AmericanMineralogist, Volume64, pages 893-901,1979
Miissbauerspectra of iron in l:l phyllosilicates
Isn,tnl RozrNsoN Departmentof Geology,The Hebrew Uniuersity Jerusalem.Israel
Entrn R. Bnuuncnn RacahInstitute of Physics,The Hebrew Uniuersity Jerusalem.Israel
nNO LIsn HNLTSN-KALLAI Departmentof Geology,TheHebrew Uniuersity Jerusalem.Israel
Abstract
One sampleof chamositeand a numberof kaolinites,dickites, and serpentineswere examinedby theM0ssbauer effect technique at roomtemperature. Some of thespectra were alsorecorded at 77" and4oK. Only one type of octahedralsite was found in thedioctahedral mineralsand in antigorites.Two different sites were distinguished in chamosite, lizardites, and chrysotiles.Fd+ occupiesoctahedral sites randomly in chamosite,but different site preference wasfound in variouslizardites. Fes+ occurs in oneor two octahedralsites, and in at leasttwo samplesalso in tetrahedralcoordination. Thetemperature-dependence ofthe quadrupolesplittings ofboth Fe'+ doublets shows that theMl doublet,with the smallerquadrupole splitting, is associatedwith theless distorted sites. p"s+1F€+ ratiosdetermined by M0ssbauerspectroscopy differ from thosepreviously establishedby chemicalmethods, casting some doubt on thevalidity of proposedcorrelations betweenthese ratios and the structure of serpentines.
Introduction serpentinespresents some difficulties, because of the poor crystallinityof the samples.Serpentine minerals This studywas undertaken to establishthe correla- are divided into three main groups-antigorites, tion betweenthe Mdssbauerspectra of iron in serpen- chrysotiles,and lizardites-accordingto the adjust- tines and their structure.The spectraof chamosites, ment of the misfit betweenthe tetrahedraland oc- kaolinites, and dickites were examined for com- tahedral sheets:corrugation of the layers in anti- parison. gorites,formation of cylindricallayers and buckling Little is known about the structureof chamosites, of the octahedralsheets of chrysotiles,and flat layers beyondthe factthat they are l: I trioctahedralphyllo- with more stronglybuckled octahedral sheets in liz- silicates,the iron-rich analoguesof serpentines.Ser- ardites. Wicks and Whittaker (1975) reviewedthe pentineminerals have a fairly narrow rangeof chem- information available from X-ray diffraction mea- ical composition,and their iron content does not surements,and the correlation betweenstructural exceeda ie* p"r""nt. The dioctahedrall: I phyllosili- featuresand IR spectraof serpentineswas investi- cateskaolinite and dickite do not departappreciably gatedby Yariv and Heller-Kallai(1973). In the pres- from the ideal composition,and their iron contentis ent study a correlationbetween the different struc- very low. turesand the Mdssbauerspectra of iron is attempted. The structuresof kaolinite and dickite have been It should be stressedthat due to the low iron content established,but detailedstructure determination of of serpentines,the information derived from the onp.3-.wx/ 79 / u 08-0893$02.00 894 ROZENSON ET AL.: MOSSBAUER SPECTRA
spectra is strictly confinedto iron and doesnot neces- tion effects.12 obtainedin the computerfits ranged sarily apply to other ions occupyingcrystallograph- from 200 to 270. ically equivalentsites in the structure. Results Experinental Sometypical Mdssbauerspectra are shown in Fig- Materials ure l, and the parametersobtained from the least- squarescomputer fits aresummarized in TablesI and The following minerals were studied: chamosite 2. Isomer shiftsare given relativeto Fe metal.Some Bl, labelled"bertierine, Dielette, Mancho." from the of the sampleswere also examinedat 77" and 4oK, collection of S. Caillere; dickite samplesNo. 14 and the parametersobtained are included in TablesI (Dl4), from Ouray, Colorado and Dl6 from St. and2. Therewas no changein the line width at lower George,Utah, suppliedby Wards Natural Science temperaturesand the isomer shifts changed by Establishment;eight different samplesof kaolinites 0.10+0.02 mm,/sec, compatible with the change includingone from OnealPit, Macon, Georgia (O.p.) causedby the second-orderDoppler shift. suppliedby Wards, and sampleIZ2 from Mizpe Ra- mon, Israel. Most of the serpentinesamples were Chamosite describedin previouspapers (Yariv and Heller-Kal- A singlesample of chamosite(Bl) was available lai, 1973;Heller-Kallai et al., 1975).Specimens with for this study. The experimentalspectrum was re- the prefix F are from the collection of Faust and solvedinto threedoublets, two correspondingto Fd+ Fahey (1962).Two additional analyzedsamples of and one to Fe8+in high-spinoctahedral coordina- chrysotiles,Nos. 12176and 12180,from the Takaka tion. This is in agreementwith the resultsreported by IgneousComplex, N. W. Nelson,New Zealand.were Yershovaet al. (1975). providedby B. F. Kohn. Antigorites Method All the antigoritesexamined except one (Fl) give All the sampleswere examinedin their natural riseto spectraconsisting of two doublets,compatible form. Somekaolinites and serpentineswere also in- with Fe2+and Fe8+in high-spinoctahedral coordina- vestigatedafter 2 hours treatmentwith hot l:6 HCI tion (Fig. la and Tablel). The parametersobtained solution to removeiron-containing surface coatings. for all the Fe2+ doublets are very similar [e.S. Two ground samplesof serpentines(HUl76 and (298"K) : 2.73+0.03mmlsec and I.S. (298'K) : HUI1759) weretreated for 48 hours with 20 percent Ll3+0.01 mm/secland the linewidths are relatively HrO, solution.The sampleswhich servedas absorb- narrow (-0.36+0.01 mmlsec), indicatingthat the erswere ground to a thin powder.About 20-40mg of environmentof the Fe2+ions in all the samplesis powder, which contained about 2 mg iron, were similar. The quadrupolesplittings are strongly tem- placedinto a lucite holder of about 2 cm diameter. perature-dependent.The parametersof the Fe3+dou- utCo The sourceconsisted of 50 mCi in Rh at room bletsare less uniform, the line widths aregreater, and temperature.The Mdssbauerspectra were obtained the temperature-dependenceof the quadrupolesplit- with a conventionalconstant acceleration drive (Co- tings is small,as expected. hen et al., 1963)coupled to a Harwell proportional The Mdssbauerspectrum of sampleFl (Fig. lb) counter and a 200 or 256 multi-channel analyzer showsno indicationof the presenceof Fe2+.It can be working in the multiscalermode. The absorption fitted by two doublets,corresponding to tetrahedrally spectrawere automatically folded in the multichannel and octahedrallycoordinated Fe8+. analyzer(Nadav and Palmai, 1967).The velocity scalewas calibrated using a 20 mg/cm2(25 pm) thici Chrysotilesand lizardites Fe-metalfoil as absorber.The widths of the individ_ The Mdssbauerspectra of theseminerals are more ual absorptionlines in the calibration spectrawere complex than those of the antigorites.They were about0.25 mm/sec. generallyresolved into three doublets,as shown in The experimentalspectra were analyzedby least- Figureslc and d. The parametersobtained are given squarescomputer fits to two, three,or four doublets. in Table l. Resolutioninto only two doubletsgives The individual linesof eachdoublet were assumed to riseto largeline widths of eitherthe Fe3+or the Fe2+ have the sameheight and width. The good fits ob- doublets.Fes+ doublets are frequentlybroad, but the tained indicatethat there wereno preferredorienta- line widths of about 0.6 mmlsec are greater than ROZENSON ET AL.: MOSSBAUER SPECTRA 895 thoseusually observedfor phyllosilicates,including antigorites.In two spectra,resolution of the Fd+lines into two doublets leads to reasonableFd+ line widths,accompanied by a significantreduction in 12. The Fe8+doublets remain broad evenwhen the Fe8+ linesare resolvedinto two subspectra,but suchreso- lution is justifiedfor somespectra, because it leadsto considerablylower valuesof 12.Resolution into four doubletsdoes not reduce12 further. The spectrawere thereforeresolved into a maximumof threedoublets, though the presenceof an additional site cannotbe excluded. Only two specimens,chrysotile HU326l and aliz- ardite from New Zealand(not includedin the table), IJJ gaverise to the characteristicmagnetic spectrum of k present G a-FqOs. The iron in theseserpentines was (9 entirelyin the trivalent form. z zF Treatmentwith HCI or HzOz solutions ol C) (kaolinites HCI treatmentof someof the samples UJ and serpentines)did not affect their Mdssbauer spectra,which showsthat the effectsobserved were kJ UJ not due to surfacecoatings. G, HrO, treatmentof two samplesof serpentinesdid not lead to oxidation of Fd+, indicating that it is most unlikelythat any changesin compositionof the specimenshad occurreddue to prolongedstorage in the laboratory. Kaolinitesand dickites The spectraof all the kaolinitesbut one (lZ2) and of the two samplesof dickiteshowed a singledoublet, correspondingto Fes+ in octahedralcoordination, which,although relatively broad, could not be mean- ingfully resolved.All the kaolinitespectra except that of lZ2 were similar and resemblethose reported in the literature(Jefferson et al., 1975\.The data for a singlerepresentative sample are presented in Table2. VELOCITY(mmztecl The spectra of the two samplesof dickite closely Fig. l. Room-temperatureMdssbauer spectra of serpentines.(a) resemblethose of kaolinites.Kaolinite sampleIZ2 antigorite(Fla); (b) antigorite(Fl); (c) chrysotile(12lE0); (d) showed a seconddoublet, correspondingto Fe2+, lizardite(11759); (e) kaolinite(lz2). A-Fe8+in M(2), B-Fd+ in which is also relativelybroad (Fig. le). M(2), C-Fe8+in M(l), D-Fd+ in M(l), E-Fe8+in tetrahedral sites,F-Fe8+ and G-Fe'z+in singleoctahedral site.
Discussion for the samplesmeasured at77"K the ratio remained the sameas at 298oK. The ratio Fd* / Fe* It appearsthat thereare considerable discrepancies This ratio wasderived from the Miissbauerspectra betweenthe ratios determinedby Mdssbauerspec- on the assumptionthat the recoil-freefraction is troscopyand by chemicalanalyses, despite the fact equalin all sitesand that the ratio is thereforegiven that most of thesewere carried out on aliquotsof the by that of the areasunder the correspondingdou- same samples.The similarity betweenthe chemical blets.This assumptionis supportedby the fact that analysesof sampleswith prefix F performedby Faust E96 ROZENSON ET AL,: MOSSBAUER SPECTRA
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Mossbauerparameters of kaolinites and dickites o.P.(K) o.4o (r) 0.65 (1) o.58 (2) 0.48 (1) 0.66 (t' o.@ (4) L.21 (L) 2.85 G' O.t4t'(2) o.?9 (2) 0.66 (r) 0.60 (r) 1.13 (l) 2.6 (2) 0.41 (2) o.r9 (t) o.55 Q) 0.63 (1) o.39 (r) 0.61 (1) O.5t (1) Abbrcvlatloa ag for Table t ard K - kaollnlte, D - dicklte and Fahey (1962) and in our laboratory was pre- + Alros) ratio and lizarditesthe lowest.Though this viously discussed(Yariv and Heller-Kallai, 1973). conclusionseems justified for most of the samples Possibleerrors in the determinationof Fes+/Fez+ examined,there are someexceptions, like the anti- ratios from Mdssbaueranalysis may be due to the gorite sample Fl discussedabove, which contains presenceof somemagnetic phases or to paramagnetic only Fes+,and the lizarditesF47 and HUl1759, in relaxation effects, which may cause appreciable which more than 70 percent of the iron ions are broadeningof the linesin the Mdssbauerspectra. The divalent.The Fe8+/Fe2+ratios determinedby Miiss- spectrawere checkedfor thesepossibilities, and the bauer analysisof thesethree samples,two of which inaccuraciesin the Fe8+,/f's'+ratios determinedby were included in the selectionof Whittaker and Mdssbaueranalysis were estimatedto be lessthan 5 Wicks, suggestthat the criterion they proposedfor percent. the classificationof serpentineminerals should be In the courseof chemicalanalysis, some oxidation treatedwith caution. of the specimensmay occur, thus increasingthe chemically-measuredps8+/ps2+ ratio. The opposite The Mbssbauerparameters effect,in which higher M0ssbauerthan chemicalval- All the parametersobtained for l: I phyllosilicates ues are obtained for Fes+/Fd+, as observedmost are compatiblewith thosereported for 2: I phyllosili- strikingly with sample Fl (Fig. lb), is difficult to cates.The line widths of the Fe2+doublets are rela- explain.In the Mdssbauerspectrum of sampleFl no tively narrow (0.32-0.40mm/sec) and fall within the doubletcorresponding to Fez+is seen.The limit ob- rangegiven by Bancroft(1974) for silicates.The Fe8+ tained from the Mdssbaueranalysis shows that less doubletshave a much broaderline width (0.40-0.75 than 3 percentof the total iron is divalent,in com- mm,/sec),even when Fes+ions are assignedto two plete contradictionto Faust and Fahey,who found differentsites. This is a commonfeature of phyllosili- that the iron waspresent entirely in the divalentform. cates(paper in preparation).No significantchanges Oxidation of the serpentinesduring storagein the in the line widths wereobserved in spectrarecorded laboratory can be discounted,in view of the resis- at 77"K. The larger line widths of the Fes+doublets tanceof the groundspecimens to oxidationon drastic may be due to someinhomogeneity of the Fes+envi- HrO, treatment.It should,however, be notedthat the ronment or to paramagneticspin-spin relaxation ef- total concentrationof iron is very low (0.35percent fects,which arecommonly observed for Fe8+in sam- FeO, according to Faust and Fahey), which may ples with low iron content (Mdrup, 1974). The causeinaccuracies in the chemicalanalysis. crystal-fieldsymmetry is usually sufficientlylow to Whittaker and Wicks (1970)discussed the chem- renderthe electronicground stateof Fd+ a singlet, ical differencesbetween serpentines and concluded which cannot have a magnetic moment. Para- that antigoriteshave the highestFeO/(FeO * FqOg magneticrelaxation effects are thus not expectedto ROZENSON ET AL,: MOSSBAUER SPECTRA affect Fe2+spectra (Lang and Marshall, 1966)and The configurationof the octahedralsheets could, therefore,explain the observeddifferences in Phyllosilicatesin generalhave two octahedralsites, line width betweenthe Fe8+and Fez+ doubletsin Ml andM2,in the ratio I :2. Structuredetermination phyllosilicates.Note that glauconites and non- showedthat thesesites differ in kaolinites(Zvyagin, tronites, which are rich in Fd+, give rise to much 1967,p. 256) and dickites(Newnham, 1969). Kunze narrowerFee+ doublets than phyllosilicateswith low ( 196l) inferredthat the two sitesare indistinguishable Fe3+ content (Rozensonand Heller-Kallai, 1977). in antigorites,and this was confirmed by their IR The changesin isomer shift from room temperature spectra(Yariv and Heller-Kallai, 1973).In chryso- to77"K are0.ll+0.01 mm,/sec,as anticipated from tiles and lizarditestwo distinct sitesoccur. Sincethe the second-orderDoppler shift (Josephson,1960). unit layers are buckled, in lizarditesmore than in Fd+ in tetruhedral coordination The presenceof chrysotiles(Wicks and Whittaker, 1975),different tetrahedralFd+ wasinferred for two of the samples distortions of the two octahedralsites may be ex- studied(12176 and Fl) on the basisof the isomer pected. No structure determinationof chamosites shift, which resemblesthat of ferri-phlogopite(An- seemsto havebeen reported. nerstenet al., 1974).The spectraof samples12180, Chamosites.Yershova et al. (1975)found that Fe2+ 6(2) orthochrysotile,and HU904 showFe8+ doublets ions occupy the two different octahedral sites of with isomer shifts smallerthan are generallyfound chamosite(sample 152) in the ratio 65:35,with the for octahedrally-coordinatedFe8+ in silicates,but higher proportion of Fe2+in the sites with higher larger than thosegenerally associated with tetrahe- quadrupolesplitting. We deriveda ratio of 100:48 drally-coordinatedFes+. The parametersfall in the for sampleB I (Table I ). Theseratios, which areclose rangeattributed to tetrahedrally-coordinatedFes+ in to 2, suggestthat Fe2+in chamositesoccupieB sites nontronitesby Goodmanet al. (1976).Itseems pos- M2 and Ml non-preferentially,as reportedfor bio- sible that thesedoublets in serpentinesare due to tites (e.g. Annersten, 1974).On this assumptionit both octahedraland tetrahedralFe8+, but that their followsthat the doubletwith higherquadrupole split- quadrupolesplittings are too similar to permit fur- ting correspondsto Fe2+in M2 sites.This assignment ther resolution. receivessome support from considerationof the In calculatingstructural formulae from the experi- structural formulae of the samples,provided that mentally-determinedcomposition, it is customaryto theseare chemically pure. The FeOcontent of sample postulateAl-for-Si in preferenceto Fe8+-for-Sisub- Bl, inferred from the Mdssbauerspectra, is 35 per- stitution. While this is justified from similarity of centi.e. it is similar to that of sample152, described ionic radii, the misfit betweenthe octahedraland by Yershovaet al. (1975),for which a structural tetrahedralsheets of trioctahedrallayer structuresis formula with 1.7 Fe2+ions per formula unit was more effectivelyrelieved by Fe8+-for-Sithan by Al- deduced.Since Ml sitescan accomodatea maximum for-Si substitution.Replacement by trivalent ions in of one cation per formula unit, and the populationof both the tetrahedraland octahedralsheets of serpen- the site with larger quadrupolesplitting exceedsl, tinesgives rise to characteristicIR absorptionbands this doublet must be assignedto M2 sites. at about 3400cm-1, which persistafter elimination of Yershovaet al. reportedonly a singleFe8+ doublet adsorbedwater (Heller-Kallaiet al., 1975).Such ab- for all the-natural samplesstudied. The ratio Fe8+/ sorptionsappear in the IR spectraof samples12176 Fez+is 0.18 and 0.25 for samples152 and Bl respec- andHUl1759 but not in the spectrumof sampleFl tively, and accordinglythe Fe8+doublets are weak. Accordingto the chemicaland microprobeanalyses, Yershovaet al. reporteda narrow Fe8+doublet with this samplecontains no more than 0.08Fe ions per largeQ.S. We observeda smallerQ.S. and broader formula unit. The amountof tetrahedralFes+ present line widths, as is usual for phyllosilicateswith low is thus necessarilyvery small. Moreover, it is to be Fes+content.Resolution of the Fe8+doublet into two expectedthat Fe8+-for-Si.+substitution causesa doubletsdid not decreasethe value of 1'z,but this smallerincrease in the polarity of the layerthan Als+- does not prove that Fe8+occupies only one type of for-Si{+substitution, and minor amountsof tetrahe- site selectively.Yershova et al. found that spectraof dral iron may not sufficefor the formation of the samplesof heatedchamosite, in which the Fe2+was interlayer H bonds which give rise to the band at partly or completelyoxidized, could be resolvedinto about3400 cm-1. four doublets,two Fe2+and two Fe8+.They inferred ROZENSON ET AL.: MOSSBAUER SPECTRA that Fe8+originally occupiedone type of site only largerquadrupole splitting in the spectraof lizardites and that the other becamepopulated by Fe3+derived and chrysotilesshould be assigned to Fez+in M2 sites from oxidizedFe2+. In interpretingthe spectra,how- and that the singlesite in antigoritesresembles M2. ever,the problemof maintainingelectrical neutrality In the mineralogicalliterature it hasgenerally been of the sampleon oxidationof Fe2+seems to have assumedthat larger quadrupolesplittings of Fe2+ beenignored. This could occur by partial deprotona- doublets indicate smaller distortion (e.9. Bancroft, tion of the specimens,which may modify the corre- 1974).As is well known, the quadrupolesplitting is spondingsites. mainly due to an electricfield gradient (EFG), caused Serpentines.The Mdssbauerspectra of antigorites by (a) the asphericalcharge distribution of the 'va- show that only one type of octahedralsites is occu- lence'electrons of the Mdssbaueratom and (b) the pied by Fez+ or Fe8+ions, in completeagreement chargeson ions surrounding the Mdssbaueratom with the conclusionsreached by X-ray and IR stud- in the crystal. The first contribution to the EFG is ies. Most lizarditesand chrysotilescontain Fez+or usuallydenoted by qu,,,the secondby q,",. For Fes+, Fes+in two types of octahedralsites. Despite their an S stateion, qvar: 0 and the Q.S.is mainly due to differentcrystal structures, the Mdssbauerspectra of q61. The theory for the Q.S. of Fez+was developed antigorites,Iizardites, and chrysotilesall show one by Ingalls (196/-).The Q.S. is expressedby E(T; = Fe2+doublet with similar parameters-quadrupole AE'F(T). The bare quadrupole coupling constant splittingsat 298oKranging from 2.70to 2.79mm/sec AEo is definedas (2.70to 2.76 mm/secfor nine of the elevensamples examined)and isomershifts of l.l2 to I .I 5 mm/sec. AEri(r;rsr)e'Q Only the Mdssbauerspectrum of 6(2) orthochrysotile doesnot showthis doublet.but the doubletobserved where(r;rsr) is an effectivevalue for the 3d electronson hasa relativelylarge Q.S. and its assignmentis uncer- the ferrous ion, including both covalencyand anti- tain. shieldingeffects, and Q is the quadrupolemoment of The secondFd+ doublet seenin the Mdssbauer Fe67'.The reductionfactor F(T) takesinto account spectraof chrysotilesand lizarditeshas a much wider the thermal population of the crystal field levelsof range of parameters.The differencesbetween the the Fe2+ions, spin orbit couplingand q1,1.The values configurationof sitesin the varioussamples probably of AEo vary from compound to compoundand are dependon the type of serpentineand its chemical mainly influencedby effectsof covalency(Ingalls, composition.Thus it is possiblethat the two Fd+ 1969).qr"t is temperature-independent.Its sizeis usu- doublets in sample HUl1759 differ less from each ally much smallerthan that of evar,and in mostcases other than thosein HU4050. becausethe misfit be- it has the oppositesign (Nozik and Kaplan, 1967). tweenthe octahedraland tetrahedralsheets is partly The temperature-dependenceof F(T) is mainly dic- relievedby Al8+-for-Sir+substitution. This reduces tatedby the crystalfield splittingsA1 and Ar, and to a the distortion of the octahedralsheets and renders lesserextent by the spin orbit couplingA. Ml sitesmore similar to M2. Furtherconfirmation of The conclusionthat largerquadrupole splittings of this hypothesisis required,but no other suitableser- Fe2+doublets indicate smaller distortion (e.9. An- pentineminerals were available for study. nersten,1974) was based on the assumptionthat Asis The spectraof serpentinesare reminiscentof those greater than approximately800 cm-l and any de- of biotite(Annersten, 1974), but the Q.S.of the outer creaseobserved from the maximum possiblequad- Fe2+doublets are about 0.1-0.2 mm/sec greater. This rupole splitting of Fe2+doublets is due to a negative may be due to the differentligands: O2(OH). in ser- contributionof et., (Fig. 6-9, Bancroft, 1974).It did pentinesand O(OH), in biotites.Assignment of the not take into account major changesin AEo and doubletsto the two octahedralsites, Ml andM2. is F(T). On the other handthe temperature-dependence difficult.The structuredeterminations reported in the of the quadrupolesplittings measures in principlethe literatureare not sufficientlyaccurate to provide the crystal field splittings41 and Az and hencedepends exactconfiguration of the octahedrafor correlation on the distortion-the smaller thesesplittings, the of distortion with the quadrupolesplittings. Com- smaller the distortion, and the larger the temper- parison of the Mdssbauerdata for serpentinesand ature-dependence.In the present casethe relative chamosites(Table l) suggeststhat the doublet with changeof the quadrupolesplitting with temperature ROZENSON ET AL.: MOSSBAUER SPECTRA in the lizarditesmeasured is larger for Fe2+ascribed The temperature-dependenceof the Q.S. of Fe2+ to the Ml than for Fe2*ascribed to the M2 sites.This doubletsof kaolinitesis smallerthan that of serpen- is clearlyseen for sample11759, where Q.S. (300'K)/ tines,indicating that the correspondingsites are more Q.S.(4"K) : 0.91and 0.83 for theM2and Ml sites distorted.This is in agreementwith the dataobtained respectively,thus indicating that the M2 are more for 2:l layer silicates-the temperature-dependence distorted than the Ml sites,though the quadrupole of Fe2+doublets of dioctahedralminerals, e.8'. mus- splittingof the M2 is largerthan that of the Ml sites. covite,is smallerthan that of trioctahedralminerals, Becauseof the similarity of the M2 parametersin all e.g.biotite (paperin preparation). the serpentinesmeasured, it is plausibleto assume that in the other samplestoo the M2 site is more Comparisonof Mdssbauerparameters of Fd+ distortedthan the Ml site. in I: I and I:2 phyllosilicates With this assumption,the assignmentof the Fe8+ Both I : I and I :2 phyllosilicatescontain two dif- doublets to the Ml and M2 sitesmay be based on ferentoctahedral sites, but the ligandsdiffer-4(OH) that of the Fe2+doublets. If Ml sites are lessdis- groupsin l: I and 4(O) and 2(OH) groupsin l:2 torted than M2, i.e. the q1"scontribution is smaller,it phyllosilicates.Moreover, while the distribution of follows that the quadrupolesplitting of Fe8+in such the ligandsin Ml andM2 sitesin l:l phyllosilicatesis sitesis expectedto be smallerthan that of Fes+in M2 similar, OH groups in l:2 phyllosilicatesoccur in sites.It seems,therefore, that Fe3+doublets with the eithertrans (Ml) or cis(M2) arrangements. Iargerquadrupole splittings should be assignedto M2 It would be desirableto compare the spectraof sites.This assignmentis supportedby the spectraof serpentineswith thoseof talc, but all the samplesof antigorites,in which the quadrupole splittings of talc examinedto dateshow only a singleFe2+ doublet doubletscorresponding to Fe8+ions occupyingthe assignedto M2 sites (Rozenson,unpublished). tn singlesite, like thoseof Fe2+ions, resemblethose of biotitestwo Fe2+doublets were distinguished, resem- the correspondingions assignedto M2 sitesin the bling thoseof serpentines,but the quadrupolesplit- other serpentines. tings of the outer doubletsare smallerby about 0.1- It is impossibleto decidewhether the Fe8+present 0.2 mm/sec,possibly due to the differentligands. The in the chamositeand serpentinesis primary or is due temperature-dependenceof the quadrupolesplitting to oxidation of structuralFe2+. The associationwith of the two Fe2+doublets in biotite is very similar hematite in two samplesin which iron is present (Annersten,1974), whereas in lizarditesthe smaller entirelyin the trivalentform (not includedin Table l) quadrupolesplitting, associated with Ml doublets,is suggeststhat someoxidation may haveoccurred. much more temperature-dependentthan the larger Kqolinitesand dickites. Structure determinations of one.This is attributedto the greaterdistortion of M2 kaolinite (Zvyagin, 1967) and dickite (Newnham, sitesin serpentines.In contrast,the differencein the l96l) showedthat Al occupiestwo crystallograph- quadrupolesplittings of the two Fe2+doublets in I :2 ically-distinct sites in the octahedral sheets.Only phyllosilicatesis mainly due to the differentgrat con- single Fe8+and Fe2+doublets, however, were ob- tribution arisingfrom the differentdistribution ofthe servedin the Mdssbauerspectra. The interatomic ligands,while the distortion of the sites,and hence distancesand the distortion of the two typesof sites the temperature-dependenceof the quadrupolesplit- are similar,as are the ligandsand their spatialdistri- tings, is similar. bution. It thereforeseems possible that the Mdss- bauer parametersof iron in the two sitesare very Conclusions similar. The presenceof only one doublet for either Mdssbauerspectra of some of the l: I phyllosili- Fe8+or Fe2+cannot therefore be attributedto selec- cates examinedshowed doublets correspondingto tive occupancyof one type of site.The doubletsare Fe2+or Fe8+ions in two differentoctahedral sites. relativelybroad. This may be attributedto relaxation Sincethe distribution of ligandsin the Ml and M2 effects for Fet*, but may also be due to in- sitesis similar in theseminerals, in contrastwith the homogeneityof the sitesoccupied by either Fe2+or cl'sand lrazs distribution in 2: I phyllosilicates,the Fe8+. differencesin Q.S. betweendoublets associated with The similarityof the Mdssbauerspectra of dickites the two typesof sitesreflect differences in distortion. and kaolinitesshows that differentmethods of layer Although structuredeterminations of kaoliniteand stackingdo not appreciablyaffect the geometryof the dickite show that Al occupiestwo differentoctahe- octahedralsites. dral sites,only one type of sitecould be distinguished ROZENSON ET AL,: MOSSBAUER SPECTRA 901 by Mdssbauerspectroscopy, probably due to the Goodman, B. A., J. D. Russell,A. R. Fraser and F. W. D. small differencebetween their Mdssbauerparame- Woodhams(1976) A M6ssbauerand IR spectroscopicstudy of ters. In antigorites,also, only one type of site was the structureof nontronite. Claysand Clay Minerals,24,53-59. Heller-Kallai,L., (1975) found. In contrast.in chamosites S. Yariv and S. Gross Hydroxyl-stretch- and lizarditesFd+ ing frequenciesof serpentineminerals. Mineral. Mag., 40, 197- occursin two clearlydistinguishable sites, with well- 200. definedranges of quadrupolesplittings, reflecting dif- Ingalls, R. (1964)Electric-field gradient tensor in ferrouscom- ferent distortions. In the chrysotilesexamined it pounds.Phys. Reu., 1 33,'187-795. - seemsto be the Fe8+and not the Fe2+ions which (1969)Comments on divalentFe6"- quadrupolar coupling occupy two different constants.Phys. Reu.,,l8E, 1045-1046. octahedralsites. Interpretation Jeflerson,D. A., M. J. Tricker and A. P. Winterbottom(1975) of the Fes+spectra is difficult, however,due to the Electronmicroscopic and M6ssbauerspectroscopic studies of large line width of the doublets and the possible iron-stained kaolinite minerals. Clays and Clay Minerals, 23, presenceof Fe8+in tetrahedralcoordination. 355-360. In general,the Mdssbauerparameters of Fe2+in Josephson,B. D. (1960)Temperature dependent shift ofX-rays by M2 sitesand their temperature-dependence solids. Piys. Reu. Lett., 4, 341-342. are very Kunze,G. (1961)Antigorit: StrukturtheoretischeGrundlagen und similar for the sampleof chamositesand all the ser- ihre praktischeBedeutung ftir die weitereSerpentin-Forschung. pentinesstudied. The correspondingparameters of Fortschr.Mineral., 39, 206-324. the Ml sites differ appreciablyand show a larger Lang, G. and W. Marshall(1966) Mdssbauer effect in somehae- temperature-dependenceof the This indicates moglobin compounds.Proc. Phys. Soc.,87, 3-34. Q.S. Mdrup, (1974)Mdssbauer that thesesites are lessdistorted than S. effectstudies of electronicrelaxation M2 sites,de- in ferric compounds.In I. J. Gruverman et al., Eds.,Mbssbauer spitethe smallerQ.S., in disagreementwith the usual Efect Merhodology, Yol. 9, p. 127-149. Plenum Press, New interpretationin the mineralogicalliterature, which York. correlates a smaller Q.S. of Fe,+ doublets with Nadav,E. and M. Palmai(1967) Automatic folding in Miissbauer greatersite dlstortion. experimentsthrough forward-backward operation of the multi- Fe2*occupies octahedral sites channelanalyzer. Nucl. Inst. M ethods,56, 165-169. randomlyin chamo- Newnham,R. E. (1961)A refinementof the dickitestructure and sitesand lizarditeHUI1759, whileM2 sitesare occu- some remarkson polymorphismin kaolin minerals.Mineral. pied preferentiallyin lizardite HU4050 and exclu- Mag., 32, 683-704. sively in F47. These distributions may reflect Nozik, A. J. and M. Kaplan (1967) Significanceof the lattice differencesin energy associatedwith the site dis- contribution to Mdssbauerquadrupole splitting. Phys. Reu., tortions and may throw somelight on the history 159,273-275. of Rozenson,I. and L. Heller-Kallai(1977) Mdssbauer spectra of the samples.Further study is required. dioctahedralsmectites. Clays and Clay Minerals, 25, 94-l0l . Whittaker,E. J. W. and F. J. Wicks (1970)Chemical differences Acknowledgments among the serpentine"polymorphs": a discussion.Am. Min- eral., 55, 1025-1047. We are indebtedto Dr. G. T. Faust,Dr. B. F. Kohn, Mlle. S. Wicks, F. J. and E. J. W. Whittaker(1975) A reappraisalof the Caillere,Mr. C. D. Rushten,and the SmithsonianInstitution for structuresof the serpentineminerals. Can. Mineral., 13, 227- donationof specimens. 243. Yariv, S. and L. Heller-Kallai( 1973)The relationshipbetween the References i.r. spectra of serpentinesand their structures.Clays and Clay Minerals. 23. 145-152. Annersten,H. (1974)Mdssbauer studies of natural biotites.,42. Yershova,2.P., A. P. Nikitina, Yu. D. Perfilevand A. M. Babesh- Mineral., 59. 143-151. kin (1975) Study of chamositesby gamma-resonance(Mtiss- Bancroft, G. M. (1974) MbssbauerSpectoscopy. McGraw-Hill, bauer)spectroscopy. Proc. Internatl. Clay Conf.,Mexico,2ll- New York. 220. Cohen,R. L., P. G. McMullin and G. K. Wertheim(1963) High Zvyagin, B. B. (1967)Electron-Difraction Analysis of Clay Miner- velocitydrive for Mdssbauerexperiments. Reu. Sci.Instr., 34, a/s. PlenumPress. New York. 671-673. Faust,G. T. and J. J. Fahey(1962) The Serpentine-Group Minerals. Manuscript receiued,July 28, 1978; U.S. Geol. Surv. Prof. Pap.384-A. acceptedfor publicationOctober 27, 1978