Oxidation-reduction mechanism of iron in dioctahedral smectites: 2. Crystal chemistry of reduced Garfield nontronite

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1 Americn Minerlogist, 85, 53-7, Oxidtion-reduction mechnism of iron in diocthedrl smectites:. Crystl chemistry of reduced Grfield nontronite A. MANCEAU,, * V.A. DRITS,,. LANSON, D. CHATEIGNER, J. WU, 3 D. HUO, 3 W.P. GATES, 3,, AND J.W. STUCKI 3 Environmentl Geochemistry Group, LGIT-IRIGM, University Joseph Fourier nd CNRS, 38 Grenole Cedex 9, Frnce LPEC, Université du Mine-Le Mns, v. Olivier Messien, P Le Mns cedex, Frnce 3 Deprtment of Nturl Resources nd Environmentl Sciences, University of Illinois, W-37 Turner Hll, South Goodwin Avenue, Urn, Illinois 68, U.S.A. ASTRACT The crystllochemicl structure of reduced Grfield nontronite ws studied y X-ry sorption pre-edge nd infrred (IR) spectroscopy, powder X-ry diffrction, polrized extended X-ry sorption fine structure (P-EXAFS) spectroscopy, nd texture goniometry. Untreted nd highly reduced (>99% of totl Fe s Fe + ) nontronite smples were nlyzed to determine the coordintion numer nd the crystllogrphic site occuption of Fe +, chnges in in-plne nd out-of-plne lyer structure nd mid-rnge order etween Fe centers, nd to monitor the chnges in structurl nd dsored OH/H O groups in the structure of reduced nontronite. Contrry to erlier models predicting the formtion of fivefold coordinted Fe in the structure of nontronites upon reduction, these new results reveled tht Fe mintins sixfold coordintion fter complete reduction. In-plne P- EXAFS evidence indictes tht some of the Fe toms occupy trns-sites in the reduced stte, forming smll triocthedrl domins within the structure of reduced nontronite. Migrtion of Fe from cisto trns sites during the reduction process ws corroorted y simultions of X-ry diffrction ptterns which reveled tht out 8% of Fe + ctions exist in trns sites of the reduced nontronite, rther thn fully cis occupied, s in oxidized nontronite. Out-of-plne P-EXAFS results indicted tht the reduction of Fe suppressed sl oxygen corrugtion typicl of diocthedrl smectites, nd resulted in flt sl surfce which is chrcteristic of triocthedrl lyer silictes. IR spectr of reduced nontronite reveled tht the diocthedrl nture of the nontronite ws lost nd nd ner 363 cm formed, which is thought to e ssocited with triocthedrl [Fe + ] 3 OH stretching virtions. On the sis of these results, structurl model for the reduction mechnism of Fe 3+ to Fe + in Grfield nontronite is proposed tht stisfies ll structurl dt currently ville. The migrtion of reduced Fe ions from cis-octhedr to djcent trns-octhedr is ccompnied y dehydroxyltion rection due to the protontion of OH groups initilly coordinted to Fe. This structurl modifiction results in the formtion of triocthedrl Fe + clusters seprted y clusters of vcncies in which the oxygen lignds residing t the oundry etween triocthedrl nd vcncy domins re gretly coordintion understurted. The chrge of these O toms is compensted y the incorportion of protons, nd y the displcement of Fe + toms from their idel octhedrl position towrd the edges of triocthedrl clusters, thus ccounting for the incoherency of the Fe-Fe nd Fe-Fe distnces. From these results, the idel structurl formul of reduced Grfield nontronite is N.3 [Si 7. Al.78 ] [Fe Al.3 Mg. ]O 7.93 (OH) 5 in which the incresed lyer chrge due to reduction of Fe 3+ to Fe + is stisfied y the incorportion of protons nd interlyer N. INTRODUCTION Reduction-oxidtion rections of lyer silictes in soils nd sediments influence environmentl processes such s wethering, microil ctivity, nd digenetic trnsformtions (Egshir *E-mil: Alin.Mnceu@os.ujf-grenole.fr And Geologicl Institute of the Russin Acdemy of Sciences, 7 Pyzhevsky Street, 97 Moscow, Russi. And Environmentl Geochemistry Group, Grenole. Present ddress: CSIRO Lnd nd Wter, Privte Mil g No., Glen Osmond, SA 56, Austrli nd Ohtsuo 983; Ernstsen 998; Stucki et l. 987). oth physicl nd chemicl properties of smectites re modified y the reduction of structurl Fe 3+, including ction exchnge cpcity (Stucki et l. 98), specific surfce re (Ler nd Stucki 985), swelling ehvior (Gtes et l. 993; Ler nd Stucki 989), nd texture (Gtes et l. 998; Stucki nd Tessier 99). Although we cn predict to some extent the effect of Fe oxidtion stte on smectite chemicl nd physicl properties, we do not know how the surfce properties of reduced smectite re controlled due to the lck of moleculr level understnding of the reduction mechnism of Fe 3+. The present study exmines the chemicl nd structurl environment of octhedrl Fe in

2 MANCEAU ET AL.: REDUCED NONTRONITE Texture nlysis Quntittive texture nlysis (goniometry) ws pplied to the films following EXAFS mesurements using Huer texreduced Grfield nontronite through polrized extended X-ry sorption fine structure (P-EXAFS) spectroscopy, X-ry sorption pre-edge nd infrred (IR) spectroscopy, X-ry diffrction (XRD), nd texture goniometry, nd develops structurl model which ccounts quntittively for the chnges of physico-chemicl properties descried in the literture. EXPERIMENTAL METHODS Reduction The N-exchnged Grfield nontronite (R. Gleser), hs structurl formul of [N.8 (Si 7. Al.78 )(Fe Fe+.Al.3 Mg. )O (OH) (Tle in Mnceu et l. ). Smples were reduced with N dithionite following modifictions of the method of Komdel nd Stucki (988). Smples (65 mg) were suspended in ml deionized Milli-Q wter, to which 3 ml of citrte-icronte (C) uffer ( prt M NHCO 3 :8 prts.3 M N 3 C 6 H 5 O 7 H O) were then dded. Then mg solid powder N-dithionite (N S O ) ws dded to the cly suspension nd the rection ws llowed to proceed t 7 C under inert tmosphere purge for hours (smple ). Prt of this tch ws then fully oxidized y uling pure oxygen gs through the suspension for hours, wshed free of excess slts using high purity wter, nd reduced gin s descried ove (smple OR). The level of reduction ws mesured y digesting portion of ech smple in HF-H SO nd nlyzing the digestte for Fe + y the method of Komdel nd Stucki (988). Chemicl nlysis indicted tht more thn 99% of the octhedrl Fe 3+ ws reduced to Fe +. The totl content of N in determined y tomic sorption spectrometry is 3.5 ±. wt%. Preprtion of films Highly textured self supporting films of reduced smples were prepred under dry inert tmosphere (N or Ar gs) y filtering the suspensions onto.5 mm pore-size Millipore memrne filter. The resulting oriented cly film ws then quickly trnsferred to glove ox hving H O nd O prtil pressure of out 6 tm. Virtionl modes due to structurl O-H stretching nd deformtion nds were mesured in selfsupporting cly films contining pproximtely. mg of smple per cm. Within the glove ox, the reduced, dry selfsupporting films destined for IR spectroscopy were plced inside seled vcuum cell fitted with ZnSe windows (Angell nd Schffer 965) nd pumped to 6 tm. For P-EXAFS mesurements, the mss of cly sedimented onto the Millipore filter ws clculted to otin n sorption jump cross the edge ( µ ) of typiclly.8 t the experimentl α ngle of 6 (Stern nd Kim 98). Films were mounted in vcuum chmer (P = tm) for polrized X-ry sorption mesurements. Preprtion of reference mterils Reference smples for pre-edge spectroscopy include nontronite dehydroxylte, iotite, nd FePO, which were used s stndrds for V Fe 3+ (Drits et l. 995), VI Fe +, nd IV Fe 3+, respectively. The untreted Grfield nontronite film (, Mnceu et l. ) served s VI Fe 3+ reference. Nontronite powder ws heted to 5 C for 3 hours to produce the nontronite dehydroxylte, nd then stored in seled tues filled prtilly with silic gel desiccnt. Powdered mounts of the references were prepred so s to prevent thickness effects on X- ry sorption mesurements ( µ <, Mnceu nd Gtes 997; Stern nd Kim 98), nd spectr were recorded with the smple rotted t 35 with respect to the electric field vector to eliminte texture effects (Mnceu et l. 99). X-ry diffrction Powder X-ry diffrction ptterns for nd were recorded on Siemens D5 diffrctometer equipped with Si(Li) solid-stte detector. CuKα rdition ws used with counting time of 5s per. θ step. The smple ws mounted in vcuum chmer (P = 5 6 tm) to prevent reoxidtion. The intensity distriution nd profiles of hk nds were clculted using the mthemticl formlism of Plnçon (98), Skhrov et l. (98, 98), nd Drits nd Tchour (99). Structure fctors for were clculted from the derived structurl formul N.3 [Si 7. Al.78 ][Fe Al.3 Mg. ]O 7.93 (OH) 5 (see Discussion ). As smll vritions in the tomic coordintes hve no significnt influence on the intensity of oth the 3 nd the 3 nds (Mnceu et l. ), the sme tomic coordintes were used for oth nd smples. Interlyer N ws shifted y.5 Å from the middle of the interlyer spce towrd the nontronite siloxne surfce in order to otin typicl distnces etween interlyer N nd the O toms defining the hexgonl cvity [d(n O) =..5 Å, iley 98]. Unit-cell prmeters nd = 6d(6) nd d() were determined from XRD. Turostrtic lyer stcking ws simulted y introducing % proility of rndom stcking fults, nd the size nd proportion of coherent scttering domins (CSDs) were determined y fitting the - nd profile nd ssuming tht the 3 diffrction line ws Lorentzin shped. Infrred spectroscopy Infrred spectr of the structurl OH stretching nd deformtion regions for oxidized () nd reduced () Grfield nontronite smples were otined on self-supporting films oriented perpendiculrly to the em direction. The window mteril (ZnSe) of the vcuum cell ecomes opque to IR rdition t out 65 cm, which overlps prt of the O-H deformtion region for the cly smples nd therefore defines the lower frequency limit for the nlysis. Infrred spectr were collected t resolution of.5 cm using Midc lser interferometer equipped with Kr emsplitter nd DTGS (deuterted triglyceride sulfte) detector. Dt collection nd nlysis were performed using the Grms/386 progrm. The spectrometer ws constntly purged y vporized liquid N to void H O nd CO contmintion. To mke comprisons mong the spectr, holders with hole of 5 mm dimeter were used to msk ech film, the pek intensities were normlized to mg/film, nd the offset ws djusted for some spectr. No seline djustment nd curve smoothing were done.

3 MANCEAU ET AL.: REDUCED NONTRONITE 3 ture goniometer in reflectnce mode with monochromtized FeKα rdition point focused to mm. Pole () figures were mesured in increments of 5 for tilt (ρ) nd zimuth (ϕ) with ngulr rnges etween ρ 85 nd ϕ 36. Possile smple inhomogeneities were suppressed with n oscillting smple holder. The method used to normlize the () pole figures nd express them in distriution densities ws detiled in the compnion pper (Mnceu et l. ). XAS spectroscopy Polrized (P-EXAFS) nd pre-edge spectr were recorded in LURE t Orsy, Frnce, on the D sttion. Dt cquisition nd reduction were s reported y Mnceu et l. (). Rdil structure functions (RSFs) uncorrected for phse shifts were otined from the Fourier trnsform (FT) of k- or k 3 -weighted EXAFS spectr (kχ or k 3 χ) to emphsize the contriution from the lower or higher k region, respectively. Intertomic distnces (R) nd numers of tomic neighors (N) for the first three shells (O, Fe, Tet) were determined using experimentl phse shift nd mplitude functions derived from polrized EXAFS spectr of (Mnceu et l. 998). In this minerl Fe is surrounded y six nerest O toms t n verge distnce of. Å, three nerest Fe toms t /3 = 3.5 Å, nd four nerest (Si,Al) toms t Å. From this reference, uncertinties for R nd N of pproximtely. Å nd % for the O, Fe, nd Tet shells cn e expected, ut the reltive ccurcy is etter. ecuse of the lrge reciprocl rnge explored in this study (3 k.5 Å ), nd the very high qulity of P-EXAFS spectr, the sensitivity to vritions in intertomic distnces is s little s. Å for these three shells. Functions for the long distnce (~5 Å) Fe shell were clculted initio y the FEFF7. code (Rehr et l. 99), nd their vlidity ws tested on Grfield nontronite. All reported N vlues refer to crystllogrphic vlues otined y correcting effective numers (N eff ) from the ngulr dependence term <3 cos θ > (Mnceu et l. 998). The sttic disorder in reduced smples is expressed s σ, the difference of root-men-squre stndrd devitions of distnces etween the reduced smples nd the oxidized Grfield reference. Norm. Intensity.3.. FePO -5 iotite OR Energy (ev) FIGURE. Fe K pre-edge spectr for oxidized (), reduced () nd re-oxidized/re-reduced (OR) Grfield nontronite, s compred to stndrds including IV Fe 3+ (FePO), % V Fe 3+ (-5 ), nd VI Fe + (iotite). RESULTS Pre-edge spectroscopy The oserved shift in energy (Fig. ) etween the iotite ( VI Fe + ) nd the ( VI Fe 3+ ) references reltes to differences in inding energies of the s electrons, nd to destiliztion of ntionding moleculr oritls s the forml vlency of the metl ion increses (Mnceu nd Gtes 997). Additionlly, the differences in the spectrl intensity etween iotite nd reflects the different degree of filling of the 3d electronic oritls for Fe 3+ (d 5 ) nd Fe + (d 6 ) ions, ecuse pre-edge spectroscopy directly proes the density of sttes of 3d-like electronic oritls. For given oxidtion stte, the pre-edge intensity depends on the symmetry of 3d moleculr oritls, nd on the tomic coordintion nd geometry (Mnceu nd Gtes 997). It increses from six- to five- to fourfold coordintion (Frges et l. 997). This evolution is pprent in Figure in order of unheted ( VI Fe 3+ ), heted ( V Fe 3+ ) Grfield nd FePO ( IV Fe 3+ ). During heting, phyllosilictes re dehydroxylted (rindley nd LeMitre 987; Drits et l. 995), nd the VI Fe sites of nontronites re trnsformed to fivefold sites t 5 following the rection OH structurl H O vpor + O structurl. The presence of V Fe in the nontronite structure decreses the crystl field splitting of the e g - nd t g -like components of the 3d electronic oritls (Dougls et l. 99), which explins the roder shpe of heted vs. non-heted smples. oth the OR nd preedge spectr re similr to tht of iotite nd re consistent with the presence of structurl VI Fe + (Fig. ). Without suitle reference, we cnnot definitively reject the presence of V Fe + species in the reduced smples. However, one my nticipte from theory, nd from results otined on heted ferric nontronites, tht the presence of fivefold Fe + species would hve resulted in significnt increse in the preedge Fe + intensity compred to iotite, nd to decrese of the splitting of 3d oritls. As will e shown elow, the sence of V Fe + is lso supported y the Fe + -O EXAFS distnce. The strong similrity of the spectr for iotite nd the reduced nontronites suggests tht essentilly ll Fe in the structure of nd OR is in the reduced stte. The mximum mount of unreduced iron ws estimted from liner comintions etween oxidized Grfield nd iotite spectr to e 5% s compred to >99% with the wet-chemicl nlysis. X-ry diffrction Powder XRD ptterns of nd (Fig. ) disply only l nd two-dimensionl hk nds typicl of pure turostrtic lyered structure (rindley nd rown 98). Thus, reduction of Fe 3+ to Fe + does not increse noticely the three-dimensionl ordering of nontronite. As expected from the lrger size of Fe + ions, the unit-cell prmeter of (9. Å) ws lrger thn tht of the oxidized smple (9.3 Å). The d() distnce of the reduced smple (9.73 Å) equls tht of indicting tht ll interlyers were collpsed in the dehydrted stte. The I - /I -3 rtio of remrkly decreses compred to (Fig. ). The intensity rtio of these two nds depends on the distriution of octhedrl ctions etween M nd M sites within the sme octhedrl sheet (Drits et l. 98; Mnceu et l. ). An

4 MANCEAU ET AL.: REDUCED NONTRONITE optimum fit ws otined with 8% of totl iron ions in M sites (Fig. 3). The M site occupncy ws vried to estimte the sensitivity of the method for determining the ction distriution over M nd M sites within the sme octhdrl sheet. Even smll increse of the M site occupncy from 8% to 33% of totl Fe in M sites (Figs. 3 nd 3, respectively) significntly modified the intensity rtio etween the - nd the -3 nds. All octhedrl Al ctions re ssumed to remin in M sites in the smple. Fitting of the -3 nd profile indicted tht lyers hve n orthogonl symmetry, with / rtio equl to 3.3. The ssumption of hexgonl lyer symmetry (/ = 3) yielded poorer fit with the simulted -3 nd eing shifted to higher d vlues (lower θ ngle) reltive to the experimentl nd (Fig. 3c). Three disk-shped CSDs were necessry to reproduce the profile of the - nd. They hd rdii of Å, Å, nd 6 Å nd their reltive undnce ws :5:6, respectively. In, two CSDs of Å nd Å rdii nd reltive undnce of :. were sufficient to fit the profile (Mnceu et l. ). Consequently, the reduction of nontronite results in significnt decrese of CSD dimeters. This decrese of the structurl order within the lyer plne cn e oserved in Figure 3d which compres the experimentl pttern for to the optiml simultion otined for : the - nd is clerly rodened in the reduced stte. Finlly, XRD results indicte tht 8% ± 5% of octhedrl Fe ctions, initilly entirely present only in M sites of the untreted nontronite, re present in M sites. The presence of occupied M (trns) sites implies tht Fe ions migrted from M to M sites in the nontronite structure due to the reduction of Fe 3+ to Fe +. IR spectroscopy O-H stretching region. For the oxidized minerl, the structurl O-H sorption nd ws reltively shrp nd centered ner 357 cm (Fig. ). This position is typicl for OH coordinted predominntly with two Fe toms ([Fe 3+ ] ) in the octhederl sheet of diocthedrl smectites (Russell et l. 979) nd is rther symmetricl, indicting high Fe 3+ content. Notice the very low sornce of H O stretching nds etween 3 nd 35 cm, confirming tht the method for minimizing interference due to dsored wter ws successful. FIGURE. Experimentl powder X-ry diffrction ptterns for the oxidized (, ove) nd reduced (, elow) Grfield nontronite smples. Diffrction mxim re indexed, nd d(6) is indicted for ech smple. FIGURE 3. Comprison etween experimentl nd clculted XRD ptterns. () Optimum fit otined for reduced Grfield nontronite () ssuming 8% of totl iron in trns sites nd 7% in cis sites. All luminum ctions re supposed to remin in cis sites. () XRD pttern clculted for with 33% of totl iron occupying trns sites. (c) XRD pttern clculted for ssuming n hexgonl lyer symmetry with = 3 = 5.37 Å insted of 5. Å in the optimum cse. (d) XRD pttern clculted for smple y using 5:6: rtio, s for smple, etween CSDs hving Å, Å, nd 6 Å rdii, respectively. Experimentl profiles re shown s crosses, nd the clculted ptterns re plotted s solid lines. All clcultions were perfomed y using the chemicl composition derived from the proposed model, nd tomic coordintes from Mnceu et l. (). For ll clcultions = 5. Å, = 9. Å, nd d() = 9.73 Å. Except for Figure 3d, :5:6 rtio etween CSDs hving Å, Å, nd 6 Å rdii, respectively, ws used.

5 MANCEAU ET AL.: REDUCED NONTRONITE 5 Reduction of structurl Fe chnged the entire sorption envelope (Fig. ). The OH signture is now rod locted t out 353 cm on more prominent nd centered ner 337 cm. We tenttively ttriute the ltter nd to strongly sored or ound H O tht survived dehydrtion. The chnge in position nd intensity of the primry structurl OH ctionic nd suggests mjor rerrngement in the OH environment fter reduction. Further evidence confirming this is the ppernce of shrp pek t 363 cm (spectrum ), ecuse this position is unchrcteristic for OH surrounded y ferric Fe in diocthedrl phyllosilictes (esson nd Drits 997; Slonimsky et l. 986). urns nd Strens (966) reported the existence of fundmentl O-H stretching virtions of [Fe + ] 3 OH t 365 cm nd 365 cm in two mphiole series (tremolite-ferroctinolite nd cummingtonite-grunerite). In iotite the center of the complex OH stretching nd ppers ner 36 cm (rshd nd Kishk 968). sed on the XRD evidence for 8% trns-site occupncy in the reduced nontronite, we propose tht the sorption nd t 363 cm is ttriuted to [Fe + ] 3 OH domins of triocthedrl Asornce Asornce Wvenumer (cm - ) A A Wvenumer (cm - ) FIGURE. Infrred spectr of oxidized (A) nd reduced (, h t 7 with dithionite) Grfield nontronite. () structurl hydroxyl stretching region; () structurl M-O-H deformtion region. chrcter. The nd ner 363 cm revels tht reprotontion occurred fter formtion of the triocthedrl chrcter. M-O-H deformtion region. The oxidized Grfield (spectrum A, Figure ) hs the clssic pek t 83 cm, ssigned to [Fe 3+ ] OH deformtion. Less certin is the interprettion of the pek t 83 cm. Goodmn et l. (976) ssigned it to either AlFe 3+ OH or OH lirtions ssocited with trns Fe. Stucki nd Roth (976) found tht the shift in this pek upon deutertion corresponds closely to tht expected if the oscilltors hd the msses of reduced Fe nd OH/OD, ut the sme effect would e oserved if the ending virtion is highly constrined. However, in spite of the uncertin ssignment of these nds, they together re typicl for, nd even dignostic of, nontronites (Russell et l. 979; Frmer, 97). Upon Fe reduction the 95 7 cm region ecme virtully fetureless, with oth [Fe 3+ ] OH lirtions disppering, nd thus presenting further strong evidence for the sence of diocthedrl chrcter in the reduced smectite. Huo (997) reported tht the disppernce of the [Fe 3+ ] OH nds in reduced nontronites ws ccompnied y the growth of new pek t 656 cm, which is ner the region where [Fe + ] 3 OH deformtion modes re expected (Wilkins nd Ito 967), thus further supporting the formtion of triocthedrl Fe + domins s demonstrted y XRD. Film chrcteriztion y texture nlysis From pole figures, the mximum density of orienttion equls.6 mrd for nd 3.6 mrd for OR (Fig. 5). These re less thn the vlue of 37. mrd otined for the oxidized smple () ut similr to tht of.3 mrd for the Wshington nontronite SW- (Mnceu et l. ). The two reduced smples hve FWHM of.9 () nd 38.8 (OR) s compred to 9.6 for nd 38. for SW-. Thus, the dispersion from perfect lignment of prticles in the film plne of the reduced smples is lmost identicl to tht otined for SW-. Mnceu et l. (998, 999) showed tht the difference etween N eff, estimted from the crystllogrphic structure ssuming n idelized texture, nd the vlue mesured y P- EXAFS (N exfs ) is equl to % t α = 9 for the Oct-Tet pir, nd to % t α = for the Oct-Oct pir ssuming continuous inclintion of ± of the c* xis symmetriclly round the film norml. Consequently, the dispersion of crystllites in nd OR ( ρ ± ) remins firly cceptle nd should only mrginlly ffect N eff vlues. P-EXAFS spectroscopy Angulr dependence of EXAFS spectr nd RSFs. The vrition of the EXAFS spectr (Fig. 6) s function of experimentl ngle (α) is pronounced in the.5 Å rnge, confirming the successful preprtion of highly oriented films in the reduced stte, just s ws otined for the oxidized smples (Mnceu et l. ). However, mrked difference from the EXAFS spectr of oxidized Grfield (see Fig. 7 in Mnceu et l. ) is the loss in mplitude of k 3 χ(k) ove Å, reflecting decresed short-rnge order in reduced nontronites. Due to this loss in spectrl mplitude, the ngulr vrition of the mplitude is diminished nd ecomes progressively smered out in the noise.

6 6 MANCEAU ET AL.: REDUCED NONTRONITE FIGURE 5. Pole figures for the reflection (left) nd corresponding rdil distriution densities (right) for () nd OR (). The distriution mxim re for the centre of the pole figures (ρ = ). The mrd level (perfectly rndom powder) is indicted y dshed line. Liner density scles nd equl re projections re used for the pole figures. The out-of-plne (α = 9 ) spectr (Fig. 6) were clculted using liner regression method descried elsewhere (Mnceu et l. 988, 998). The qulity of this lineriztion cn e ssessed visully y overplotting experimentl nd reclculted EXAFS spectr t ny prticulr experimentl ngle ( α 6 ), nd sttisticlly y clculting profile reliility fctors (R p, Tle ) etween experimentl nd reclculted spectr. This nlysis (Figs. 7 nd 7; Tle ), shows tht the experimentl k 3 χ(α = ) spectrum for is indistinguishle from tht reclculted from regression. The men vlue of R p (<R p >) verged over the 5 experimentl ngles ws.9 3 () nd.3 3 (OR). These firly low vlues re similr to tht otined for the oxidized smple (.8 3 ), nd provide confidence for the clcultion of k 3 χ(α = 9 ) theoreticl spectr. R p vlues (Tle ) lso give n estimte of the precision for the mesurement of EXAFS spectr. The experimentl k 3 χ(α = ) nd reclculted k 3 χ(α = 9 ) spectr nd OR (Figs. 7c nd 7d) er strong resemlnce t oth α = nd α = 9, which indictes tht the two reduced smples possess essentilly the sme in-plne nd outof-plne structures. Severl oscilltions of OR hve slightly higher mplitude thn, suggesting tht the two smples hve slightly different structurl order. Note tht this difference of spectrl mplitude etween nd OR cn not e interpreted y difference of texture strength ecuse OR, which hs the highest texture, would hve lrger ngulr dependence nd, consequently, higher spectrl mplitude t α = nd lower mplitude t α = 9 thn. The oscilltions peking t k = 6 nd 6.5 Å (Figs. 7c nd 7d) show tht OR hs insted higher spectrl mplitude for the two ngles. Aprt from this smll difference, interpreted elow, the two smples oviously hve similr structure, nd for this reson we focus on. The comprison of oxidized () nd reduced (/OR) P-EXAFS spectr is lso insightful. EXAFS spectr hve higher frequency in the reduced thn in the oxidized stte regrdless of the orienttion ngle. This phse shift reflects difference in Fe-O nd Fe-Fe distnces cused y the lrger size of Fe + compred to Fe 3+. If one excludes this difference of wve phse, the k 3 χ(α = 9 ) spectr (Fig. 7d) of the oxidized

7 MANCEAU ET AL.: REDUCED NONTRONITE α ngles k 3 χ(k) OR 6 8 FIGURE 6. k 3 -weighted Fe K-edge P-EXAFS spectr for () nd OR () t α ngles of,, 35, 5, 6, nd 9, where α is the ngle etween the electric field vector nd the film plne. The 9 spectrum hs een otined y regression of the experimentl mplitudes for α 6. The mplitude of χ decreses with incresing α t 6. Å, nd increses with α t 5 Å. Note the presence of isosestic points where χ(k,α) is independent of k. k 3 χ(α= ) exp k 3 χ(α= ) reclc k 3 χ(α= ) exp k 3 χ(α= ) reclc 5 3 k 3 χ(k) k 3 χ(k) -5 Q = 3 - α = 8 c 6 8 OR - 8 d 8 OR k 3 χ(k) k 3 χ(k) α = α = FIGURE 7. () Experimentl nd reclculted k 3 χ (α = ) functions for. () Zoom of (). (c) Comprison of experimentl k 3 χ (α = ) functions for, OR, nd. (d) Comprison of experimentl k 3 χ (α = 9 ) functions for, OR, nd.

8 8 MANCEAU ET AL.: REDUCED NONTRONITE TALE. Profile reliility fctor (R p ) etween experimentl nd reclculted P-EXAFS spectr α = α = α = 35 α = 5 α = 6 < R p > OR Notes: R p is the figure of merit for the spectrl fitting, R p = Σ (k 3 χ exp - k 3 χ th ) / Σ (k 3 χ exp). nd reduced nontronite hve similr shpe, wheres k 3 χ(α = ) spectr (Fig. 7c) do not. The only sustntil difference in the k 3 χ(α = 9 ) spectr is the ppernce of shoulder t 3. Å (rrow in Fig. 7d), indicting tht less severe structurl modifictions my e expected in the tetrhedrl thn in the octhedrl sheets of reduced nontronite. In k 3 χ(α = ) for nd OR the wve mximum t. Å is lost wheres the mximum t 5. Å is mplified (see rrows), nd the reltive intensity of the two mxim t 7. Å nd 8. Å is reversed in the two oxidtion sttes. Lstly, the loss in mplitude of k 3 χ(k) with incresing k noted previously in Figure 6 is more mrked in the inplne thn in the out-of-plne orienttion when we compre oxidized nd reduced smples. This oservtion suggests tht the source of disorder origintes from distnt Fe-Fe pirs ecuse the high k rnge is dominted y the contriution of Fe toms (see Fig. in Mnceu et l. ). All spectrl modifictions noted in the prllel orienttion indicte tht the reduction of Fe impcts the structure of the octhedrl sheet of nontronite nd, specificlly, the coherency of Fe-Fe interctions. Six peks re seen in the RSFs corresponding to k- nd k 3 - EXAFS spectr for (Fig. 8) (lso referred to s FT[kχ] nd FT[k 3 χ]). Pek A cn e ssigned to the nerest oxygen shell (O) ; pek results from the comined effect of the nerest Fe, the nerest (Si,Al = Tet) nd the next-nerest oxygen shells (Fe + Tet + O); pek D is ssigned to the next-nerest (Si,Al) shell (Tet); nd pek E to the next-nerest Fe shell (Fe) (Mnceu et l. 998; see Fig. in Mnceu et l. ). The polriztion dependence of ech pek is similr to tht found in (Mnceu et l. 998), nd conforms to the structure of phyllosilicte. Two dditionl peks, C nd F, re not oserved in oxidized Grfield. The former hs mximum intensity t α = 9, nd the second t α =. These two peks deserve specil considertion nd they will e nlyzed elow, following the nlysis of the O shell. First oxygen shell nlysis. The first oxygen shell contriution of (Pek A) is much lower in mplitude thn tht of (see Fig. in Mnceu et l. ). In the oxidized stte the mplitude of pek A is 3 (ritrry unit) compred to in the reduced stte. This mplitude reduction is not ssocited with loss of oxygen nerest neighors ecuse preedge spectroscopy indictes tht Fe + is locted in sites coordinted y six O toms. Insted, the decrese in mplitude suggests tht verge Fe + -O distnces in reduced nontronite re more incoherent thn verge Fe 3+ -O distnces in oxidized nontronite. To estimte the chnges in the Fe-O distnces with reduction, pek A for ws Fourier ck-trnsformed in the [..] Å rnge nd, y lest-squres procedures, the verge Fe + -O distnce nd the numer of O toms coordinted to Fe + FT(k 3 χ) FT(kχ) α= peks A,, E nd F α=9 α= peks A,, E nd F α=9 Fe-O A A Fe-Fe Fe-Si Fe-O C D E were determined. The precision of N nd R ws estimted y ltering the spectrl djustment y R p. The est spectrl fit (R p = -3 ) ws otined y ssuming 5.3 ±.8 O toms t. ±. Å ( σ =. Å) (Fig. 9). A Fe-O ond length of. Å is chrcteristic of VI Fe + species. oth the. Å increse in the differentil disorder term ( σ), nd the decrese of N O from 6. to 5.3, reltive to, re significnt nd originte from the greter spred in the verge Fe-O ond distnce in the reduced stte. ecuse N nd σ re interdependent (Teo 986), the incoherency of Fe-O distnces nd the pprent loss of O neighors cn oth e ccounted for y simultneous increse in σ nd decrese in N O. The mximum mount of unreduced VI Fe 3+ in ws estimted y performing two-shell fits with differing N O vlues for the su-shells t. Å (Fe 3+ - O) nd. Å (Fe + -O), while fixing the other prmeters to their vlue in the reference ( E = σ =.). The ddition of 7% VI Fe 3+ (N O =.) resulted in douling of R p (Fig. 9), nd indictes tht introducing smll mount of Fe 3+ lters the mtch etween the two phses in the 9 Å < k < 3 Å intervl, s well s ffects the mplitude envelopes. We cn conclude from Fe-O 3 C Fe-Si Fe-Fe E Fe-Fe 3 FIGURE 8. k 3 -weighted () nd k-weighted () Fe K-edge polrized RSFs for t α ngles of,, 35, 5, 6, nd 9. The mplitude of peks A,, E, nd F decreses with incresing α. F F

9 MANCEAU ET AL.: REDUCED NONTRONITE 9 this nlysis tht Fe + represents t lest 93% of the totl Fe in the reduced Grfield smple, which is in greement with preedge nd chemicl nlysis results. Anlysis of out-of-plne contriutions. Appliction of P- EXAFS to lyer silictes is powerful technique which llows the isoltion nd sutrction of the minor tetrhedrl sheet contriution from the predominnt octhedrl sheet contriution (Mnceu et l. 998; 999). Certin spectrl fetures originting from the tetrhedrl sheet provide structurl informtion. Comprison of FT[k 3 χ(α = 9 )] nd FT[kχ(α = 9 )] functions shows tht the Fe-Tet contriution (pek ) hs lower mplitude in the reduced smple (Figs. nd ). The loss in mplitude my originte either from decrese in the numer of Fe-Tet pirs (for exmple, s consequence of migrtion of Fe + out of the cly structure) or from wider distriution of the Fe-(Si,Al) distnces. Determintion of which fctor is responsile cn e chieved y compring clculted nd experimentl envelopes of the Fourier filtered EXAFS contriutions [χ Tet (α=9 )]. As result of structurl loss of Si,Al nerest neighors (first possiility), the wve mplitude will decrese, ut its shpe will e preserved [A Si (k) function]. On k3χ(k) k 3 χ(k) Experimentl Theoreticl % VI Fe + α = 35 k 3 χ Fe-O Experimentl Theoreticl 93% VI Fe + +7% VI Fe 3+ α = 35 k 3 χ Fe-O 6 8 FIGURE 9. Fourier-filtered k 3 χ Fe-O (α = 35 ) contriution to EXAFS spectrum of. () est spectrl fit (R p = 3 ) otined y ssuming 5.3 ±.8 O toms t. ±. Å ( σ =. Å). () Spectrl simultion ssuming mixture of 7% VI Fe 3+ nd 93% VI Fe +. the contrry, wider distriution of the Fe-Tet distnces (second possiility) increses the wve dmping, or signl loss, in k spce (i.e., increse of σ). Figure shows tht the second interprettion is the more pproprite explntion for the loss in mplitude of the Fe-Tet contriution (pek, Fig. ) ecuse the k 3 χ Tet (α = 9 ) spectr for nd hve similr mplitudes ner Å ut not t greter k nd the reltive difference in mplitude increses progressively from ~5 Å to ~ Å. Also, hs slightly higher wve frequency, indicting tht the Fe-Tet distnces re lrger in the reduced stte. Assuming single Fe-Tet distnce provided n pproximte fit to this spectrum (R p =.7, Fig. ). However, the phse mismtch t higher k vlues suggested the presence of t lest two discreet Fe-Tet distnces. Thus, two-shell fit to k 3 χ Tet (α FT(k 3 χ) FT(kχ) FT(kχ) c Fe-Si Fe-O 3 C Fe-Si Fe-O 3 C C D D D α = 9 α = 9 iotite α = 9 FIGURE. RSFs functions in the norml orienttion. () k 3 - weighted RSFs for nd. () k-weighted RSFs for nd. (c) k-weighted RSFs for nd triocthedrl iotite smple.

10 MANCEAU ET AL.: REDUCED NONTRONITE = 9 ), ssuming R Tet = 3.8 Å, N Tet = 3. ±.5 ( σ =. Å) nd R Tet = 3. Å, N Tet =.9 ±. ( σ =. Å) yielded etter greement over the entire k rnge (R p =., Fig. c). The precision on N ws estimted from R p vrition of the fit qulity. Pek C is sent in (Fig. ), ut is present in the reduced smples. Comprison of the FT[k 3 χ(α = 9 )] (Fig. ) nd FT[kχ (α = 9 )] (Fig. ) RSFs functions shows tht the mplitude of pek C is incresed y k-weighting reltive to peks (Fe-Tet) nd D (Fe-Tet), which indictes tht it is k3χ(k) k3χ(k) k 3 χ(k) c α = 9 k 3 χ Fe-Tet 6 8 Experimentl Theoreticl α = 9 k 3 χ Fe-Tet One shell fit 6 8 Experimentl Theoreticl α = 9 k 3 χ Fe-Tet Two shell fit 6 8 FIGURE. Fourier-filtered k 3 χ Fe-Tet (α = 9 ) contriutions to EXAFS spectr. () Comprison of prtil EXAFS spectr for nd. () One shell fit for. (c) est two-shell spectrl fit for. ssocited with out-of-plne oxygen toms (Mnceu et l. ). Mnceu et l. (998) showed y initio FEFF clcultions tht this O3 tomic shell is locted on the sl plnes of cly lyers (see Fig. in Mnceu et l. ). Pek C is sent in diocthedrl cly structures, ecuse of corrugtion of the sl oxygen plne (Lee nd Guggenheim 98), which increses the verge spred in the four Fe-O3 distnces. Triocthedrl structures generlly hve less corrugted oxygen sl plne nd, consequently, less difference is found in the distnces of the four third-nerest O3 toms from the centrl Fe tom. In the triocthedrl phyllosilicte iotite, this O3 shell is inclined y ~37 off c* (Tked nd Ross 975), thus its contriution is gretly enhnced in the perpendiculr orienttion. This effect is displyed in Figure c, comprison of the RSFs for nd iotite (no. 3 in Mnceu et l. 99). The similrity of the two FT[kχ (α = 9 ] functions in the 3. Å < R + R <.5 Å intervl is remrkle, nd constitutes strong rgument in fvor of the formtion of triocthedrl Fe + clusters in the octhedrl sheet of reduced Grfield s implied y XRD nd IR nlyses. Anlysis of in-plne contriutions Assignment of RSFs peks. The in-plne orienttion (Fig. ) mplified the contriutions from successive Fe shells. In ddition to the prominent Fe-Fe contriutions, O3 (pek C), nd Tet (pek D) shells from the tetrhedrl sheet re still detected, ecuse these pirs re not orthogonl to the plne (Fig. in Mnceu et l. ). The O shell () is detected to the right of the Fe-Fe () contriution in when the Fourier trnsform is performed on kc (Fig. ), ut not in. In the reduced stte Fe + + -Fe () nd Fe + -O () contriutions overlp for steric resons, nd re too close to seprte y Fourier trnsform. Up to three Fe-Fe contriutions (, E, nd F in Fig. ) cn e identified in the RSF for. Successive Fe shells re represented in Figure 3 y circles of rdius R. Pek, t n pprent (uncorrected for phse shift) distnce of R Fe + R =.8 Å, nd pek E, t R Fe + R = 5 Å, correspond to the nerest Fe shell t R Fe /3 = 3.7 Å, nd to the next-nerest Fe shell t R Fe = / 3 = 5.3 Å, respectively, s for (Mnceu et l. 998). In pek F is t R + R = 5.9 Å, so tht R ~R Fe. This simple nlysis indictes tht pek F corresponds to third Fe shell t R 6. Å. The Fe3 shell contriution is more intense in triocthedrl lyer structures (Fig. c) owing to the incresed lignment long [], [3] nd [3 ] of Fe, Fe, nd Fe3 toms, which mplifies multiple scttering (MS) pths of the photoelectron (Fig. 3) (Mnceu et l. 998). This focusing effect is commonly oserved in lyered metl oxides nd hydroxides (O Dy et l. 99), nd occurs when toms re ligned with little vrition in the ction-ction distnces. In triocthedrl cly structures ll the odd shells (Fe, Fe3, Fe5 ) re ligned long [], [3], nd [3 ] with periodicity of /3 = Å, nd, s such, their contriution is systemticlly reinforced. This contrst with even shells, which re ligned long [], [], nd [ ], nd re seprted y much longer distnces, pproximtely = 5.3 Å. The Fe6 shell is thus too fr (~.6 Å) to e detected. The Fe shell is t shorter distnce (~8. Å)

11 MANCEAU ET AL.: REDUCED NONTRONITE thn Fe6, nd might e detected, ut does not occupy focussing position s there is no Fe tom locted etween Fe nd the centrl Fe sctterer. Consequently, Fe is the only even shell detected in the RSF of phyllosilictes (pek E, Fig. ). In diocthedrl structures such s, the Fe3 shell (pek F) is not detected ecuse t the Fe rdil position long the Fe-Fe-Fe3 MS pth no tom is present (Fig. 3) (Mnceu et l. 998). The ssignment of pek F to Fe shell is confirmed y inverse Fourier trnsform nlysis (see elow). The ppernce of this Fe3 contriution, nd the persistence of the Fe contriution fter reduction of structurl Fe 3+ provides evidence for the formtion of triocthedrl Fe + domins within the octhedrl lyers of reduced nontronite. This result is consistent with the nlysis of the out-of-plne structure (cf. previous section), from which this structurl trnsformtion ws inferred from the loss of corrugtion of the oxygen sl plne through the ppernce of the sl plne Fe-O3 contriution (Fig. ). The nlysis of Fe-Fe contriutions t the in-plne orienttion lso offers clue for understnding the structurl mechnism Triocthedrl structure [3] FT(kχ) FT(kχ) FT(k 3 χ) c Fe-Fe + Fe-Si Fe-O 3 Fe-Si C D Fe-Fe Fe-O C E Fe-Fe Fe-Fe E F Fe-Fe 3 F α = α = Mic α = [] Fe 5 Fe 3 Fe Fe Fe [3] Diocthedrl structure Fe Fe 5 Fe 3 Fe Fe [] 3 C E F FIGURE. RSFs functions in the prllel orienttion. () k 3 - weighted RSFs for nd. () k-weighted RSFs for nd. (c) k-weighted RSFs for nd triocthedrl iotite smple. FIGURE 3. Representtion of the successive Fe shells in the octhedrl sheet of triocthedrl () nd diocthedrl () lyer silicte.

12 MANCEAU ET AL.: REDUCED NONTRONITE of the reduction process in nontronite. Given the structurl importnce of the modifiction oserved in the in-plne RSF, whether or not the Fe3 shell cn e detected on rw EXAFS spectr would e of interest to ssess its reliility. This nlysis ws performed y compring the full experimentl EXAFS spectrum for to n inverse Fourier trnsformtion of the prtil RSF over R + R rnge of [ 5. Å] y excluding pek F. Figure shows tht exclusion of the Fe-Fe3 contriution sustntilly modifies the spectrl shpe. This modifiction is sttisticlly significnt ecuse the R p vlue etween the two spectr is equl to.36, which is pproximtely n order of mgnitude greter thn the estimted precision for the spectrl mesurement (R p =, Tle ). The most spectculr modifiction is oserved t k = 7.9 Å, where the ppernce of Fe-Fe3 pirs t R ~6. Å yields mrked node pttern tht reduces the mplitude of the wve oscilltion. Given the high qulity of EXAFS spectr, this spectrl feture hs undoutedly structurl origin. A similr nlysis ws conducted for the perpendiculr orienttion. This tretment reveled tht the Fe-O3 contriution is responsile for k3χ(k) k 3 χ(k) α = Q= α = Q = FIGURE. () Experimentl k 3 -weighted EXAFS spectrum for t α = (solid line), nd Fourier filtered k 3 χ(α = ) function for in the [, 5. Å] R + R intervl (dotted line). () Experimentl k 3 - weighted EXAFS spectrum for t α = (solid line), nd Fourier filtered k 3 χ(α = ) function for in the [ Å] R + R intervl (dotted line). the shoulder t k = 3. Å oserved in Figure 7d. The loction of the spectrl signture for the O3 shell t lower k (3. Å ) thn the Fe3 contriution (7.9 Å ) results from the difference in shpe of A O (k) nd A Fe (k) functions s shown in Figure of Mnceu et l. (). Another importnt difference in the in-plne FT[k n χ(α = ] functions for nd ws identified ove 6 Å. Within this more distnt rnge (Fig. 5) pek t R + R = 8.6 Å (G) is oserved for, which is pproximtely twice s intense s the surrounding ckground signl. Note tht this pek is sent in. The structurl relity of this pek ws further ssessed y compring the full experimentl k 3 χ(α = ) function with tht reclculted from the RSF y ck-trnsforming the [ 8.3 Å] rnge. The two EXAFS spectr differ only y the intensity of severl oscilltions (dt not shown). The lck of distinct spectrl feture in k spce ttriuted to pek G, in contrst to tht oserved previously for pek F (Fig. ), my e explined y the overll low contriution tht this tomic shell mkes to the whole EXAFS spectrum. Experimentl nd reclculted spectr differed, however, y R p =, which is.5 times higher thn the estimted experimentl precision (8, Tle ). This difference is due to RSFs peks, which were not included in the ck FT window, nd tht re locted in the [8.3 Å] rnge. In this domin, RSFs peks rise from the Fourier trnsformtion of the medium to high frequency signl in the EXAFS spectrum. This signl origintes prtly from the sttisticl high frequency counting noise of the mesurement, ut still includes some structurl contriutions from distnt Fe shells. Thus, for the ske of completeness in our ssessment of the structurl origin of pek G, the entire EXAFS spectrum of ws reclculted over the whole [ Å] R + R window, nd compred to the experimentl spectrum (Fig. ). The mplitudes of oscilltions re similr, the only difference eing tht the reclculted spectrum hs no high frequency noise. The two spectr in Figure differ y R p = 5, which should e priori identicl to tht determined from the lineriztion procedure (8, Tle ). One reson for the difference oserved is tht the [, Å] rnge contins minor Fe-Fe contriutions tht re progressively lost in ckground noise, nd which re not tken into ccount when the Fourier trnsform is performed in the [ Å] rnge. From these considertions, pek G is likely structurl nd cn e ttriuted to the Fe5 shell t R = = 9.5 Å (Fig. 3). The presence of this pek in the oxidized nontronite, despite the long distnce, is explined y the existence of high order MS pths long the Fe-Fe-Fe5, nd Fe-Fe3-Fe5 chins. Consequently, the disppernce of pek G in indictes tht the reduced smple hs no, or very few, Fe toms in the 5th Fe shell. Thus, tht the formtion of triocthedrl Fe + clusters is glolly relized y occupncy of formerly vcnt octhedr in the Fe shell, nd y concomitnt deprture of Fe toms from the Fe5 shell. The Fe + domins therefore hve limited lterl extension, nd the migrtion of Fe + toms should e restricted to short distnce, for instnce from cis-occupied to djcent trns-vcnt sites. Structurl prmeters of reduced nontronite. Three Fe shells were identified in the RSF of reduced nontronite t α = : Fe (), Fe (E), nd Fe3 (F) (Fig. ). Eqution 9 in Mnceu

13 MANCEAU ET AL.: REDUCED NONTRONITE 3 et l. () shows tht for given tomic pir ij (i.e., given φ ij function), the wve frequency of χ ij is determined y the distnce (R) seprting toms i nd j. Thus, the comprison of wve frequencies for Fe-Fe nd Fe-Fe pirs in oxidized nd reduced smples provides insight on the modifiction of distnces to different Fe shells during the redox rection. This nlysis is generlly chieved y superimposing ptril EXAFS spectr otined y Fourier ck-trsnsforming specific RSF peks. Alterntively, comprison of the imginry prts of the forwrd (k R) Fourier trnsform my e used (Teo 986). The imginry prts for nd closely overlp t R + R =.7 FT(k 3 χ) FT(k 3 χ) FT(kχ) 8 6 E Fe-Fe Fe-Fe 3 F Fe-Fe 5 OR c Fe-Fe + Fe-Si G α = Fe-Fe E Fe-Fe 3 F Fe-Fe + Fe-Si Fe-O 3 Fe-Fe E Fe-Fe 3 F α = α = FIGURE 5. () Expnded view of the Å intervl of k 3 - weighted RSFs functions in the prllel orienttion for, nd OR. (, c) Modulus nd imginry prt of Fourier trnsforms for nd t α =. () k 3 -weighting. (c) k-weighting. Å (rrow in Figs. 5 nd 5c) nd.7 Å (rrow 3), ut the phse of progressively shifts to the right when R increses (rrows nd ). In oxidized Grfield, Fe toms in the Fe nd Fe shells re locted t the coherent distnces of 3.5 Å (/3) nd 5.8 Å (), respectively, from the centrl Fe. Consequently, the imginry prt elow peks nd E in possess unique frequency. Thus, the progressive shift in phse oserved in indictes tht Fe-Fe distnces re incoherent within the Fe nd Fe shells. Pek E in is doulet (Fig. 5), which suggests imodl distriution of the Fe-Fe distnces. The incoherence of the Fe-Fe, distnces is source of structurl disorder, nd explins the wekening of peks nd E (Fig. 5) in the reduced stte reltive to the oxidized stte. However, the mplitude loss is more pronounced on FT[k 3 χ(α = ] thn on FT[kχ (α = ] functions (Figs. 5 nd 5c). To understnd this difference, we compre k 3 χ Fe nd k 3 χ Fe functions in the two redox sttes (Figs. 6 nd 7). The wve envelopes of these two functions differ sustntilly. For, the k 3 χ Fe nd k 3 χ Fe spectr hve the chrcteristics of single Fe shell contriution with n mplitude mximum t 8 Å s predicted from theory (see Fig. in Mnceu et l. ). The ssumption of unique structurl distnces of 3.5 Å (/3) nd 5.8 Å () in Fe nd Fe shells yielded good fits to these spectr with R p vlues of. (Mnceu et l. ) nd. (Fig. 6). However, the two k 3 χ Fe contriutions of exhiit different spectrl shpe with mximum t low k ( 6 Å ) followed y rpid dmping of the mplitude t incresing k (Figs. 6 nd7). A similr oservtion ws mde for k 3 χ Tet in (Fig. ) s result of the incoherence of the Fe-(Si,Al) distnces. Thus, the dispersion of intertomic distnces oserved t α = 9 clerly is not limited to the Tet shell, ut extends lso to the Fe nd Fe shells. This in-plne nd out-of-plne structurl disorder detected on k 3 χ Tet nd k 3 χ Fe, functions ccounts for the severe dmping in mplitude of the whole EXAFS spectr (Figs. 7c nd 7d) for reduced smples reltive to. For instnce, the mplitude of k 3 χ Fe for equls.8 (ritrry unit) t.8 Å (rrow in Fig. 6), nd the mplitude of the whole EXAFS spectrum t the sme k vlue is.8 (Fig. 7c). This comprison indictes tht the Fe shell lone contriutes s much s 56% of the entire EXAFS signl in the high k region t α =. Therefore, the high wve-vector rnge of the rw EXAFS spectr must e dominted y Fe-Fe interctions t α = (i.e., inplne), nd y the Fe-Tet t α = 9 (i.e., out-of-plne). This nlysis indictes, therefore, tht the lrge dmping of the EXAFS signl in the reduced stte results from the vrition of intertomic Fe-Tet nd Fe-Fe distnces in. The dmping of the Fe-Fe signl is more pprent on k 3 - weighted thn k-weighted Fourier trnsforms (Fig. 5). The reson for this ehvior is now evident when we compre k 3 χ Fe, spectr for nd (Figs. 6 nd 7). FT[k n χ] functions re otined y integrting k n χ spectr over k (Eq. in Mnceu et l. ) nd, consequently, the structurl Fe-Fe disorder, pprent t k > ~6 Å, is emphsized when FTs re performed on k 3 χ. In contrst, multipliction of χ y k decreses the weight of the high k region reltive to the low k region. In this cse, hs slightly higher χ Fe, mplitude thn. Thus, less emphsis is given to in-plne structurl disorder in the k-weighting.

14 MANCEAU ET AL.: REDUCED NONTRONITE Oviously, the Fe nd Fe shells in include severl Fe-Fe distnces. The ssumption of single Fe shell of.9 Fe t <R> = 3. Å ( σ =.3 Å) provided resonle fit to χ Fe (R p =., Fig. 6c). In this simultion, the distriution of distnce ws modelled y ssuming Gussin distriution of distnces ( σ), nd the Fe shell ws fitted y.9 Fe locted t n verge distnce of 3. Å with continuous distriution of.3 Å hlf-width. The distriution of the Fe-Fe distnces k3χ(k) k3χ(k) k3χ(k) Experimentl Theoreticl c α = k 3 χ Fe-Fe α = k 3 χ Fe-Fe Experimentl Theoreticl α = k 3 χ Fe-Fe 6 8 FIGURE 6. () Comprison of Fourier filtered χ Fe-Fe (α = ) functions for nd. () est one shell spectrl fit of χ Fe-Fe (α = ) for. (c) est one-shell spectrl fit of filtered χ Fe-Fe (α = ) for. is too smll to e resolved, nd the fit would e insignificntly improved y ssuming two discrete Fe-Fe distnces. The precision of N Fe ws estimted y vrying σ: σ =. Å yielded N Fe = 3.9 nd R p =., nd σ =. Å yielded N Fe = 6. nd R p =.. When R p ws.5 to 3. times greter thn its optiml vlue, the spectrl fit ws visully degrded, leding to the conclusion tht the intervl represents the mximum rnge for the vrition of N Fe (±%), N Fe likely eing etween.5 nd 5. (±%). Individul Fe-Fe distnces re less overlpping in the Fe thn in the Fe shell, s ttested y the splitting of pek E (Fig. 5), nd the node pttern t ~9 Å (Fig. 7). This wve node ws stisfctorily reproduced y ssuming two discrete su-shells composed of ~.7 Fe t.86 Å ( σ =.3 Å) nd ~3. Fe t 5.38 Å ( σ =.3 Å) (Fig. 7). In generl, the precision of N decreses with distnce from the centrl tom, nd in the present cse hs een estimted to e ± Fe. The reltive precision is etter, nd we my estimte tht the long distnce su-shell t 5.38 Å contins to 3 times more Fe thn the closer shell t.86 Å. The wve envelope of the third Fe shell (k 3 χ Fe3 ) for (corresponding to pek F in Fig. 5) shown in Figure 7c differs unexpectedly from k 3 χ Fe, ut resemles k 3 χ Fe (Fig. 7), indicting tht the Fe-Fe3 distnces re coherent. This finding ws confirmed y overlying k 3 χ Fe nd k 3 χ Fe3 (Fig. 7d), nd y clculting the theoreticl mplitude function k 3 A Fe (k) = k 3 F Fe (k) exp ( σ k ) exp [ R/λ(k)] for single Fe shell contriution with the FEFF 7. code (detils re in Mnceu et l. ). The experimentl k 3 χ Fe,3 curve envelopes nd k 3 A Fe (k) hve the sme line shpe (Fig. 7d), which proves tht the Fe3 shell in smple cnnot e split into two su-shells, s ws the cse for the Fe shell. Therefore, the structurl disorder oserved in the Fe shell, nd predominntly in the Fe shell of, is less importnt in the Fe3 shell. ecuse the Fe nd Fe3 shells re ligned long the [], [3], [3 ] directions, wheres the Fe shell is oriented long the [], [], [ ] directions, the disorder induced y reduction of structurl Fe is nisotropic nd is more pronounced in directions of the Fe shell (Fig. 3). Structurl differences etween nd OR smples nd OR hve similr in-plne nd out-of-plne EXAFS spectr (Figs. 7c nd 7d), differing only y slight increse in mplitude of the twice reduced smple. To interpret these spectrl differences we compre (Fig. 8) the two RSF functions t α = nd 9. The two RSFs hve, for ech orienttion, the sme peks t the sme distnces, which mens tht the two reduced smples hve the sme overll structure. ut, s ws predicted from mplitude differences of k 3 χ (Fig. 7), OR hs generlly more intense RSF peks, nd thus higher shortto mid-rnge structurl order thn. The fct tht peks A nd re more intense in OR thn in oth t α = nd 9 implies tht this must e lso the cse t α = 35 nd, consequently, tht these RSFs differences hve structurl rther thn texturl, origin. The increse in pek mplitude does not uniformly ffect the vrious tomic pir contriutions nd is predominntly oserved for the O, Fe, Fe, Fe3, nd to some extent the Si, shells, which suggests tht Fe toms re prtitioned differently in the two smples. Proper structurl inter-