MÖSSBAUER SPECTROSCOPY OF IRON COMPOUNDS PRESENT IN THE PROFILE OF AN URANIUM DEPOSIT

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MÖSSBAUER SPECTROSCOPY OF IRON

COMPOUNDS PRESENT IN THE PROFILE OF AN URANIUM DEPOSIT

C. Saragovi-Badler, F. Labenski, E. Frank

To cite this version:

C. Saragovi-Badler, F. Labenski, E. Frank. MÖSSBAUER SPECTROSCOPY OF IRON COM- POUNDS PRESENT IN THE PROFILE OF AN URANIUM DEPOSIT. Journal de Physique Collo- ques, 1974, 35 (C6), pp.C6-563-C6-568. �10.1051/jphyscol:19746121�. �jpa-00215734�

JOURNAL DE PHYSIQUE Colloque C6, supplhent au no 12, Tome 35, Dgcembre 1974, page C6-563

M~SSBAUER SPECTROSCOPY OF IRON COMPOUNDS PRESENT IN THE PROFILE OF AN URANIUM DEPOSIT

C. SARAGOVI-BADLER and F. LABENSKI

Departamento de Radiaciones At6micas y Moleculares, Area de Investigacihn Desarrollo y Servicios, Comisi6n Nacional de Energia Atbmica, Bs. As., Argentina

E. FRANK (*)

Departamento de Radiaciones At6micas y Moleculares, Area de Investigaci6n Desarrollo y Servicios, Comisi6n Nacional de Energia Atbmica, Bs. As., Argentina

and

Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Argentina

R6sum6. - Plusieurs Cchantillons prelevks dans le profil d'un gisement uranifkre dp district de la Sierra Pintada (Mendoza, Argentine) ont BtC BtudiCs par spectroscopie Mossbauer. La prksence de spectres hyperfins magnetiques a certaines profondeurs qui sont plus riches en uranium, confirme le mCcanisme de transport d'uranium et la rCduction bactkrienne suggCrks prkckdemment.

Abstract. - Several samples taken in the profile of an uranium deposit of the Sierra Pintada district (Mendoza, Argentina) were studied by means of Mossbauer spectroscopy. The presence of magnetic hyperfine spectra at certain depths, which are richer in uranium, confirm previously suggested uranium transport mechanism and bacterial reduction.

1. Introduction. - One of the most striking pro- blems in modern life is to determine the energy resources which are available. This is a field which requires the joint effort of several branches of science.

It is our purpose to show that Mossbauer spectroscopy can also provide useful information in this field, even if its application is not always straightforward. In this paper we report Mossbauer results of iron compounds present in the profile of an uranium deposit and attempt an explanation on its origin. The present work arose in a multidisciplinary effort to help to understand the genesis of some type of uranium deposits, and, there- fore, the possible extension of the mineral bed. We have studied several samples, taken at different depths in the Sierra Pintada district (Mendoza, Argentina).

Numerous field observations which can be found in the literature [I] have shown that highest concentra- tions of uranium ores (pitchblende) on sandstones are

(*) Member of staff Consejo Nacional de Investigaciones Cientificas y Tkcnicas, Argentina.

accompanied by strong reddish, brownish or violet pigmentations due to iron oxides or hydroxides or their derivatives. The study of those ores has shown that most of them were originally associated with simple mineralogical conformations and that the complexity was introduced on later stages of develop- ment. The original uranium bearing solutions were subject to oxidations and/or reductions, depending on local conditions. Therefore any ion present in the solutions may act as a probe and provide information on what has happened to the uranium ions. The fre- quency of iron in the earth mantle has made it a most appropriate element for such studies, and therefore the Mossbauer effect is an obvious choice as a spectrosco- pic tool.

To analyze the origin of uranium deposits it is convenient to study : i) the origin of uranium ; ii) how it was transported from one site to another, and iii) concentration phenomena. Point i) is not relevant for our purposes.

The main uranium bearing primary mineral is ura-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19746121

C6-564 C. SARAGOVI-BADLER, F. LAEIENSKI AND E. FRANK ninite, UO,, a well crystallized material which due to

oxidation is transformed into very soluble compounds which may be transported by means of ground waters to reduction zones where it may precipitate as pitch- blende, a microcrystalline mineral (USiO, . nH,O).

Furthermore it may be added that only oxidation states + 4 and + 6 of uranium are of geochemical interest as the + 5 state is unstable in an aqueous medium :

The oxidation/reduction potential for the half- reaction U(1V) P U(V1) lies in the normal range of values to be found in geologically interesting envi- ronments

Three points may be stressed : a) U4+ forms extre- mely insoluble hydroxides ; b) the equilibrium concen- tration of U4+ in the presence of UO, is negligible ; c) pure water is an extremely poor transport agent for uranium. Solubility increases in the presence of CO, as (U02(C0,),2 H20)' - and (U02(C03)3)4- are formed. Both are extremely soluble and are the trans- port agents towards the reduction zones, where they are precipitated and decomposed. Uranium is repre- cipitated as pitchblende. This is the standard mecha- nism for U 6 + transport which can be altered depending on particular situations.

If the medium has iron ions, several reactions may occur :

and if the medium contains oxygen :

Those process :s are responsible for the pigmentation of sandstones which has been pointed out above.

Reaction (1) is more complete in basic media in agreement with what has been found in superficial waters [2].

In uitro work has shown that iron bearing solutions in contact with quartz particles, at rest and exposed to air, form simultaneously red and ochre oxides. The red ones develop as a coating on the surface of the quartz particles, whereas the ochre precipitate in the bulk of the solution. X-ray studies show that the red pigments are decomposition products of y-Fe203. H,O (lepido- crocite), such as maghemite (y-Fe203), whereas the ochre product is a-Fe,O, . H,O (goethite). Maghemite is unstable and may pass to hematite (a-Fe,O,) [3].

It has been found as well that the y form tends to show up on the surface as there the partial pressure of CO, is lower. Therefore ground-waters on their upper level (smallest CO, pressure) may oxidize Fe2+ to Fe3+.

Simultaneously or afterwards, dissolved uranium reduces and precipitates. Reduction may be due to bacteria which live in poor CO, media. Bacterial mechanism may be direct or indirect. The first one is due to the taking up of oxygen, therefore reducing the medium. Indirect mechanism involves sulphur and hydrogen sulphide equilibria. Direct reduction due to the oxidation of Fez+ to Fe3+ is not very probable as the uranium ions may arrive after precipitation of the solid oxides.

2. Experimental. - The analyzed samples were taken at following depths : 49,63,77,94,102 and 116 m.

Samples are ochre except the 63 and 102 m ones which are reddish. Samples were isolated through manual picking up of the quartz particles (0.1 to 0.4 mm) on which they were deposited as a very thin film. Further separation of the coloured material was not possible.

X-ray diagrams were made but the identification was unsuccessful as the quartz lines mask the diagram. At this stage the Mossbauer spectroscopy seemed appro- priate to give some information [4-81.

Samples were encapsulated between acrilic discs.

An Elron Mossbauer spectrometer in the constant acceleration mode was used together with a 20 mC 57.Co source in Pd matrix and an Ar/C02 filled Reuter Stokes RSG 61 proportional counter and conventional nuclear instrumentation.

Spectra were fitted by means of a least square fitting program [9] which provided as well the plotted output.

Spectra were taken at room and liquid nitrogen temperatures.

Counting statistics was up to lo6 counts/channel, and therefore the statistical error N'/' was lo3 or less and the percentage statistical error 0.1 % at the best, whereas the weakest peak is 0.16 %. Improvement of these figures, which correspond to the worst case, was not possible, as each of those runs required nearly 4 days counting and we were not able to guarantee the stability of our equipment for longer periods. Tempe- rature stability at room temperature was better than 2 OC and better than 0.2 OC at liquid nitrogen tempera- ture over the whole run.

3. Results. - Some typical room temperature spec- tra are shown in figures 1 to 6 and fitted data for both, room and liquid nitrogen temperatures are collected in Tables I and 11. Preliminary chemical analysis data are collected in Table 111. As may be seen the iron content is extremely small. This fact, together with the presence of other heavy elements is responsable for the uncertainty in some of our results.

The first off-hand interpretation of the spectra -namely the existence of a phenomenon of superpara- magnetism- had to be rejected as a major contribution as the whole set of samples has ferrous iron besides the ferric ions and these show up in magnetically split spectra in the Fe3+ rich samples. Therefore an inter- pretation based on the presence of chlorite-like

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FIG. 1. - Mossbauer spectra at room temperature. Sample taken at 49 m.

It .. I * * + *

; - .

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21 91 63

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FIG. 2. - Mossbauer spectra at room temperature. Sample taken at 63 m.

FIG. 3. - Mossbauer spectra at room temperature. Sample taken at 77 m.

C6-566 C. SARAGOVI-BADLER, F. LABENSKI AND E. FRANK

FIG. 4. - Mossbauer spectra at room temperature. Sample taken at 94 m.

1:

, . . + . . . .

. . _{. .}

. - * . .+ *. . : . . . . .- .:_. . ^.f

,... ...:... ... ^?-%

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9

O . .

I

FIG. 5. - Mossbauer spectra at room temperature. Sample taken at 102 m.

. . . . . . . . . .

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y

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:-

. .

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o. .

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4 ; ; : .:, ^{;, , ! ,:, ,, >.o ,, ,,m ,, ,, , , , , 0 2 >?} _{., .*W"} ^{l a -} _",.

FIG. 6. - Mossbauer spectra at room temperature. Sample taken at 116 m.

(($) experimental points ; o fitted points ; full circles, both coincide). (The number 219 previous to the depth in meters refers to the drilling site).

Sample (depth, m )

- 49 63 77 94 1 02 116

Mossbauer Effect Parameters of Iron compounds present in the profile of an uranium deposit from the Sierra Pintada district (Mendoza, Argentina).

Room temperature

Superimposed Hyperfine Spectra

(*) Referred to SNP at room temperature.

(**) ^E = e2 Qq/8.

Mossbauer Effect Parameters of Iron compounds present in the profile of an uranium deposit from the Sierra Pintada district (Mendoza, Argentina).

Liquid Nitrogen temperature Sample

(depth, m)

-

lost lost 0.5 f 0 . 1 0.485 + ^0.08

0.48 + 0.1

Superimposed Hyperfine Spectra

-

lost lost 0.8 f 0.1 0.81 + ^0.06

0.8 + 0.1

(*) Referred to SNP at room temperature.

(**) E = e2 Qq/8.

C6-568 C. SARAGOVI-BADLER, F. LABENSKI AND E. FRANK

TABLE 111 Mossbauer svectra at room temverature whose Chemical data analysis for the studied samples

(preliminary results) Depth

( 4 49 63 77 94 102 116

structures (aluminium silicates. with various degrees of substitution of the cationic sites (Mn2+, Fe2+, Fe3 ⁺ )) [8] in agreement with previous geochemical knowledge seems quite plausible [2].

There are some striking facts in our results : the 49 m species spectra show only 2 peaks which corres- pond to ferrous iron. The ^{X 2} value for room tempera- ture (254 for 250 channels with 7 parameters) indicates a high degree of confidence, whereas the nitrogen temperature spectra is quite poor. This may be due to the existence of several sites for ferric iron which do not show up at room temperature but appear to be more important at low temperature. The 77 m sample spectra show both ferrous and ferric iron but again at low temperature the data show consistently fits which are not straighforward to interpret. The 94 m species are well behaved and show both ferric and ferrous iron. Both the 77 m and the 94 m samples have

Fe3 +/Fez' ratios are consistent with those provided by chemical analysis.

The 116 m sample has spectra which again are not straightforward to analyze. The room temperature data are well fitted with only 2 peaks whereas the low temperature ones require 4 peaks.

Finally we may say that the 63 m and the 102 m samples have spectra which show up hyperfine magnetic interactions. Relevant data do not allow to choose among the several possibilities : maghemite, magnetite or hematite [6].

Even if our Mossbauer data are not conclusive for the identification of the compounds, several conclu- sions may be drawn : a t 63 and 102 m d e ~ t h s the appearance of magnetic hyperfine spectra suggests the existence of a vein where ground-waters have reduced U(V1) to U(1V) and ferrous iron has been oxidized to ferric iron and magnetic compounds were formed as described above. This is in agreement with the relative concentrations of both iron species as found by che- mical analysis and with the morphological description of the deposit [2]. These veins are the richest in uranium content and therefore our results support. previous hypothesis about the uranium transport and oxida- tion/reduction mechanism as explained above.

Acknowledgments. - We would like to thank Dr. H. Nicolli of the Comisi6n Nacional de Energia At6mica who suggested the subject, provided the samples and supplied the chemical analysis data prior to publication.

References

[I] RAYLEIGH, L., Proc. R. SOC. l86A (1946) 41 1.

[2] NICOLLI, H., Proc. Vth Argentine GeologicaI Congress (Cbrdoba) (1973) (in press).

[3] HOFER, L. J. E. and WELLER, S., Science (1947) 470.

[4] HERZENBERG, C. L., Miissbauer Effect Methodology, I. Gru- verman, ed. (Plenum Press) Vol. 5 (1969) 209.

[5] GOLDANSII, V. I. and HERBER, R. H., Chemical Applications of Mossbauer Spectroscopy (Academic Press, N . Y.) 1968.

[6] GREENWOOD, N. N . and GIBB, T. C., Miissbauer Spectroscopy (Chapman and Hall Ltd. London) 1972.

[7] HERZENBERG, C. L. and RILEY, D . L., Development in Applied Spectroscopy, Grove E. L., ed. (Plenum Press, N. Y.) Vol. 7 1970 277.

[8] FRANK, E., LABENSKI, F. and SARAGOVI-BADLER, C., Radio- chem. Radioanal. Letters 14 (1973) 349.

[9] ARGONNE NATIONAL LABORATORY, Appl. Math. Div.

Program Library 2031/PHY 296 with several modi- fications.