This paper reports preliminary results of 57Fe Mössbauer spectroscopy and X-ray diffraction (XRD) studies
of metamict davidite samples (La,Ce,Ca,Th)(Y,U,Fe)(Ti,Fe,Mn)20(O,OH)38 after high temperature annealing in an argon atmosphere. The Mössbauer spectra show a gradual decrease of quadrupole splitting and line width values of an Fe3+ doublet with increasing annealing temperature. Rather unexpected feature of these spectra for an Fe2+ doublet is a considerable increase of the line width with progressive crystallinity and a simultaneous decrease of both quadrupole splitting and isomer shift values. Changes of the hyperfine parameters as a function of the annealing temperature appeared as sensitive indicators of the thermal recrystallization process of metamict davidite similar to metamict silicates

Frąckowiak, Janusz

Galuskin, Evgeny

Malczewski, Dariusz

Nukleonika

English

Repozytorium Uniwersytetu Śląskiego RE-BUŚ

             Title: 57Fe Mossbauer spectroscopy of annealed metamict davidite  Author: Dariusz Malczewski, Janusz Frąckowiak, Eugeniusz V. Gałuskin  Citation style: Malczewski Dariusz, Frąckowiak Janusz, Gałuskin Eugeniusz V. (2007). 57Fe Mossbauer spectroscopy of annealed metamict davidite. "Nukleonika" (Vol. 52, supl. 1 (2007), s. 81-84).  NUKLEONIKA 2007;52(Supplement 1):S81−S84 PROCEEDINGSIntroductionDavidite (multiple oxides) −(La,Ce,Ca,Th)(Y,U,Fe)(Ti,Fe,Mn)20(O,OH)38 belongsto the group of metamict minerals [3, 7]. Metamictminerals are a class of natural amorphous materialswhich were initially crystalline but self-radiation damagemainly from alpha decays of 238U, 232Th, 235U seriesinside the structure can produce partially or fully amor-phization (metamictization) of these minerals [1, 2].Investigations of these materials are important becauseminerals containing large concentrations of uraniumand thorium can serve as natural analogues for radiationeffects in high-level nuclear waste form (HLW) [6, 8].Davidite is isostructural with the crichtonite – a groupof minerals having a structure based on a close-packedanion framework with the mixed stacking sequenceABCBCACABA with rare earth elements REE M(0),occupying a site in the anion framework and five cationsites M(1)-M(5) [3]. According to this refinement, U4+ions do not substitute for REE in M(0), but occupiesan octahedral site M(1) along with Y and small REE(Er, Yb). The remaining small metal ions, mainly Ti4+and Fe3+ occupy a tetrahedral site M(2) and three otheroctahedral sites M(3), M(4) and M(5) [3]. Investigateddavidite samples, massive grayish black with vitreous-metallic lustre, were found in granite pegmatite from57Fe Mössbauer spectroscopyof annealed metamict daviditeDariusz Malczewski,Janusz F. Frąckowiak,Evgeny V. GaluskinD. Malczewski, E. V. GaluskinFaculty of Earth Sciences,University of Silesia,60 Będzińska Str., 41-200 Sosnowiec, Poland,Tel.: +48 32 3689370, Fax: +48 32 2915865,E-mail: malczews@us.edu.plJ. F. FrąckowiakFaculty of Computer and Materials Science,University of Silesia,12 Bankowa Str., 40-007 Katowice, PolandReceived: 20 June 2006Accepted: 3 November 2006Abstract. This paper reports preliminary results of 57Fe Mössbauer spectroscopy and X-ray diffraction (XRD) studiesof metamict davidite samples (La,Ce,Ca,Th)(Y,U,Fe)(Ti,Fe,Mn)20(O,OH)38 after high temperature annealing in anargon atmosphere. The Mössbauer spectra show a gradual decrease of quadrupole splitting and line width values ofan Fe3+ doublet with increasing annealing temperature. Rather unexpected feature of these spectra for an Fe2+ doubletis a considerable increase of the line width with progressive crystallinity and a simultaneous decrease of both quadrupolesplitting and isomer shift values. Changes of the hyperfine parameters as a function of the annealing temperatureappeared as sensitive indicators of the thermal recrystallization process of metamict davidite similar to metamict silicates.Key words: davidite • metamict minerals • recrystallization • Mössbauer spectroscopyS82 D. Malczewski, J. F. Frąckowiak, E. V. GaluskinPermian granitoid massif Bektau-Ata, Kazakhstan.Basic characteristics of the davidite sample are givenin Table 1.ExperimentalThe concentrations of 232Th, 238U and 235U were calcu-lated based on gamma-ray activities of 228Ac, (232Th),226Ra, 214Pb, 214Bi (238U) and 235U. Gamma-ray spectrumwas collected using a characterized HPGe detector(30% efficiency) and analyzed using the Genie 2000v.3 software package.Untreated sample and samples after annealing inargon at 673, 873, 1073 and 1273 K were powdered andprepared in the shape of a thin disc absorber. TheMössbauer transmission spectra were recorded at roomtemperature using a constant acceleration spectro-meter, a multichannel analyzer with 1024 channels anda linear arrangement of 57Co/Rh source (= 50 mCi),absorber and detector. The spectrometer velocity wascalibrated with a high purity α-Fe foil. X-ray powderdiffraction (XRD) patterns were obtained usinga SIEMENS D5000 diffractometer in the Θ-Θ systemand CuKα radiation in the scan mode with step size0.02° and step time 5 s at each point.Results and discussionThe Mössbauer spectra of the davidite samples afterone-hour annealing in argon at 673, 873, 1073, 1273 K,and untreated sample with the corresponding X-raypowder diffraction patterns are shown in Fig. 1. Thehyperfine parameters derived from fitting procedureare given in Table 2.The Mössbauer spectrum of davidite (Fig. 1,Table 2) can be fitted to two quadrupole doubletsassigned to Fe2+ (doublet no. 1) and Fe3+ (doublet no. 2)in octahedral positions. From the crystalline structureof davidite, results that these doublets correspond toFe2+ and Fe3+ in octahedral M(3) site. As can be seenin Fig. 1, the corresponding XRD pattern shows a highdegree of amorphization (metamictization) of thismineral according to high-absorbed α-dose in thestructure (Table 1).For the Fe2+ component, Mössbauer spectra of theannealed davidite samples at 673, 873 and 1073 K showdecreasing quadrupole splitting (QS) of this doublettogether with increasing half-width (Γ) and increasinglypoor determination of the doublet’s contribution (Fig. 2,Table 2). This ferrous component completely vanishesafter heating at 1273 K. The results are the some kindof unexpected because with progressive recrystallizationthe spectral lines should become narrow as a metamictstructure is reconstituted [5]. This, however, was notthe case (Fig. 2). On the other hand, similar behaviorof an Fe2+ component was presented in an extensivestudy devoted to alpha-decay damage in titanite [4]. Thisseems similar to metamict titanite because for metamictdavidite the damage process is accompanied by thereduction of Fe3+ to Fe2+, which resides in the aperiodicdomains. These damage regions can incorporate muchmore hydrogen (as OH) than is contained in crystallinestate, presumably as a result of post damage diffusionof H into the structure. Conversely, recrystallizationprocess is accompanied by intense dehydroxylation(as H2O) with simultaneous oxidation Fe2+ to Fe3+.Table 1. Basic characteristics of the davidite sample fromBektau-Ata, KazakhstanAge Fe U Th Calculated doseb(Ma) (wt.%) (wt.%) (wt.%) (α-decay/mg)270(20)a 19.0 1.47 0.31 1.47(17) × 1016a Permian granitoid massif.b  Dose has been calculated from the equation: D = 8 × N238(etλ238– 1) + 7 × N235(etλ235 – 1) + 6 × N232(etλ232 – 1), where N238, N235 andN232 are the present number of atoms of 238U, 235U and 232Th permilligram, λ238, λ235 and λ232 are the decay constants of 238U, 235Uand 232Th, t is the geologic age.Table 2. List of 57Fe hyperfine fit parameters of all spectra (Fig. 1). Isomer shift values IS are given relative to the α-Festandard at room temperature. The uncertainties are given from fitting procedure using the MOS MOD and MEP softwareT Doublet χ2 IS QS Γ Assignment A**(K) no. (mm/s) (mm/s) (mm/s) (CN)* (%)Untreated 1.0sample1 0.94(2) 2.23(1) 0.22(2) Fe2+ (6)   13.1(18)2 0.366(3) 0.766(6) 0.279(4) Fe3+ (6)   86.9(16)  673 1.01 1.01(4) 1.78(6) 0.36(5) Fe2+ (6)   18.5(42)2 0.346(5) 0.712(9) 0.270(6) Fe3+ (6)   81.5(29)  873 1.11 0.84(6) 1.30(8) 0.49(9) Fe2+ (≥6)   18.9(73)2 0.365(4) 0.691(9) 0.239(6) Fe3+ (6)   81.1(46)1073 1.31 0.81(7) 1.02(8) 0.46(9) Fe2+ (≥6)   18.8(81)2 0.375(7) 0.688(4) 0.223(5) Fe3+ (6)   81.2(57)1273 1.12 0.374(2) 0.644(6) 0.232(4) Fe3+ (6) 100   * Coordination number.   ** Relative area.S8357Fe Mössbauer spectroscopy of annealed metamict daviditeFig. 1. Room-temperature Mössbauer spectra and corresponding X-ray diffraction patterns of davidite samples after annealingat given temperatures.S84 D. Malczewski, J. F. Frąckowiak, E. V. GaluskinThe quadrupole splitting (Fig. 2, Fe2+) and isomer shiftvalues (Table 2, doublet no. 2) were observed todecrease with progressive annealing most probablyindicating decreasing coordination number of theseFe2+ positions.As may be expected, the changes of Γ and QS valuesof the predominant Fe3+ doublet demonstrate a fallingtendency. As can be seen in Figs. 1 and 2, with progressiveannealing the half-width of this doublet narrowedtogether with decreasing quadrupole splitting. In oppositeto the Fe2+ component isomer shift of the Fe3+ doubletvaries in relatively narrow range from 0.35 mm⋅s−1(673 K) to 0.37 mm⋅s−1 (1273 K). These results indicatethat Fe3+ octahedron become more regular as thecrystalline structure of davidite is reconstituted. Fittingthe spectra with one Fe2+ and one Fe3+ doublet indicateda relatively constant Fe2+ to Fe3+ ratio, i.e., 0.15(2),0.22(5), 0.23(9) and 0.23(9) from the untreated a sampleto sample heated at 1073 K, respectively.ConclusionsThe annealing behavior of metamict davidite is similarto that observed in metamict titanite. Crystalline daviditeusually has iron in the trivalent state. However, theMössbauer spectrum for untreated davidite shows thatFe is present in both Fe2+ and Fe3+ states with relativecontribution of 0.13 and 0.87, respectively. The Fe2+doublet vanishes completely after annealing at 1273 Kin argon. It seems reasonable to conclude that theformation of significant amounts of Fe2+ in metamictdavidite is associated with self-radiation damage fromalpha decays of 238U, 232Th, 235U series inside the structure.Changes of the hyperfine parameters vs. annealingtemperature of the Fe3+ component can serve as sensi-tive indicators of the metamictization stage of daviditespecimens.Acknowledgment  This work has been supported bythe State Committee for Scientific Research, Poland, grantNo. 2P04D06229.References  1. Ewing RC (1994) The metamict state: 1993 − thecentennial. Nucl Instrum Methods Phys Res B 91:22−29  2. Ewing RC, Haaker RF (1980) The metamict state:implication for radiation damage in crystalline wasteform. Nucl Chem Waste Management 1:51−57  3. Gatehouse BM, Grey IE, Kelly PR (1979) The crystalstructure of davidite. Am Mineral 64:1010−1017  4. Hawthorne FC, Groat LA, Raudsepp M et al. (1991)Alpha-decay damage in titanite. Am Mineral 76:370−396  5. Malczewski D, Janeczek J (2002) Activation energy ofannealed metamict gadolinite from 57Fe Mössbauerspectroscopy. Phys Chem Miner 29:226−232  6. Meldrum A, Boatner LA, Weber WJ, Ewing RC (1998)Radiation damage in zircon and monazite. GeochimCosmochim Acta 62:2509−2520  7. Pabst A (1961) X-ray crystallography of davidite. AmMineral 46:700−718  8. Weber WJ, Ewing RC, Catlow CRA et al. (1998) Radiationeffects in crystalline ceramics for the immobilization ofhigh-level nuclear waste and plutonium. J Mater Res13:1434−1484Fig. 2. Variations of the half-widths and quadrupole splittingsas a function of the annealing temperature (black spheres)for Fe2+ doublet (top) and Fe3+ doublet (bottom). Whitecircles − projection on the T-Γ plane; black and white circles− projection on the T-QS plane; black circles − projection onthe QS-Γ plane. The uncertainties are within the black spheresizes.

57Fe Mossbauer spectroscopy of annealed metamict davidite

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57Fe Mossbauer spectroscopy of annealed metamict davidite

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