Journal of Minerals and Materials Characterization and Engineering, 2013, 1, 347-352
Published Online November 2013 (ht t p:/ / )
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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: *
Received September 20, 2013; revised October 21, 2013; accepted November 5, 2013
Copyright © 2013 Frank Elliot Sahoa et al. This is an open access article distributed under the Creative Commons Attribution Li-
cense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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.
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]:
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.
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
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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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-
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-
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