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.

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Altschuler, Z. S., Clarke, R. S., Jr. and

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Brown, K. B., Schmitt, J. M., Hurst, F. J.

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Ewing, R. A., Lutz, G. A. and Bearse, A. E.,

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Frederickson, A. F., 1948, Some mechanisms

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McKelvey, V. E., Everhart, D. L. and Garrels,R. M.

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349