cl)) is known to exist in the Gassaway Member
of Chattanooga shale. Uranium was found to
be concentrated in some apatite grains up to
180 ym in length. Figures 6 and 7 show a
Figure 6. Apatite grain, Chattanooga
shale sample number 3 (500X)
Figure 7. X-ray counts characteristic to
uranium (apatite) .
representative grain of apatite at a mag
nification of 500X, and the x-ray counts
characteristic of uranium, respectively.
The lightly colored particle in the apa
tite grain is mostly pyrite. Figure 8
shown an elemental scan of one uranifer
ous apatite grain. Note the two x-ray
intensity peaks for calcium, K and Kfl..
ot p
By comparison with an uranyl nitrate
346
Ca,
Ca/
Figure 8. Elemental scan of apatite grain
(intensity peaks) .
standard, the composition of uranium within
the apatite grain was estimated to be 0.5
percent by weight. Altschuler and others
(1976) proposed that uranium can replace cal
cium in the apatite structure. The similaro
ionic radii for tetravalent uranium (1.05A)o
and divalent calcium (1.06A) make substitu
tions likely. He suggests that uranium is
typically 0.00X to 0.01X percent of sedimen
tary marine apatite, somewhat lower than esti
mated by our studies on the electron micro
probe.
Within Chattanooga shale, Mutschler
(1976) notes that phosphates occur in scat
tered nodules in the top few feet of the Gas
saway Member and in sparse,finely- divided
particles of apatite. Apatite comprises less
than one percent of the shale matrix.
Multiple Oxides
Titanium-and uranium-bearing grains
were found in Chattanooga shale sample number
2. An elemental scan showed the concentra
tion of titanium was high in these multiple
oxides, while that of uranium was low. Iron
in trace quantities was also seen. Hakkila
(1977) found TiO- minerals in shale from
both the Mahogany Zone and West Virginia.
TiO- can exist as rutile; however, the pre
sence of uranium makes such an occurrence
doubtful. Brannerite ((U,Ca,Fe,Y,Th)3(Ti,Si) _0, 6) and davidite contain uranium