57Fe MOSSBAUER SPEGTRA of Perovskite and TITANITE
Conadian Minerologist Yol.22, pp. 689-694(1984)
57FeMOSSBAUER SPEGTRA oF PERoVSKITEAND TITANITE
IAN J. MUIR, JAMES B. METSON AND G. MICHAEL BANCROFT Deryrtmentof Chemistry,University of llestern Ontario, London, OntarioN6A 587
ABSTRAC"I & Povondra 1972,Staatz et al. 1977,Exley 1980) have reported a wide range of substitution for Ca The 57FeMdssbauer spectra of natural perovskite, and and Ti, and little, if any, for Si. Recently, Hol- four natural and two syntheticsamples oftitanite containing labaugh & Rosenberg(1983) have documentedthe small amounts of iron, have been recorded. In the natural substitution of Ti for Si in Sideficient synthetic samples,most of the iron is presentas Fe3+ in the octa- titanite. The spacegroup of perovskiteis Pm3m, and hedral Ti sites,but Fd+ in the Ti site is also presentin the unit cell is comprisedof a large Ca2* site sur- some samples.In synthetic samples,Fe3+ is presentin rounded by eight TiOu octahedra et al. 1966). approximately equal amountsin both the octahedralTi and @eer tetrahedral Si sites. An unusual degreeof line-broadening Perovskitemay also accepta wide range of substi- prob- tution into its structure (Ringwood et al, 1979). is observedin the spectrum of natural titanite, and is 57Fe ably due to structural defects from radiation damageand The technique of Mdssbauerspectroscopy compositional or next-nearest-neighboreffects. has been used widely to determinethe site prefer- ence and oxidation state of iron in minerals @an- Keywords: titanite, perovskite, isomer shift, quadrupole croft 1973,Hawthorne 1983).However, there have splitting, octahedralsite, tetrahedral site. been no Mdssbauerstudies on natural samplesof titanite or perovskite, perhapspartly becauseoftheir SoMMerne low iron content. As pointed out by Higgins & Ribbe (1976), such Mdssbauer studies of titanite are pr6sente 57Fe p€rov- On le spectreMdssbauer du d'une required to confirm the oxidation state and assumed naturelle quatre skite et de six 6chantillonsde titanite, d'ori- preference paper, gine naturelle et deux pr6par6sde mdlangescontenant des of iron for the Ti site. In this we petites quantitds de fer. Dans les 6chantillons naturels, la report the Mdssbauerspectra of one natural perov- plupart du fer sepr6sente comme Fd+ dans les sites octa- skite, four samplesof natural titanite and two of syn- ddriques du Ti; le F*+ y est aussi dans certains cas. Dans thetic titanite. les 6chantillonssynthdtiques, le Fe3+ se trouve en parts approximativement dgalesdans les sites octaddrique du Ti ExpsnINIrNIAL et tetra6drique du Si. Un €largissementanomale des pics du spectrede la titanite naturelle serait d0 d un endomma- The compositionof the perovskite(Nesbitt er a/. gement structural par la radiation et i un effet de compo- 1981)and of the natural samplesof titanite is given sition caus6par l'influence des deuxidmesplus proches voisins. in Tables I and 2. The Sebastapoland Gjerstad sam- ples were kindly suppliedby Dr. F.J. Wicks of the Cfraduit par la Rddaction) Royal Ontario Museum, and all of the sampleswere quantitatively analyzedusing the electronmicroprobe Mots-c6s: titarite, pdrovskite, d6placementisomdre, s€pa- and qualitatively analyzed using SIMS (Secondary ration quadrupole, site octa6drique, site t€traedrique. Ion Mass Spectrometry). The synthetic sampleswere prepared by combin- INTRoDUCTIoN ing molar ratios of TiO2, SiO2and CaCO3with a proportional decreasein the amount of TiO, to Since the proposal was made to use perovskite compensatefor Fe substitution.Iron-metal powder CaTiO3 (Ringwood et al. 1979) and titanire was used, and two synthetic compositions were pre- CaTiSiO, (Nesbitt er a/. 1981, Hayward & pared, one containing2 wt.Voof iron, and one con- Cecchetto 1982) as possible hosts for the immobili- taining 0.8 wt.Vo, 9590 enrichedin 57Fe.HF was zation of nuclear-fuel-reprossssing wastes, there has also addedin an attempt to balancethe chargediffer- been a renewed interest in the crystal chemistry of ence between Tia+ and Fe3*. CO, was released these minerals. The space group of titanite is P21/a, upon heating, and the oxides were then melted in a and the unit cell is comprised of TiO6 octahedra, platinum crucible at l400oC for three hours. The SiO4 tetrahedra and large irregular CaO, polyhedra molten material was poured to form a glass slug, (Speer & Gibbs 197Q. Previous investigators (ternf which wasthen crushedto a fine powder. The pow- 689 690 THE CANADIAN MINERALOGIST
TABLE3. MOSSBAUERPARAT€TERS (nn s.I)i Sptalinn oridatlon q.S. I.S. (perovsklte) (tltanlte) (tltanlte) (tltsnlte) (tltanlte) State si0, 0.80 29.8 30.1 29,5 29.8 llagnet Cove Fe'' 0.34 0.35 0.59 Ca0- 38.9 27.3 27.7 27.2 27.4 perovsklte Tl02 4?.3 32.0 Fe-' 0.92 0.39 0.94 - 1.20 0.96 1.73 1.86 6.50 GJerstad FeD 5.90 10n tE 3.40 0.44 tltanite Na20 0.12 0.29 0.06 Mq0 0.14 0.02 0.19 0.25 oloa Sebastapol * 0.94 0.39 0.84 K)o 0.04 0.01 0.01 tl tanlte F;2' 1.72 l.l8 0.95 Mn-0 0.t6 0.06 0.17 oirs l{b205 5.90 0.18 0.52 Cardlff '--7+ 0.74 0.44 0.74 t 0.77 Total 95.34* 96.27+ 98.I 7t 94.76r 93.21 titinite te- 2.17 1.05 r"3+ 1 0.37 0.75 * * Eear Lake 0.91 ln Plght Z oxldes. Qualitative analysls by SII'1Sshows the titanl te Fe.' I.61 0.81 0.64 prcsenceof Y, Zr, La andCe in slgnificant arcunts (:0.1r). * f Qualltatlve analysls by SI s shos the presenceof Y, zr, La, ce, (l100oC) 0.41 0.81 U, Th, Pb and F jn slgniflcant arcunts (: 0.lg). Gjerstad Fe& 1.13 tl tanite 'I B€ar Lake Fe-' .05 0.30 0.64 ( ceramlc ) ti tanlte 2, Fe Synthetlc F":: 0.84 0.39 0.52 SDeclren' Beor Lake Sebastopol GJemtad Cardlff ilagnet Cove :. (tltanlte) (tltonlte) (t{tanlte) (tltanit€)(perovsklte) tltanite Fe" l.4l 0.28 0.51 57Fe 0.40 Ca 1.01 1.03 0.949 1.07 synrhetic fll :" 't.520.84 0.53 I'ln 0.002 0.005 0.006 0.005 Mn 0.004 tlunlte re- 0.29 0.61 K lrace - trace K 0.001 l{a 0,006 Na 0.019 L'1.04 : olozr * I 5i site. T:03- 0.955 h--tz T:m- Fe ls in octahedral Tl sitea Fe ls ln tetrahedral I Errcn are apPrcximtely 0.05 m s-1. lt 0.918 0.877 0.978 0.885 l1 0.818 AI 0.200 0.035 0.036 0.125 Al 0.01I te 0.054 0.073 0.092 0.013 te 0.127 llg 0.010 0.013 0.004 0.037 llg 0.001 t{b 0.002 L : 0.002 Nb 0.034 T30- 0.998 l.lt II€- si 0.021 The small quadrupole splitting indicates that the '1.04 Tm- 1.02 0.965 t.00 Fd* is presentin a site of closeto cubic symmetry. Ir2- l:r4- 6.365 l:00- The Fe3" can thus be assignedto the octahedralTi site. dered glass was then sintered in a furnace for three daysat I lm'C to producea polycrystallinecer:mic. A similar process was also carried out to make a polycrystalline ceramicfrom a natural titanite. These samples were then characterized by X-ray powder 0.0 diffraction and M6ssbauerspectroscopy. The Mdssbauersamples were preparedby grind- ing the sample to a fine powder and transferring it z. to a plexiglasssample cell to obtain a maximum con- F centration of approximately l0 mg Fe,/cm2.Mdss- E. bauer spectrawere obtained at 10 K and 295 K using o the methods described in Bancroft (1973) and 57Co F Bancroft & Brown (1975). A 25-mC source z. annealed in palladium was used. Several million td 1.0 G, counts were accumulated for each spectrum. The LJ spectra were fitted using a computer program o- adaptedby L.P. Aldridge from that of A.J. Stone (in Bancroft et al. L967), which complies with the criteria found in Bancroft (1973). The isomer shifts are quoted relative to Fe metal.
Rssurrs ANDDrscussroN -2. u 0.0 Z.U The Mdssbauerspectra are shownin Figuresl-9, and the Mdssbauer parameters are summarized in VELOCITY MM/S Table 3. The Mdssbauer spectrum of perovskite (Frg. l) showsa simpledoublet with an isomershift Frc. l. Mdssbauerspectrum of the natural perovskite from characteristicof octahedralFe3* @ancroft 1973). Magnet Cove. MOSSBAUER SPECTRA oF PBRovsKITE AND TITANITE 691
The spectra of the natural samples of titanite @igs.2-5) are broad and lesswell defined. The Gjer- stad spectrum (Fig. 2) is best fit to a broad doublet, but the fit is poor near the peak maxima, and there is obvious broadening in the peak shoulders. Once again, the isomer shift is characteristicof octahedral Fe3+,and we can assignthe Fd+ to the octahedral z. Ti site. as previously postulated (Higgins & Ribbe F 1976,Cer-n!& Povondra 1972, o_ Statz et al, 1977). G. o The larger quadrupole splitting (0.92 mm s-t) is <, attributed to signifisanl distortion of the TiOu poly- co hedron from octahedral symmetry, consistent with F z. lhe F2r/a spacegroup. l! E. The Sebastapol,Cardiff and Bear Lake samples t_! (Figs. 3-5) show some absorption at 2 mm s-r, o- indicative of Fd* in thesesamples. The fitting of two doublets to these spectra yields similar Fe3* isomer shifts and quadrupole splittings to those obtained for the Gjerstad spectrum.The isomer shift and quadrupole splitting of the Fd+ doublet indi- cate that Fd+ also is presentin the Ti site. In all four spectra,the linewidths are usually large -2.0 0.0 2-0 (0.7-0.9 mm s'r), and the fits to the Fe3* peaks especially are not satisfactory. The line shapesare VELOCITYMM/S clearly non-Lorentzian. There are three possible Frc. 2. Mcissbauerspectrum of the naturaltitanite from major causesof this broadening.Firstly, relaxation Gjerstad,Norway. effects (either spin-spin or spin-lattice) are known to contribute to broad non-Lorentzian lines in Fe3+ compounds(WiCna[ 1966,Bancroft & Sham 1976), but such line broadeningis not known for Fe3* in minerals. We initially thought that the low concen- tration of Fe3+ in these samplesmight be a con- tributing factor to the reduced spin-spin relaxation rate and the resultingbroadening. However, we do not observe increase linewidth an in with decreasing 0.0 Fe content (Tables l, 3) for thesetitanite samples, as would be expectedif spin-spin relaxation was z. important. For example, the Cardiff titanite, with by far the smallestFe content, has the smallestFd+ d- linewidth of the four natural samples. Spin-lattice E.
z z. <) F F d- o- E. €. U> co o
F F z. z L! L! O t E t! L! o_ L
-Z.U 0.0 2.0 -2.0 0.0 2.0
VELOC] TY I,lM/9 VELOCITYMM/S
Frc. 4. Mtissbauer spectnrm of the natural titanite from Frc. 6. M6ssbauerspectrum of tie natural titanite from Cardiff Township, Ontario. Gjerstad,Norway, annealed at I l00oCfor threehours.
z. z. o o
F F d_ o- t E. U>
F z. z. t! E. E. U Li.J o_ o-
-a.u u,u l.u -2.0 0.0 2.0 1.0 \/tr|naTTV MM/C VELOCITYMN/S
Frc. 7. Mdssbauer spectrum of the natural titanite from Frc. 5. Mtiss'bauer spectrum of the natural titanite from Bear Lake, Ontario, melted at 1450oCand sintered at Bear Lake, Ontario. 1250"Cto form a ceramic. MOSSBATJER SPECTRA OF PEROVSKITE AND TITANITB 693
& Karl 1981,Aldridge et ol. L978).Varyine electric- field gradients(and resulting line-broadening)would un ' nu also arise from structural defectsor metamictization in these radiation-damaged materials. The experi- mentsdessribed below demonstratethat theseeffects give significant line-broadening. z. Figure 6 shows the effect of annealing the Gjer- c) stad samplefor 3 hours at ll([oC. Such annealing F d- should help remove metamictization or structural E. in linewidths (0.81 o defects. A significant decrease o versus0.94 mm s-r) and better line-shapesresult. In another experiment,to further removeany structural z. l-J defects, the Bear Lake sample was melted to form <-) E, a glass and then crushed to a fine powder and sin- t! tered at 1250"Cfor one week to produce an annealed o- polycrystalline ceramic titanite of natural composi- tion. The spectrum (Fig. 7) shows a decreasein linewidth of about -0.1 mm s-r from tle untreated Bear Lake sample (Table 3). Metamictization and structural defects would thus appear 1o be signifi- cant contributing factors to the linewidths. To examinethe next-neaxest-neighbormntribution -2.0 0.0 2.0 to the linewidths,two syntheticsamples of closeto ideal CaTiSiO5 in composition were prepared. The VELOCITYI1M/S spectra@igs. 8, 9) consistof two doubletswith sig- quadrupole nificantly different isomer shifts and Frc. 8. Mdssbauerspectrum of the synthetictitanire doped splittings; they have smaller linewidths than eventhe with 2 wt.9oFe. annealednatural samples.These smaller linewidths show that next-nearest-neighboreffects are indeed a significant cause of line broadening. To obtain consistent Fd* octahedral parameters for all titanite samples,the inner two and outer two peakswere matched. The isomer shift and quadru- pole splitting for the inner doublet agreewith the sameparameters for Fe3* in the octahedralTi site in natural samples(Iable 3). The outer doublet gives a significantly lower isomer shift of 0.28 mm s-r z. and a larger quadrupole splitting of l.4l mm s-r. O
Theseparameters are just those expectedfor Fe3+ F o_ in the Si site (Hafner & Huckenholz 1971,Bancroft t o 1973).Indeed, these spectra are very similar to that <, of fenidiopside (Hafner & Huckenholz 1971). In both synthetic samples,close to one half of the z. iron residesin the tetrahedrat Si site. This was ini- trl d: tially rather surprising, but cation substitution for U Si in titanite hasalso beendiscussed by Hollabaugh d- & Rosenberg(1983). They noted that the substitu- tion of Ti for Si is due to a Si deficiencyin the syn- thetic samples.This substitution into the tetrahedral site may explain the larger-than-expected cell parametersof many specimens(to be published).
Coxcr-usroNs -2.0 0.0 2.0
From this work, it is evidenttfiat considerablecare VELOClTYMM/S must be taken itt .satrolling the stoichiometryof syn- thetic titanite. Along with the acceptedsubstitution Frc. 9. Mcissbauerspectrum of the qynthetictitanite doped of Fe3* into the octahedralsite, M6ssbauerspec- with 0.8 wt.9o57Fe. 694 THE CANADIAN MINERALOGIST
troscopy has shown that Fe3+ will also substiture Exlsy, R.A. (1980):Microprobe studies of REE-rich into the tetrahedral sites in synthetic titanite. accessoryminerals: implications for Skyegranite However,substitution into the tetrahedralsites was petrogenesisand .R.E'Emobility in hydrothermalsys- not seen in the natural samples examined, pre- tems,Earth Planet, Sci.Lett.48, 97-110. sumablybecause stoichiometric amounts of Si were available at the time of their formation. Substitu- GnnsNwooo,N.N. & Grss, T.C. (1971):Mdssbauer Spectroscopy. tion into the tetrahedral site may be the reason Chapmanand Hall Ltd., London. behind the enlarged cell-parameters that have been HarNsR,S.S. & Hucrrwsou, H.G. (1971): observedin many Mrissbauer specimens. spectrumof syntheticferri-diopside. Noture Phys. Natural titanite also showssome substitution of Sci.233,9-ll. divalent iron into the octahedral site, along with broadening of the linewidths. This broadeningis due Hawrnonr.n,F.C. (1983):The crystalchemistry of the both to defects or metamictization from radiation amphiboles.Can. Mineral. 27, 173-48A. damage and compositional next-nearest-neighbor effects. Havwanp,P.J. & Cnccrsrro, E.V. (1982):Develop- ment of sphene-basedglass ceramics tailored for CanadianWaste disposal conditions. .In Scientific Basisfor NuclearWaste Management (S.V. Topp, AcrNowlsnceMENTs ed.). Elsevier,New York.
We thank Dr. F.J. Wicks of the Royal Ontario Hrccns,:J.8.& RrBss,P.H.(197Q: The crystal chemis- Mlqeum for supplyingsome of the natural samples try and space groups of natural and synthetic of titanite used in this study, Mr. G.S. Henderion titanites,Amer, Mineral. 61, 878-888. for assistancein computer-fitting the spectra, and Hollasaucu, C.L. (1983): AECL and NSERC for financial assistance. & Rosnvsrnc,P.E. Substi- We also tution of Ti for Si in titanite and new end-member thank two anonymousreferees for their comments. cell dimensionsfor titanite. Amer. Mineral. 68, 177-180.:
REFERENCES Nrssrrr, H.W.., BaNcnonr,G.M., FyrB, W.S., KamnaNrs,S.N., Nrsnrrnraa,A. & Snrr, S. (1981): ALDRTDGE,L.P., Bancnorr, G.M., Fr.srr, M.E. & Thermodynamicstability and kineticsof perovskite HeRzasnc, C.T. (1978): gmphacite studies. II. dissolution.Nature ?,flg,358-362. Mdssbauer spectra of C2,/c ard p2/n omphacites. Arner. Mineral, 63, I 107-ll 15. Rrspsr,E. & KARI,R. (1981):Mdssbauer studies of thiospinels.III. The systemFeCr2So-FeIn2Sa. "/. Bercnorr, G.M. (1973): Mdssbauer Spectroscopy: An Solid StateChem. 38, 4047. Introduction for Inorganic Chemists qnd Geochemists. McGraw-Hill, Maidenhead, England. Rrr.rcwooo,A.E.,Knssox, S.E., Wenr, N.C., Hrannn- sox, W. & Maron, A. (1979):Immobilisation of & Bnowr.r,J.R. (19?5): A Mdssbauer study of high level nuclear reactor wastesin SYNROC. coexisting hornblendes and biotites: quantitative Nature 278,219-223. fs3+/pg2+ ratios. Amer. Mineral. 60.265-272. Spnsn,J.A. & GrsBs,G.V. (1970:The crystalstructure +, Maooocr, A.G., ONc, W.K., pnrNcr, R.H. & of synthetictitanite, CaTiOSiOa,and the domain SroNe,A.J. (1967): Mdssbauer spectra of iron (III) textures of natural titanites. Amer. Mineral, 61. diketone complexes. J. Chem. Soc.A. l966li7I. 238-U7.
& Snev, T.K. (1970: Mdssbauerstudies of Sraanz,M.H., Cowxr-ru,N.M. & Bnowxrrnr.o,I.K. electron spin relaxation and radiolyic effects in (1977):Rare earths, thorium, and other minor ele- diluted tris(acetylacetonato)FenI. /. Chem. Soc. mentsin sphenefrom someplutonic rocks in west- Farad. Trans.I1.72, 1706.1715, centralAlaska. J. Res.U.S. Geol. Sum. 5, 623-628. tsRNf, P. & Povoxona,P. (1972):An Al, F-rich WrcNerl, J.W.G. (1966):Mtissbauer line broadening metamict titanite from Czechoslovakia. Nezar in trivalent iron compounds.J. Chem.Phys. 4, Jahrb. Minerql. Monatsh., 4m-4fl6. 2462-2467.
Drnn, W.A., HowE, R.A. & Zusslrlarq,L (1966):An Introduction to the Rock-FormingMinerals. Long- ReceivedAugust 2, 1983,revised manuscript accepted man, London. January 12" 1984.