Calas, G.

Clozel, B.

Dran, J.C.

Hervé, A.

Muller, Jean-Pierre

Horizon / Pleins textes

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259 GEOCHEMISTRY OF THE EARTH'S SURFACE AND OF MINERAL FORMATION 
2nd ISTERNATIONAL SYMPOSIUM, J u l y ,  2-8, 1990, Aix en Provence, France. 
KAOLINITES AS DOSIMETERS : A NEW POSSIBILITY OF 
TRACING RADIONUCLIDES MIGRATION 
CLOZEL B.*, CALAS G. *, MULLER J.-P. *,**, DRAN J.-C. *** and E R V E  A. **** 
* Laboratoire de Minéralogie-Cristallographie, UA CNRS 09, 
Universités de Pans 6 et 7,75252 Paris Cedex 05, France. 
** Département T.O.A., U.R. IG, ORSTOM, Paris, France. 
*** CSNSM/CNRS, 91406 Orsay, France. 
**** Groupe de Physique des Semiconducteurs, CENGDRF, 85X, 38041 Grenoble, France. 
Natural minerals often exhibit native radiation 
damage which can be investigated by a variety of 
methods : thermoluminescence and Electron 
Paramagnetic Resonance (EPR) for defect centers, 
fission tracks ... In most minerals, radiation damage 
arises from radionuclides trapped during growth and 
provides few informations on the environmental 
geochemistry prevailing after crystal growth. A 
notable exception concerns the surface radiation 
damage observed in some minerals (e.g. coatings on 
quartz and diamond). Clay minerals present over 
other minerals the advantage of a large specific 
surface which allows an efficient interface with 
solutions and make them sensitive to the 
environment, even after crystal growth. The specific 
advantage of kaolinite resides in its presence in a 
wide range of geological environments, including 
low temperature alteration of granites and tuffs, 
surficial weathering and sedimentary rocks. Indeed 
all natural kaolinites show a significant concentration 
of defect centers which correspond either to 
background or anomalous irradiation during or after 
growth. The purpose of this presentation is twofold: 
(i) simulation of natural radiation and thermal 
annealing in order to derive the EPR signal intensity 
vs. dose rate relationship used in a kaolinite-based 
dosimetry; (ii) application in natural systems to the 
migration of short-lived radionuclides, in relation 
with uranium exploration. 
PARAMAGNETIC DEFECT CENTERS 
IN KAOLINITES 
EPR spectra of natural kaolinites exhibit a 
complex signal arising from the presence of different 
defect centers, in addition to signals due to 
paramagnetic impurities (mostly Fe3+). The structure 
of these radiation-induced defect centers as well as 
their thermal stability has been investigated in order 
to understand the effect of extemal radiations on the 
creation of paramagnetic centers and their 
persistance with time. Three centers are clearly 
separated on EPR spectra, when the frequency of 
the spectrometer is correctly selected. Two of them 
are positive holes associated to silicon, the other one 
is a hole associated with aluminum. They have 
distinct thermal stability, according to isochronal 
heating experiments. The first positive hole 
associated to silicon is stable over geological periods 
(106-108 years at low temperatures) and is indeed 
observed in all natural kaolinites. The two other 
centers are observed only in kaolinite samples 
submitted to recent irradiation, owing to their fast 
annealing over geological times and moderate 
temperatures. In uranium deposits, it is thus 
possible to separate the irradiation arising from 
recent remobilization of short lived radionuclides 
from that due to the emplacement of uranium 
mineralization, making this a unique tool for tracing 
the dynamics of radionuclides transfer. 
DOSE RATE ESTIMATION 
We have used four radiation sources : 
calibrated x-rays (Mo-tube), y-rays (60Co source) 
and ion implantation (He+ and Pb++) in order to 
simulate the natural radiation. We have determined 
for each irradiation the kind of paramagnetic centers 
created as well as their total concentration. Various 
kaolinite samples have been chosen, representing 
260 GEOCHEMISTRY OF THE EARTH'S SURFACE AND OF MINERAL FORMATION 
2nd INTERNATIONAL SYMPOSIUM, July, 2-8, 1990, Aix en Provence, France. 
different formation conditions (hydrothermal 
alteration, weathering, sedimentary rocks) : the 
irradiation has been found to decorate pre-exis ting 
diamagnetic centers and the EPR response is thus 
sample-dependent, with some influence of the 
crystallinity and iron content of kaolinite crystals. 
For each sample the relation linking the radiation 
dose and the EPR signal intensity has been derived. 
The relative efficiency of the various sources 
employed has been carefully investigated; for 
instance, a-radiation seems the most efficient. 
Isothermal heating at various temperatures shows 
that paramagnetic defect formation is not a first order 
process. Approximate activation energies have been 
obtained. 
USE OF KAOLINITES AS "IN SITU" 
DO SIMETERS 
The knowledge of the parameters governing 
the formation and the stability of the radiation centers 
in kaolinites allows to use this mineral as a 
dosimeter. In the finely divided clay minerals, the 
surface reactions with percolating solutions are 
greatly enhanced : the radiation damage arises from 
external irradiations. The accumulated dose rate is 
calculated from the dosimetry experiments by 
extrapolating to zero-EPR signal intensity. It may be 
further converted to the total quantity of the 
suspected radionuclides if some constraints are put 
on the age of the irradiation. We have tested this 
potential use on various alteration systems 
associated with uranium concentrations as well as in 
environments devoid of major radionuclide 
transfers. Kaolinites sampled in two profiles from a 
brazilian laterite developped on U-mineralized 
albitite and on associated non-mineralized gneiss 
show a relationship between defect center and total 
uranium content. Another example has been chosen 
in a tropical soil from Cameroon and has shown that 
the accumulated dose is larger than the present 
uranium concentration by 2 to 3 orders of 
magnitude, indicating a non-equilibrium U-decay 
system during all the period of soil differentiation. 
Kaolinites associated to uranium concen- 
trations allow to trace the path of past transfer of 
shortlived radionuclides and reveal the dynamics of 
the system during alteration. In both cases the 
informations are complementary to the present day 
geochemical analyses which are only sensitive to the 
parts of the rock in which the radionuclides have 
been "blocked". 


Geochemistry of the earth's surface and of mineral formation

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