Proton non-Rutherford backscattering study of oxidation kineticsin Cu and Fe sulphides

Massimo Chiari a,b,*, Lorenzo Giuntini c, Giovanni Pratesi d, Alba P. Santo e

a Dipartimento di Fisica, Universit�a di Padova, via Marzolo 8, 35131 Padova, Italyb INFN - Laboratori Nazionali di Legnaro, via Romea 4, 35020 Legnaro (Padova), Italy

c INFN - Firenze, Largo Fermi 2, 50125 Firenze, Italyd Museo di Mineralogia e Litologia, Universit�a di Firenze, via La Pira 4, 50121 Firenze, Italy

e Dipartimento di Scienze della Terra, Universit�a di Firenze, via La Pira 4, 50121 Firenze, Italy

Abstract

Non-Rutherford backscattering spectrometry (NBS) with 2.4 MeV protons was performed for depth pro®ling of ox-

ygen in three species of copper and iron sulphides ± pyrite, chalcopyrite and bornite ± on both altered and fresh sur-

faces. The tarnished surfaces were obtained by bathing samples in H2O2 (35% vol.) for 100 and 1000 s. The spectra

collected were compared to simulations to extract quantitative data on oxygen depth distributions for the di�erent bath-

ing times. The measurements have shown that the kinetics of oxidation has completely di�erent patterns in the three

investigated minerals. Ó 1998 Elsevier Science B.V.

PACS: 61.85

Keywords: Non-Rutherford backscattering spectroscopy; Light element detection; Sulphides; Surface alterations;

Oxidation kinetics

1. Introduction

Among the numerous sulphides of Fe and Cu,pyrite (FeS2), chalcopyrite (CuFeS2) and bornite(Cu5FeS4) represent the most widespread minerals.Pyrite is commonly employed for the extraction ofsulphuric acid; chalcopyrite and bornite are used,since the Bronze Age in the Etruscan civilisation,for the production of copper.

The investigation of these minerals with thetechniques commonly available in a geological lab-oratory, such as electron microprobe or X-raypowder di�raction, is ine�ective for the study ofsurface alterations. These alterations, however,represent an attractive ®eld of research due to theirimplications in minerogenesis, environmentalproblems, industrial applications and conservationof minerals in musea [1]. For the investigation ofthe elemental composition and depth pro®les oflight elements, a technique which seems to be suit-able is Non-Rutherford Backscattering Spectrom-etry (NBS), scarcely used so far for mineralogical

Nuclear Instruments and Methods in Physics Research B 139 (1998) 202±207

* Corresponding author. Fax: ++39 49 8277102; tel.: ++39

8277246; e-mail: chiari@vaxfpd.pd.infn.it.

0168-583X/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved.

PII S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 0 3 5 - 4

studies. In fact, taking advantage of the largeprobing depth of 2±3 MeV protons and of thecross section enhancement with respect to theRutherford value in elastical scattering from lightelements, it is possible to investigate the oxygenconcentration in a bulk of heavier elements up toseveral microns in depth. Thus, in this work we in-vestigated the potential of NBS in characterisingthe oxidation kinetics in pyrite, chalcopyrite andbornite.

2. Experimental procedure

Natural alteration processes develop over along time scale; therefore to characterise the timeevolution of natural oxidation one needs to accel-erate the process in the laboratory. To this pur-pose, as a ®rst attempt, we exposed freshsulphide samples to an O2 atmosphere (at 2 ´ 105

Pa) for 24 h, but no appreciable oxidation was de-tected via NBS (performed in the set-up described

Fig. 1. Pyrite: experimental backscattering spectra and simula-

tions. (a) Fresh sample; (b) 100 s treated; (c) 1000 s treated.

Fig. 2. Chalcopyrite: experimental backscattering spectra and

simulations. (a) Fresh sample; (b) 100 s treated; (c) 1000 s treat-

ed.

M. Chiari et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 202±207 203

below). Instead, bathing in H2O2 (35% vol.)proved to be very e�ective to produce fast altera-tions. For each mineral we have then studied threepolished samples: (1) a fresh surface; (2) a surfacebathed for 100 s; (3) a surface bathed for 1000 s.The selection of the samples was performed by re-¯ected light microscope and electron microprobeanalysis, to assure a good chemical and mineralog-ical homogeneity.

The measurements were performed using themulti-angle RBS facility installed at the KN-3000Van de Graa� accelerator in Florence [2]. TheLN2-cooled silicon PIN junction detector at 170°was used in this case (IBM geometry [3], subtendedsolid angle 0.376 msr). The energy resolution was3.5 keV FWHM (see again [2]). The beam currentwas about 10 nA and the measurements lasted un-til a total beam dose of about 60 lC was collected.The sample was tilted at 60° for a better depth res-olution of the near-surface layers. The protonbeam energy was 2.4 MeV. This value was chosento enhance detection sensitivity of surface oxygenin the presence of a large sulphur bulk concentra-tion, keeping in mind the known energy depen-dence of proton scattering cross sections onsulphur [4] and oxygen [5].

3. Results and discussion

The obtained results are shown in Figs. 1±3.The results of simulations obtained with the codeDVBS [6], under the depth distribution hypothesesdiscussed below, are also shown superimposed tothe experimental spectra.

Quantitative data for the oxygen, sulphur, ironand copper depth distributions were extractedfrom the comparison to simulation; in the latter,non-Rutherford cross sections were used for oxy-gen [5] and sulphur [4]. For all three minerals,the samples with fresh surfaces were successfullysimulated merely adopting a homogeneous stoi-chiometric composition. Note the modulation pro-duced by the energy dependence of the S crosssection, which gives rise to the decrease in scatter-ing yield just below 2 MeV in the spectra.

Concerning the altered samples, it may be ob-served that pyrite and chalcopyrite are strongly af-

fected by the adopted treatments in H2O2, while inthe case of bornite the e�ect, although visible, ismuch smaller. For the ®rst two minerals, theagreement between experiment and simulation isvery sensitive to the di�erent assumptions of thelatter, so that a good modelling of the oxidationprocess can be achieved. On the contrary, in thecase of bornite the depth interested by alterationis only few hundreds of Angstrom; the excellentenergy (therefore depth) resolution in our set-upmakes it still possible to detect an oxidation e�ect

Fig. 3. Bornite: experimental backscattering spectra and simu-

lations. (a) Fresh sample; (b) 100 s treated; (c) 1000 s treated.

204 M. Chiari et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 202±207

and even to ®nd a layer structure model whosesimulated spectrum reasonably ®ts the experimen-tal one, but the information remains basicallyqualitative and a detailed model must rely on inv-estigations performed by di�erent techniques(Auger Electron Spectroscopy and X-ray Photo-electron Spectroscopy [7,8]).

An unexpected feature observed in some sam-ples, is the presence of an even remarkable carboncontent. While traces of surface carbon can be ac-counted for by residual e�ects of beam-inducedbuild-up on the target, this is not certainly the caseobserved e.g. in pyrite and chalcopyrite 100 s treat-ed. In these cases the thickness involved by the ef-fect is about 2 lm. Further investigations arerequired to ascertain whether this e�ect is to be as-cribed to physical or instrumental origin.

The main results concerning the altered miner-als are described below; for pyrite and chalcopy-

rite, the deduced elemental depth pro®les arereported, respectively, in Figs. 4 and 5.

Pyrite (FeS2): In both samples, the experimen-tal data are successfully reproduced with a relativeFe:S ratio equal to 1:2 (as in the bulk), with the ox-ygen concentration progressively decreasing fromthe surface to the inner layers. Oxygen di�usionis markedly stronger after the 1000 s treatmentthan after the 100 s one.

Chalcopyrite (CuFeS2): Again, in both sam-ples, the simulation model assumes a relative Fe:-Cu:S ratio equal to 1:1:2 (as in the bulk), withthe oxygen concentration decreasing from the sur-face to the inner layers. Note that, in this case, the®rst few tenths of micron of the samples can bewell reproduced assuming the presence of Fe andCu sulphates (FeSO4 and CuSO4), a phenomenonhypothesised but rarely observed in chalcopyrite[9]. Oxidation is faster than in pyrite, with oxygen

Fig. 4. Element depth pro®les in pyrite as obtained by the simulation model. (a) 100 s treated ± data normalised to the 100% excluding

the C content in each layer; (b) 1000 s treated.

M. Chiari et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 202±207 205

penetrating up to several microns even after the100 s treatment.

Bornite (Cu5FeS4): A much reduced oxidationis observed with respect to the other two minerals;however, it seems to be relatively fast, soon reach-ing a saturation level, as the spectra correspondingto 100 and 1000 s (see Fig. 3(b) and (c)) are repro-duced with the same composition model in thesimulation code. As anticipated, the results con-cerning this mineral are only semi-quantitative. Amodel to reproduce the alteration in bornite is sug-gested by results obtained with AES and XPS [7,8]:roughly, a ®rst layer of FeOOH + FeSO4 followedby a second one of CuSO4, produce an oxidisedover all thickness of about 200 �A; then, a �100�A layer of Cu sulphides overlays the bulk. Evenfor bornite, the formation of sulphates has beenhypothesised but rarely observed [10].

After two years of storage (under standard tem-perature and humidity conditions, similar to thosefound in museum environments) we repeated the

analysis on the untreated samples of all three min-erals. The results are reported in Fig. 6.

All the obtained scattering spectra con®rm thebehaviour described above: in pyrite the oxygenfraction grows to 57% in the ®rst �800 �A, whilein chalcopyrite a 66% fraction of oxygen can beobserved in the ®rst �150 �A; bornite does not ex-hibit detectable oxidation. Note that even thoughthe 100 s bathing time in H2O2 determines astronger alteration than that produced naturally,the two e�ects are comparable, thus con®rmingthe validity of the procedure followed to simulatea natural alteration in a period of the order ofsome years.

In conclusion, NBS analysis with 2±3 MeV pro-tons has proved to be a very useful tool to studythe oxygen depth distribution in altered sulphides.Our measurements have shown that the kinetics ofoxidation has indeed completely di�erent patternsin the three investigated minerals [11]. As pointedout by the already quoted XPS measurements

Fig. 5. Element depth pro®les in chalcopyrite as obtained by the simulation model. (a) 100 s treated ± data normalised to the 100%

excluding the C content in each layer; (b) 1000 s treated.

206 M. Chiari et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 202±207

and by our modelling of altered bornite, ``second-ary'' Cu sulphides form following Fe depletion inthe bulk. These sulphides seem to hinder the oxi-dation process; thus the di�erent quantity of cop-per in the three minerals may be responsible forthe observed kinetics.

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Fig. 6. Backscattering spectra collected on untreated samples

after two years of storage: (a) pyrite ± the nitrogen peak corre-

sponds to a content of some percent in the altered layer; (b)

chalcopyrite; (c) bornite ± the expected position of the oxygen

peak is also shown.

M. Chiari et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 202±207 207