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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
in concentrations up to 43 and 4.4 per
cent by weight, respectively (Merritt
1971). Perhaps one of these forms is
present in Chattanooga shale.
Figures 9 and 10 show the shale's
surface at a magnification of 1000X, and
Figure 9. Titanium oxide grain, Chattanooga
shale sample number 2 (1000X) .
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Figure 10. X-ray counts characteristic touranium (titanium oxide) .
347
the x-ray counts characteristic of uranium,
respectively. The lightly colored grain, 7
ym in diameter, at the center of figure 9,
contains uranium and titanium. The composi
tion of uranium within the titanium-bearing
grain was estimated to be 5 percent by weight.
Other Minerals
The possibility exists of otheruranium-
containing minerals, such as coffinite
(U(Si04)1_x(0H)4x)and aluminum phosphates
being present in Chattanooga shale. The ex
tremely small size of some particles made
the study of a single grain difficult; thus,
all minerals may not have been identified
with this preliminary work. The association
of uranium with clays is yet unknown. Urani
um-enriched grains, smaller than 4 ym in size,
yielded ambiguous x-ray scans due to elements
in adjacent grains. A nonassociation of
uranium with pyrite, however, was noted.
Scans of numerous pyrite particles in urani
ferous Chattanooga shale failed to indicate
the presence of any uranium.
Uranium Mineral Surface Area
If the uranium within Chattanooga shale
is concentrated only in grains"visible"
to
the electron microprobe, the occurrence of
such a particle can be predicted. The fol
lowing assumptions must be used:
(1) Assume most of the uranium is con
centrated as uraninite. With a uranium com
position of 47 percent by weight, uraninite
has an approximate specific gravity of 7.5.
(2) Assume apatite and multiple oxides
with titanium contribute little to the over
all uranium content of shale.
(3) Use an average uraninite grain
diameter of 6 ym.
(4) Use shale containing 74 ppm by
weight uranium. Shale has a specific gravity
of 2.3.
With these assumptions, there is an oc
currence of one uranium particle for every
18 sweeps of a 160*200 ym area. Experimen
tally, a uranium particle was found for
every 8-15 sweeps under the electron beam.
Particles smaller than 6 ym in size and the
occurrence of less concentrated uranium
grains (apatite, multiple oxides) account
for the minor discrepancy in the number
of sweeps .
CONCLUSIONS
1. Uranium-bearing minerals exist
in the Gassaway Member of Chattanooga
shale. Uraninite, uraniferous apatite,
and titanium-bearing multiple oxides have
been identified by electron microprobe
analyses. Other minerals of uranium may
exist. The small grain structure of the
shale often yielded ambiguous x-ray scans
of associated elements.
2. The oxide forms of uranium appear
in very small minerals, 3 to 20 ym in size.
Uraniferous apatite forms much larger
grains, up to 180 ym in size, though they
contain less than one percent uranium.
3. Uranium beneficiation by pulver
izing and particle size classification is
uncertain. The small size of the uranium
minerals makes physical separation unat
tractive. Dissolving out these minerals
with acid, though, may be the only prac
tical recovery method.
4. The uranium mineral surface area,
as studied on the electron microprobe,
agrees with calculated predictions, based
on Chattanooga shale containing only
small (less than 10 ym in diameter) urani
nite particles. Apatite and multiple
oxides probably contain less than 10 per
cent of the shale's uranium.
5. The electron microprobe is an
effective tool in characterizing the mi
crostructure of shales. It is also ef
fective in locating trace elements such
as uranium. Though the preliminary work
is somewhat qualitative, quantitative com
position analyses of uranium minerals are
possible by comparison to standard com
pounds .
6. Chattanooga shale is an alter
nate source of both oil and uranium. The
recovery of both fuels in concert may be
of more economic value than the recovery
348
of either resource alone
ACKNOWLEDGMENTS
The assistance provided to one of the
authors by the Michigan Memorial-Phoenix Pro
ject and a Ford Fellowship (Ford Motor Com
pany) is gratefully acknowledged, along with
the work of J. S. Leventhal, U. S. Geological
Survey, in assaying shale samples by neutron
activation analysis.
Assistance provided by D. R. Peacor and
D. V. Wiltschko, Department of Geology and
Mineralogy, and G. B. Williams, Department
of Chemical Engineering, University of
Michigan, is also appreciated.
Recovery of uranium from Chattanooga
shale: Ind. and Eng. Chem., v. 35
(12), p. 1750-3.
Stanfield, K. E. and Frost, I. C, 1949,Method of assaying oil shale by a
modified Fischer retort: RI 4477,U. S. Bureau Mines.
Yen, T. F., 1974, Facts leading to the
biochemical method of oil shale re
covery, Analytical chemistry pertain
ing to oil shale and shale oil: NSF
Conference -
workshop report,
Washington, D.C.
REFERENCES
Altschuler, Z. S., Clarke, R. S., Jr. and
Young, E. J., 1976, Uranium in apatite:
Geol. Soc. Am. Abs. Prog., v. 8, p. 160-1
Brown, K. B., Schmitt, J. M., Hurst, F. J.
and Crouse, D. J., 1950, Recovery of
uranium from oil shales: Y-564, Oak
Ridge National Laboratory.
Ewing, R. A., Lutz, G. A. and Bearse, A. E.,
1949, Recovery of uranium from shales:
BMI-JDS-210, U. S. Atomic Energy Com
mission.
Frederickson, A. F., 1948, Some mechanisms
for the fixation of uranium in certain
sediments: Science, v. 108, p. 184-5.
Hakkila, E. A., Elliot, N. E., Williams, J. M.
and Wewerka, E. M. , 1977, Electron mi
croprobe studies of Mahogany Zone and
Devonian oil shales: Div. of Anal.
Chem. and Petr. Chem., Inc., ACS New
Orleans meeting.
McKelvey, V. E., Everhart, D. L. and Garrels,R. M.
, 1955, Origin of uranium deposits:
Econ. Geol., 50th Anniversary Volume,p. 464-533.
McKelvey, V. E. and Nelson, J. M., 1950, The
characteristics of marine uranium-bear
ing sedimentary rocks: Econ. Geol.,v. 45, p. 35-53.
Merritt, R. C, 1971, The extractive metal
lurgy of uranium: Colo. Sch. Mines
Res. Inst., Golden, Colo.
Mutschler, P. H., Hill, J. J. and Williams,
B. B., 1976, Uranium from Chattanooga
shale - Some problems involved in devel
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Pollara, F. Z., Levine, N., Killelea, J. R. ,
Musa, R. C. and Hassialis, M. D., 1958,
349
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