Crystallographic Study of Uranium-Thorium Bearing Minerals in Tranomaro, South-East Madagascar

doi:10.4236/jmmce.2013.16053

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Journal of Minerals and Materials Characterization and Engineering, 2013, 1, 347-352

Published Online November 2013 (ht t p:/ / www.scirp.org/journal/jmmce )

http://dx.doi.org/10.4236/jmmce.2013.16053

Open Access JMMCE

Crystallographic Study of Uranium-Thorium Bearing

Minerals in Tranomaro, South-East Madagas car

Frank Elliot Sahoa1, Naivo Rabesiranana1*, Raoelina Andriambololona1, Nicolas Finck2,

Christian Marquardt2, Hörst Geckeis2

1Institut National des Sciences et Techniques Nucléaires (Madagascar-INSTN), Antananarivo, Madagascar

2Institute of Nuclear Waste and Disposal (INE), Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany

Email: *[email protected]

Received September 20, 2013; revised October 21, 2013; accepted November 5, 2013

cense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ABSTRACT

Studies are undertaken to characterize the uranium and thorium minerals of south-east Madagascar. Seven selected

uranothorianite bearing pyroxenites samples from old abandoned uranium quarries in Tranomaro, south Amboasary,

Madagascar (46˚28'00"E, 24˚36'00"S) have been collected. To determine the mineral micro-structure, they were inves-

tigated for qualitative identification of crystalline compounds by using X-ray powder diffraction analytical method

(XRD). Results showed that the uranium and thorium compounds, as minor elements, were present in various crystal-

line structures. Thorium, as thorianite, is present in a simple ThO2 cu bic crystalline system, whereas the uranium com-

ponent of the Tranomaro uranothorianite samples is oxide-based and is a mixture of complex oxidation states and crys-

talline systems. Generally called uraninite, its oxide compounds are present in more than eight phases.

Keywords: Tranomaro; Madagascar; Uranothorianite; Crystallography; XRD

1. Introduction

In 1907, a Malagasy mineralogist, J. B. Rasamoel, dis-

covered euxenite-bearing tertiary lake sediments near

Vinaninkarena. French interest in the source of radium

led to intensive search for primary uranium minerals in

the adjacent upland plateau [1]. The central island is one

of the regions of interest of a Manhattan District Project.

In 19 43- 1944, the Kitsamby district, located in the regio n,

was then explored for potential uranium ore deposit by

the project specialists [2]. But Moreau [3] has observed

uranium-thorium mineralization in Tranomaro, south-

east of Madagascar, where major extracting activities

were undertaken by the French Commissariat à l’Energie

Atomique (CEA) during the fifties and sixties, to support

their effort to develo p nucl e a r pr o ject .

In 1979, using new analytical technology such as

gamma spectrometry system, Raoelina Andriambololona

et al. [4] have further investigated the radioactivity char-

acteristics of uranium and thorium minerals from Mada-

gascar, in particular from the South-East region [5-7].

Rabesiranana et al. [8] reported the natural radionuclide

content in soil, focusing mainly on the comparison be-

tween on-site vs. off-site data, to detect significant dif-

ferences, taking into account spatial variability. Using

electronic microprobe analysis and X-ray fluorescence

technique, Rakotondratsima and Moine et al. [9,10]

studied the origin and concentration of uranothorianite

bearing pyroxenite content in the region, and have demon-

strated the formation of uranothorianite bearing pyrox-

enites with marble at Belafa and Marosohihy and miner-

alization without marble at Ambidandrakemba, two bil-

lion years ago. Rakotondrazafy used XRD technique and

microprobe analysis for dating and determining the foma-

tion conditions of hibonite in Madagascar. He has indicated

that hibonite mineral has been plainly confined in uranotho-

rianite bearing skarns formed 545 million years ago [11].

Recently, in order to supply worldwide increasing

need for alternative energy, and in view of future short-

age of conventional fossil fuel, there is a renew interest

in nuclear energy. Consequ ently, mining co mpanies are ex-

ploring old and newly discovered uranium-thorium pro-

vinces of the country. This is the case for the South East

region of Madagascar, where the uranothorianite deposits

of the Tranomaro province are re-investigated (Figure 1).

2. Rationale

The uranium and thorium compounds may be mobilized

*Corresponding author.

F. E. SAHOA ET AL.

348

Figure 1. Geographical location of the tranomaro study site,

south-east madagascar.

and transported into the environmental compartments,

increasing the risk of human contamination and exposure

from radioactive elements. If the mobilization is limited

for an undisturbe d g eolog ical formation, human activ ities

such as mining may accelerate the process. The disper-

sion depends on 1) the weathering/leaching efficiency, 2)

the uranium-thorium structure resistance and 3) the dis-

turbance magnitude. Rock weathering is normally insti-

gated by air and water from the earth’s surface gaining

access to minerals previously surrounded by other min-

erals. When air and water reach previously inaccessible

parts of the rock matrix, chemical reaction such as oxida-

tion and leaching can change the rock’s mineral compo-

sition [12,13].

Uranium and thorium elements occur in the 4+ oxida-

tio n st at e in p r imary igneous rocks and minerals. As s u c h ,

both elements are almost chemically immobile in the

near surface environment at low temperature. In the oxi-

dized zone of the terrestrial near-surface environment,

uranium and thorium may be mobilized, but in different

ways. According to Gascoyne [14], thorium is mainly

transported in insoluble resistate minerals or is adsorbed

on the surface of clay minerals. But uranium can be oxi-

diz e d to 5 + an d 6+ states in the near surface environ me nt .

The most probable oxidation reaction at normal envi-

ronment is [15]:

U2HOUO4H2e











The 6+ oxidation state is the most stable and forms

soluble uranyl complex ions which play an

important role in uranium transport during weathering

[14]. Finch and Ewing [16] reported that in a highly al-

tered uraninite-bearing ore body of Koongara, Australia,

partially exposed to meteoric water, alteration at depth

has resulted from interaction with groundwater having a

reduced Eh compared to the surface. Urani nite, Pb-uranyl

oxide hydrates and uranyl silicates control uranium solu-

bility at depth; uranyl phosphates and uranium adsorption

onto clays and FeMn-oxides control uranium solubility

near the surface. So, uranium may either move in solu-

tion as complex ion, or like thorium, in a sorbed or detri-

tal phase.



UO 

Most of the studies report mobilization in terms of

oxidation state. To further support the risk analysis re-

lated to the dispersion of uranium and thorium from solid

phases to the environment, crystallographic structure of

uranium-thorium compounds in pyroxenite minerals

from the Tranomaro uranium-thorium province, south-

east Madagascar, is investigated. The sample characteri-

zation has been conducted using X-ray powder diffrac-

tion analytical technique (XRD).

3. Experimental Methodology

Prior to the study, radiometric measurement within the

study area had been done by the Pan African Mining

Corporations-Madagascar team. Using Exploranium

scintillator counters, high radioactivity spots had been

localized. Sampling for the current study was subse-

quently conducted in 2009, and sampling points were

then selected according to counting level previously

found. Only samples with count rate greater than 1500

counts per second (cps) have been selected. They were

usually from abandoned mining quarries, with details

shown in Table 1. Most of the samples are crude rocks,

but one of them is a pure uranothorianite mineral care-

fully picked from a pyroxenite mineral sample.

Mineral sample is ground and sieved in order to get 10

µm to 20 µm size fine and homogeneous powder. For

low diffraction background, the powder is packed into a

sample holder made of silicon single crystal and smeared

uniformly onto a glass slide. When packing into a sample

container, care is taken to create a flat upper surface and

to achieve a random distribution of the crystalline lattice

orientations.

The XRD analysis is carried out using a Bruker D8

Advance diffractometer equipped with a copper anode,

powered at (40 kV, 40 mA), and controlled by the XRD

Commander software. The sample is illuminated by the

Cu-Kα radiation (λ = 1.5406 Å) and the diffracted beam

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F. E. SAHOA ET AL.

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349

Table 1. Collected mineral samples and their localization.

Code Surface Counts Rate (cps) Location

Longitude Latitude Mine code Name

R01M37 1610 46˚30’24’’E 24˚34’43’’S M37 Ambindandrakemba

R01M47 1650 46˚31’47’’E 24˚23’36’’S M47 Marosohihy

R02M47 2500 46˚31’46’’E 24˚23’36’’S M47 Marosohihy

R03M47 3000 46˚31’45’’E 24˚23’38’’S M47 Marosohihy

R01M52 1720 46˚31’36’’E 24˚21’47’’S M52 Belafa

R02M52 1725 46˚31’48’’E 24˚21’55’’S M52 Belafa

Uranothorianite 2800 46˚30’27’’E 24˚4’12’’S Uranium-thorium mineral

is detected with a Sol-XE energy dispersive detector. The

diffractogram is collected for 2θ varying from 5˚ to 80˚

with steps of 0.01˚ and 2 seconds counting time per step,

and the sample spinning at 15 rotations per minute. The

diffractogram displaying the diffracted intensity as a

function of the diffraction angle is processed using the

Bruker Eva software.

Qualitative analysis is carried out by comparison with

the JCPDS database (Joint Committee for Powder Dif-

fraction Studies). The more peaks match the data from

the database, the higher the confidence in phase identifi-

cation. For samples containing more than one single

ph ase, the position of the characteristic peaks ca n ov er lap .

Since each pure crystalline phase is characterized by a

specific diffraction pattern, the identification of the

phases present in the sample is enhanced by using the

Rietweld method. Essentially, it involves fitting the ob-

served diffraction pattern with a synthetic pattern which

is a sum of patterns calculated for each phase in the sam-

ple [17]. As such this method is known as a full-pattern

fitting method [18]. The difference between the synthetic

pattern and the observed pattern is minimized by an in-

teractive refinement/optimization procedure. The amounts

of the phases present in the sample are obtained from the

final value of the refined scale factor for each phase [19].

To improve the accuracy, only compounds which are

likely to be found in the studied samples are selected for

identification. From previous studies [9,20], Table 2 shows

major minerals and uranium-thorium compounds in py-

roxenite minerals from the study site. Additionally, all

uranium-thorium phase diffraction patterns are added

from the JCPDS database for identification.

4. Results and Discussion

Examples of the graphical output from the Bruker Eva

software are shown in Figure 2. Two typical diffracto-

grams are presented: Figure 2(a) shows a simple spec-

trum from pure uranium-thorium minerals, whereas Fig-

ure 2(b) shows a complex spectrum from the R01M37

sample, which is a natural uranium-thorium ore.

A summary of the processed result is presented in Ta-

ble 3. For natural ores, common major and minor mineral

Table 2. Minerals expected to be found in the studied ura-

nium-thorium bearing rocks.

Expected elements Chemical formula

Uranothorianite (Th,U)O2

Diopside pyroxenite CaMg(SiO3)2

Calcite CaCO3

Corundum CaMg(SiO3)2

Spinel FeAl2O4, MgCr2O4

Apatite Ca5(PO4)3(Cl,F,OH)

Phlogopites KMg3ASi3O10

Hibonite CaAl12O19

phases such as corundum, spinel, diopside, soddyite, cal-

cium silicate and calcite are usually present in the sam-

ples. Interestingly, uranium-thorium compounds are pre-

sent in multiple crystalline phases and systems.

 The uranothorianite sample contains thorianite as

ThO2 in cubic system.

 The R01M37 sample contains 1) Th0.5U0.5O2.04 in

cubic system, which can be considered as a mixture

of UO2 and ThO2, and 2) Fe2UO6 in hexagonal sys-

tem, which can be considered as a mixture of Fe2O3

and UO3.

 The R01M47 sample contains only UO3 in mono-

clinic system.

 The R02M47 sample contains 1) UO3 in monoclinic

system and 2) U2O5 in orthorhombic system.

 The R03M47 sample contains 1) UO3 in monoclinic

system and 2) U2P2O10 as a complex system, which

can be considered as a mixture of U2O5 and P2O5.

 The R01M52 sample contains 1) thorianite as ThO2

in cubic system, 2) uranium oxide as U3O7 in tetra-

gonal system and 3) uraninite as UO2 in orthorhom-

bic system.

 The R02M52 sample contains 1) thorianite as ThO2

in cubic system and 2) uranium oxide as UO3.

The presence of both thorium and uranium oxides is

expected. According to Frondel [21], uraninite is com-

monly present in pegmatites. Moreau explained that pre-

viously, thorium and uranium were rather concentrated in

the granitic and charnocktic zones. Precambrian granite-

F. E. SAHOA ET AL.

350

(a)

(b)

Figure 2. Examples of the graphical output from the Bruker Eva software. (a) Pure XRD spectrum from uranothorianite

minerals; (b) Complex XRD spectrum from R01M37 sample.

Table 3. Crystalline systems of uranium-thorium minerals identified in the studied samples by XRD analysis.

Sample code Identified compounds Chemical formula Crystalline systems

Uranothorianite Thorianite ThO2 cubic

R01M37 Thorium uranium oxide Th0.5U0.5O2.04 cubic

Iron uranium o xide Fe2UO6 hexagonal

R01M47 Uranium oxide UO3 monoclinic

R02M47 Uranium oxide UO3 monoclinic

Uranium oxide U2O5 orthorombic

R03M47 Uranium oxide UO3 monoclinic

Uranium oxide phosphate U2P2O10 complex

R01M52 Thorianite ThO2 cubic

Uranium oxide U3O7 tetragonal

Uraninite UO2 orthorombic

R02M52 Thorianite ThO2 cubic

Uranium oxide UO3 uncertain

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F. E. SAHOA ET AL. 351

zation is followed by the formation of the main pegma-

titic areas in Madagascar with thorium-uranium niobo-

tantalates, uraninite and beryl. In the same time, a large

uranium and thorium provinces with uranothorianite de-

posits appear within the calcomagnesian series of the

southern area of Madagascar [22]. Thorianite is an iso-

metric ThO2 crystal (found in R01M37, R01M52,

R02M52 and uranothorianite samples) in the hexoctahe-

dral class







sometimes modified to {111} or {113} [21]. The uran-

oth orianite diffractogram (Figure 2(a)) particularly shows

the dominance of ThO2 in uranothorianite sample. This

demonstrates that thorianite is the main component of the

Tranomaro uranothorianite mineral, and that uranium

may be contained in other co-existing phases.

If thorium oxide exists only as cubic system, by con-

trast, the uranium oxides are present in several and more

complex crystalline systems, generally called uraninite.

In fact, various definitions of uraninite are proposed by

different authors. According to Rogers [23], uraninite is

described as the crystalline mineral with higher specific

gravity and lower water content and pitchblende for the

massive mineraloid with lower specific gravity and

higher water content. Following the Dana classification,

uraninite is de fined as an oxide for m of uranium and tho-

rium and has an isometric crystalline structure. Accord-

ing to Wasserstein [24,25] who proposed a classification

based on the radiogenic lead content and the redox con-

ditions, α-uraninite corresponds to UO2 (found in

R01M37 and R01M52 samples), β-uraninite to U3O7

(found in R01M52 sample) and τ-uraninite to U4O9. It is

also known that UO2 is transformed into U3O8 and UO3

(found in R01M37, R01M47, R02M47, R03M47 and

R02M52 samples) under oxidizing conditions. Ebelmen

reported also that U2O5 (found in R02M47 and R03M47

samples) is an essential constituent of uraninite [26].

In the Tranomaro area, the presence of the uraninite is

closely linked to that of the thorianite. This is because

uranium and thorium are isometric and both present in

minerals at different concentrations (88.15% of uranium

in uraninite and 70% of thorium in thorianite). Some

authors have proposed a solid solution model to explain

the possibility of uranium-thorium exchange between the

uraninite and thorianite systems, but their oxidation be-

haviours differ from each another [27-29]. On one hand,

this results in a simple crystal system for thorianite: it

only exists in a cubic system. On the other hand, the ura-

nium component of the Tranomaro uranothorianite sam-

ples are oxide based and is a mixture of complex oxida-

tions states and crystalline systems. The finding is in

agreement with published results, showing that uraninite

is a complex uranium oxide compounds [24,25,27].

5. Conclusion

Due to the presence of radioactive minerals, populations

living in the Tranomaro area, south-east Madagascar may

be exposed to increased environmental radiation. The

study of seven mineral samples collected from the site,

using X-ray diffraction, confirms the existence of ura-

nium-thorium bearing minerals in the Tranomaro deposit,

and moreover, gives an insight on the crystalline struc-

ture and combination of the uranium-thorium minerals.

The uranium and thorium are present in a complex mul-

tiple mineral phases. If the thorium oxide is identified in

a simple crystalline system, on the opposite, the uranium

oxides, classified as uraninite, are present in more than

eight phases and show multiple oxidation states and

crystalline systems, including α-uraninites and β-urani-

nite. As each crystalline system is more or less subject to

dissolution or dispersion in the environment, further in-

vestigation must be done to investigate the details of their

mobilization and transport in the environment, in terms

of crystallographic phases.

6. Acknowledgements

This paper was completed within the framework of a

cooperation between the Institut National des Sciences et

Techniques Nucléaires (Madagascar-INSTN) and the

Institute of Nuclear Waste and Disposal (INE). The

German Academic Exchange Service (Deutscher Akade-

mischer Austausch Dienst: DAAD) financed a fellowship.

The Pan African Mining-Madagascar (PAM) facilitated

the access to the Tranomaro sites. Technical supports

fr om Madagascar-INSTN and INE are gratefully acknow-

ledged.

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