Clay Minerais (1993) 28, 165-184

LOCAL STRUCTURE OF FERRIHYDRITE AND FEROXYHITE BY EXAFS SPECTROSCOPY A. MANCEAU

AND

V. A. DRITS*

LG/T, Université Joseph Fourier, BP53X, 3804/ Grenoble, France and * Geological /nstitute of the Russian Academy of Science, 7 Pyzhevsky prospekt, /09017 Moscow, Russia (Received 30 March /992; revised 3 November 1992)

AB STRACT: Synthetic 2-line and 6-line ferrihydrite and feroxyhite samples prepared from ferric salt solutions have been investigated by EXAFS spectroscopy. All these mate rials have been found to be short-range ordered, consisting of Fe octahedra linked by corners, edges, and faces. Their local structures are related to those of well-crystallized (oxyhydr )oxides, and the absence of hkl reflections in sorne samples is attributed to the small size of coherent scattering domains. The presence of face sharings indicates that these materials have structural similarities with hematite. Based on Fe-Fe distances and the analysis of the static disorder, it has been concluded that the local structure of feroxyhite is close to that of hematite, whereas ferrihydrite has common structural features with both hematite (aFe20J) and œ/f3FeOOH. The local structure of ferrihydrite thus differs from that of aqueous Fe polymers obtained by the partial hydrolysis of ferric nitrate and chIo ride solutions. Differences of local structures among hydrous Fe oxides and aqueous polymers have been interpreted on the basis of a room temperature stability phase diagram established for wellcrystallized (oxyhydr)oxides.

Ferrihydrite and feroxyhite (o'FeOOH) are common oxyhydroxides of the Earth's surface (Jenne, 1968; Chukhrov et al., 1973; Schwertmann, 1988). Ferrihydrite (Fh) is found in soils, weathered rocks, sediments and in the water column of lakes, rivers and oceans. Conversely, few continental occurrences of feroxyhite (Fx) have yet been reported (Carlson & Schwertmann, 1980). Although of an elusive nature, Fx is, however, widely present on the oceanic sea fioor where it is intimately mixed with vernadite (oMn02, Chukhrov et al. , 1976; Manceau & Combes, 1988; Manceau et al., 1992a). "Ferrihydrite" is a generic term which is used to name materials with various degrees of crystallinity (Chukhrov et al., 1973b; Schwertmann & Fischer, 1973; Carlson & Schwertmann, 1981). X-ray diffraction (XRD) patterns of most disordered Fh contain only two broad hk bands at 2·5-2·6 À and 1·5 À. The absence of hkl peaks refiects extremely small sizes of the coherent scattering domains (CSD) as weIl as disorders in anionic packings (Chukhrov et al., 1988; Drits et al., 1992b). At the opposite extreme, XRD patterns of less disordered Fh contain sorne hkl peaks indicative of a 3D periodicity. Facing this quite continuous range of crystallinity, ferrihydrites are usually classified empirically according to the nurnber of XRD peaks and are often described as 2-line (2-Fh) or 6-line (6-Fh) ferrihydrite. The term "protoferrihydrite" is in sorne instances used for a rnaterial having only 2 hk lines (Chukhrov et al., 1972), whereas "ferrihydrite", sensu-stricto, has 6 hkl peaks at 2·54,2·26,1·98,1·73,1·51 and 1·48 Â. Feroxyhite, on the other hand, is a weIl identified product having always 4 hkl peaks at 2·54,2·22,1·70 and 1·47 Â (Chukhrov et al., 1976; Carlson & Schwertrnann, The lack of long-range order for sorne of these rnaterials, and their srnall particle size,

© 1993 The Mineralogical Society

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A. Manceau and V. A. Drits

causes concern about the uniqueness of structural determinations by XRD. This explains the number of structural models and Fe site occupancies proposed so far in the Iiterature for ferrihydrites (Van der Giessen, 1966; Towe & Bradley, 1967; Atkinson et al., 1968; Feitknecht etaI., 1973; Russell, 1979). Among the models proposed, the one by Towe & Bradley for 6-line Fh (formally 5Fe203' 9H 2 0) is the most widely accepted (Chukhrov et al., 1973, 1974; Schwertmann & Fischer, 1973). This model is derived from the hematite (œFe203) structure, but has more vacant Fe sites and sorne replacements of 0 by OH and H 20. Recently, Eggleton & Fitzpatrick (1988) proposed a revision of this structure, and suggested a structural model where 36% of Fe 3+ would occupy tetrahedral sites similar to that in yFe203' Mossbauer (Cardile, 1988) and XANES (X-ray absorption near edge structure, Manceau et al., 1990) spectroscopy have been used to ascertain the site occupancy of Fe3+ in Fh. Results obtained with the former method have not been conclusive overall, whereas the latter has led to a rejection of the presence of 4-fold Fe 3+ , based on a detection limit for Fe};t ions by XANES of 12 Â -1), and hence do not possess the same mean Fe-Fe distances. ln Fig. 8c, it can be se en that the partial EXAFS spectrum for 6-Fh can be fitted assuming only two Fe shells at 3·01 Â (.1as = 0·07 Â) and 3-44 Â (.1as = 0·05 Â) as was found for f3FeOOH (Table 1). However, in contrast to akaganeite, the large .1as value (0·07 Â) for the first Fe shell indicates a large distribution of distances around the weighted mean value of 3·01 Â, and, thus, to the presence of Fe atoms at short distance (i.e. =2·90-2·95 Â). In a similar way as œFe203, the first Fe-Fe contribution has been selected in the r space, back-Fourier transformed, and then fitted in order to analyse more thoroughly the asymmetry of this Fe shell. The best two-shell fitting obtained is illustrated in Fig. 8d assuming two sub-shells constituted by 1·5 Fe at 2·92 Â and 2·6 Fe at 3·05 Â (Table 1). As for Fx, both the first-shell and the two-shell analysis clearly point to the presence of short Fe-Fe pairings and, hence, to the existence of face-sharing Fe octahedra.

176

A. Manceau and V. A. Drits

5 akaganeite Fe polymers

3

..c ü

X ~ -1 -3 -5

12

6

3

15

5

b

,

3

Il

feroxyhite hematite

..c u X

~ -1 \1 \1

-3 -5

3

6

12

15

5

c

- - feroxyhite akaganeite

3

..c

u

X

~

-1 -3 -5

L -_ _ _

3

~

6

_ _ _- L_ _ _ __ L_ _ _

12

~

15

FIG. 6. Fourier filtered Fe-Fe contributions to EXAFS for Fe polymers (a) and b'FeOOH (b,c) and comparison with selected references.

Local structure of ferrihydrite and feroxyhite

a

Fe-(O,OH)

b

6-Fh

177

Fe-(O,OH)

#A feroxyhite

feroxyhite

:co x

~

t:

4

C

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5

o

2

d

#B

3

Fe-(O,OH)

2

RW

3

4

5

#c

feroxyhite

o

4

R(À)

feroxyhite

5

0

2

03

4

5

RW

FIG. 7. Fe K-RDF for 6-line (6-Fh) and 2-line (#A, #B, #C) ferrihydrites compared to o'FeOOH. RDF are not corrected for phase shift functions.

2-line fcrrihydrites

The RDF of #A and #B are plotted in Fig. 7b and 7c, respectively, together with that of Fx. The left si de of the tirst Fe peak for #B and Fx are superimposed whereas their right sides are not. As for 6-line Fh, this result indicates that the radial distribution of Fe atoms in the nearest cation shell is larger than in Fx, i.e. Fe-Fe distances are more incoherent. But the most interesting result is the similarity of partial EXAFS contributions for # A, # Band 6-1ine Fh (Fig. 9a). Their phases and amplitude envelope are identical over aH the k span even to the point of displaying the same beat-pattern at high k (10 A -1 < k < 12 A -1). The only difference concerns the overall wave amplitude of #A which is lower than the other two (see also Fig. 4). This difference is readily explained by the fact that #A has been synthesized at room temperature and is thus more disordered th an #B. But it is interesting to notice here that this disorder affects each Fe sub-shell to the same extent, otherwise neither the phase nor the amplitude envelope of waves would have been preserved in going frorn #A to #B and 6-1ine Fh. In Fig. 9b and 9c, Fe-Fe contributions for #C are compared ta those of 6-1ine Fh and feroxyhite, respectively. Over most of the k span (3-12 A-1) there is close similarity of #C and 6-Fh in terms of wave envelope shape and amplitude. In this regard, #C shows sorne sirnilarities ta #B. Onlyabove 12 À -1 do the signaIs of #C and 6-Fh become out of Comparison of #C with feroxyhite reveals the opposite case. These signaIs are in phase over

A. Manceau and V. A. Drits

178

5

4

a

b

6- Fh feroxyhite

-

3

2

:.cu

6-Fh akaganeite

:.cu

X 0

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~ -1

~ -2

,

-3

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4

~----~----~~----~----~

-5

~------------------------~ Exp. ••• Fit

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c

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3

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6

15

.-------------------------~ Exp. ••. Fit

d

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:.cu

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~

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-4

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6

12

15

-3

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3

6

9 k(A-1)

12

15

8. Fourier filtered Fe-Fe contributions to EXAFS for 6-line Fh. (a) and (b): comparison with c5'FeOOH and f3FeOOH. (c) and (d): fit. (c) r window = 2·1-2·9 Â; (d) r window = 2·10--3·4 Â.

FIG.

all the k span, but their envelopes are markedly different in the low k range (5 < k < 8 A-1). From these comparisons, the spectrum of #C can be considered as being intermediate between those of 6-line Fh and feroxyhite. This is in agreement with Fe-Fe distances which have been found to be equal to 2·89 A (L1os = 0·00 A), 3·05 A (L1os = 0·00 A) and 3·43 A (L1os = 0·05 A, Table 1, Fig. 4). The low value of the first distance unambiguously attests that this 2-line Fh has structural features common with hematite. DISCUSSION Short-range ordered structure of Fh and Fx

The RDF for Fx and 6-line Fh display two Fe peaks at distances characteristic of edge and double-corner linkages. Further analysis, however, has revealed that the static disorder of Fe atoms within the first Fe shell is much higher th an in f3FeOOH. For Fx, a one-shell analysis leads to 3·1 Fe at 2·99 A (L1os = 0·07 A), and a two-shell analysis to 1·2 Fe at 2·91 A (L1os = 0·01 A) + 1·8 Fe at 3·04 A (L1os = 0·01 A). The reliability factor of the fit (Q) does not improve greatly by adding the second shell (Table 1), and so it is concluded that EXAFS spectroscopy fails to differentiate the two following structural situations: one consisting of a continuous Gaussian distribution of Fe neighbours with a half-width equal to 0·07 A and the

Local structure of ferrihydrite and feroxyhite

179

4 - - 6-Fh

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• • • #A

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-4

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~

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c

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..c ü

0

X

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-2

-4

3

15

FIG. 9. Fourier filtered Fe-Fe contributions to EXAFS for 2-line Fh. Comparison with 6-line Fh and b'FeOOH.

180

A. Manceau and V. A. Drits

other consisting of two discrete sub-shells 0·13  apart. Based on the polyhedral approach of the structure of well-crystallized Fe (oxyhydr)oxides, it is reasonable to assume that the distribution of distances (i.e. variation in interpolyhedral bond angles) is small, and that nearest Fe are located at few discrete positions as confirmed by XRD (Drits et al., 1992a,b). But whatever the structural model used to describe the se local structures, it must be emphasized that both of them lead to the same conclusion, i.e., to the presence of Fe nearest neighbours at ~2·90  and, therefore, to the existence of shared faces. Examination of Fig. 4 shows that the distribution of Fe-Fe distances in Fx is close to that of hematite, whereas that of 6-line Fh is more intermediate between those of œFe203 and œ/f3FeOOH. There are two possible structural possibilities. The first structural model consists of intergrown domains of œ/f3FeOOH and hematite. In hematite the nearest Fe atoms are at 2·89  (F linkage) and 2·97  (E linkage), whereas in œ/f3FeOOH, these are at 3·01  (E). Thus, EXAFS analysis of the first cation shell for Fx, and also for 6-line Fh, could be interpreted as a weighted mixture of aFe203-like and œ/f3FeOOH-like local structures. But this model must be rejected for two reasons. First, the second Fe shell for Fx is as well ordered as in hematite (L1os2 = 0·00 Â), and the presence of œ/f3FeOOH domains would have led to an increase in L10s (L1os(akg.) = 0·06 Â). The same argument applies to 6-Fh: the presence ofhematite do mains in large amount would have led to a substantial reduction of L1os 2' Second, the stoichiometry of 6-Fh and Fx are very close to FeOOH. The second structural model takes into account that for a given set of Fe-Fe distances, different mutual arrangements of Fe atoms over octahedral sites can correspond. For example, despite the fact that Fx and hematite have quite identical Fe-Fe distances and, hence, similar octahedrallinkages (F, E, De), many space combinations of these linkages can exist in Fx. In contrast, only one combination exists in hematite owing to its 3D periodic distribution of Fe atoms (dioctahedral structure). The determination of the space distribution of Fe atoms for Fx and Fh is beyond the possibilities of the method and cannot be solved by this means. It will be shown in the two companion papers (Drits et al., 1992a,b) that Fx and Fh indeed possess original local structures with the same set of Fe-Fe distances determined by EXAFS. Accordingly, even though Fx and Fh are disordered on a large scale, these two structures are ordered at the local scale. Their local order involves both filling of octahedral sites and anionic stackings, but it varies in space, explaining the lack of 3D periodicity. Sometimes 2-line Fh is referred to as an "amorphous" product, as the XRD patterns display few and broad reflections. But in contrast to glasses or amorphous metals, the distribution of interpolyhedral bond angles is narrow enough to give rise to discrete cationcation pairings beyond the first coordination sphere: 2-line Fh are ordered at the local scale and the lack of hkl reflections is mainly due to the extremely small size of coherent scattering domains (~20  in the ab plane and 14  in the c direction, Drits et al., 1992b). Thus, strictly speaking, 2-line Fh cannot be viewed as "amorphous" material. In addition, their SROS may be related to those of Fe (oxyhydr)oxides and can even present a certain diversity not perceived by XRD (for further details see Manceau et al., 1992a). The SROS of #A differs from those of #B and #C as attested by their differences of nearest Fe distances. As observed in Fig. 4, the higher the temperature of synthesis, and probably also the longer the ageing time, the shorter the first Fe-Fe distances. This result is fully consistent with the ide a that 2-line Fh progressively transforms into a ferric gel with a hematite-like local structure as previously described by Combes et al. (1990). In other

Local structure of ferrihydrite and feroxyhite

181

words, the fraction of the ultradispersed hematite within the bulk material (cf. introduction and Drits et al., 1992b) grows at the expense of the ferrihydrite proper. The driving force of these structural rearrangements is certainly the dehydration and further dehydroxylation of Fe-OH-Fe bonds resulting in an increase of the density of face linkages. However, the simultaneous diffusion of cations in between octahedral sites cannot be excluded.

Stability domains of hydrous Fe oxides (HFO) The local structure of poorly crystallized Fe oxyhydroxides (aqueous polymers + hydrous oxides) depends on both the anion present and the rate of hydrolysis, i.e. the pH. Aqueous colloidal polynuclears formed during the partial hydrolysis of a chloride solution possess a f)FeOOH-like local structure (Schneider, 1984; Bottero et al., 1993) whereas the XRD amorphous hydrous ferric oxide obtained at the end of the hydrolysis, i.e. at pH = 7, possesses a 6-line Fh-like local structure (see also Combes et al., 1989, 1990). Thus, a structural transition takes place, even at room temperature, from acidic to neutral pH. This structural change corresponds to the appearance of face linkages. These changes of local structure with pH can be understood in the light of a simplified stability phase diagram established at room tempe rature for well-crystallized (oxyhydr)oxides (Schwertmann & Murad, 1983; Fig. 10). On this diagram the stability field for hematite lies near neutral pH, whereas that of goethite is at a higher (and lower in the absence of Cl-) pH. Ferric gels obtained by the full hydrolysis of a ferric salt solution (pH = 7) are in the stability field of hematite, which provides an explanation for the coexistence of ultradispersed hematite and the presence of sorne face sharings in the hydrous oxide phase proper. Structural similarities between 2-line Fh sample and hematite, combined with stability field considerations, help the understanding of macroscopic observations such as the progressive transformation of 2line Fh into hematite upon ageing at 25°C and neutral pH (Feitknecht & Michaelis, 1962; Schwertmann & Fischer, 1966; Schwertmann & Murad, 1983). Considering the preceding discussion, a second phase transition ought to take place at higher pH at the boundary between hematite and goethite stability fields. Let us now consider the low pH si de of the phase diagram. Aqueous Fe polymers, which form during the hydrolysis of FeCI3 , do possess an akaganeite structure (this work; Schneider, 1984; Bottero et al., 1993). This result is not surprising since the ageing of partly hydrolysed FeCl3 solution leads to the formation of well-crystallized akaganeite (Atkinson et al., 1977; Paterson & Tait, 1977; Cornell & Giovanoli, 1990). Conversely, the presence in the solution of nitrate anions instead of chloride is known to drive the synthesis towards goethite, and is also thought to change the local structure of Fe polymers (Combes et al., 1990). Finally, the fact that the speciation of œ- and f3-oxyhydroxides can be realized at the earliest stages of the hydrolysis process can be understood by reference to the stability phase diagram of weIl crystallized (oxyhydr)oxides (Fig. 10). The interconversion of Fe (oxyhydr)oxides has been weil studied by solution chemistry and is thought to proceed, at the atomic level, via two competing pathways: (1) dissolutionreprecipitation, and (2) solid-state rearrangement (Feitknecht & Michaelis, 1962; Schwertmann & Fischer, 1966; Fischer & Schwertmann, 1975; Cornell & Giovanoli, 1990). ln this respect, the formation of goethite from either ferrihydrite or akaganeite in alkaline media involves dissolution and precipitation from soluble ferric species. ln contra st , hematite is expected to form from ferrihydrite by an internaI dehydration-rearrangement mechanism. The former mechanism is sustained by our structural results since the

182

A. Manceau and V. A. Drits CI.

FeOOH

~

CI.

FeOOH

CI.

FeOOH

Fe polymer

Fe polymer (P FeOOH)

4

7

10

pH

FIG. 10. Schematic representation of stability domains for hematite, akaganeite, goethite, and associated hydrous oxides formed by the hydrolysis of a Fe(II1) solution at room temperature. Bold text: well-crystallized minerais. Plain text: local structure of their corresponding poorly crystallized phases (when existing).

transformation of Fh to goethite necessitates (i) the breaking (i.e. hydrolysis) of face linkages and, (ii), deeply altering the anionic packings in going from ABACAB ... to ABABAB ... stackings. Indeed, the displacement of 0 atoms from aC-type to aB-type position seems hardly feasible at low temperature and pressure without dissolving the structure. The solid-state transformation, as deduced from solution chemistry experiments, of Fh into hematite could at first sight be explained by the presence of ultradispersed hematite grains and by the existence of sorne faces linkages in ferrihydrite proper. However, the transformation of Fh proper to hematite again necessitates the moving of 0 atoms belonging to Fh proper from C to B position, and thus dissolving the Fh proper component. That the formation of hematite and goethite from Fh appears macroscopically to follow two different pathways can be accounted for by the fact that the se two transformations dîffer in the extent to which the dissolution-reprecipitation process affects the ferrihydrite framework. This process would operate solely during the formation of goethite, whereas both dissolution and the solid-state transformation would take place during the formation of hematite. ACKNOWLEOGMENTS The authors thank the staff of the LURE for the synchrotron radiation facility. This research was supported by CNRS/INSU through the DBT program (contribution no. 496). REFERENCES ATKINSON R.J., Po SN ER A.M. & QUIRK J.P. (1968) Crystal nucleation in Fe (III) solutions and hydroxide gels . .1. Inorg. Nucl. Chem. 30, 2371-2381. ATKINSON R.J., POSNER A.M. & QUIRK J.P. (1977) Crystal nuc\eation and growth in hydrolysing iron(III) ch lori de solutions. Clays Clay Miner. 25, 49-56. BERNAL 1.0., DASGUPTA D.R. & MACKAY A.L. (1959) The oxides and hydroxides of iron and their structural interrelationships. Clay Miner. Bull. 4, 15-30. BLAKE R.L., HESSEVICK R.E., ZOLTAI T. & FINGER L. (1966) Refinement ofthe hematite structure. Am. Miner. 51, 123-129. BONNIN D., CALAS G., SUQUET H. & PEZERAT H. (1985) Site occupancy of Fe H spectroscopic study. Phys. Chem. Miner. 12,55-64.

in Garfield nontronite: a

BOTTERO J.Y., MANCEAU A., VILLIERAS F. & TCHOUBAR D. (1993) Structure and mechanism of nucleation of FeOOH(Cl) polymers. Langmuir (submitted).

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CAROlLE C.M. (1988) Tetrahedral Fe·H in ferrihydrite: 57Fe Müssbauer spectroscopie evidence. Clays Clay Miner. 36, 537-539. CARLSON L. & SCHWERTMANN U. (1980) Natural occurrence of feroxyhite «(j'FeOOH). Clays Clay Miner. 28, 272-280. CARLSON L. & SCHWERTMANN U. (1981) Natural ferrihydrites in surface deposits from Finland and their association with silica. Geochim. Cosmochim. Acta 45, 421-429. CHUKHROV F. V. (1973) On mineralogical and geochemical criteria in the genesis of red beds. Chem. Geol. 12,67-75. CHUKHROV F. V., ZVYAGIN B.B., ERMILOVA L.P. & GORSHKOV A.l. (1972) New data on iron oxides in the weathering zone. Proc. Int. Clay Conf. Madrid, 333-341. CHUKHROV F.V., ZVYAGIN B.B., ERMlLOVA L.P. & GORSHKOV A.T. (1974) Über die natur der eisenoxide in geologisch jungen bildungen. Chem. Erde 33, 109-124. CHUKHROV F.V., ZVYAGIN B.B., GORSHKOV A.I., YERMlLOVA L.P. & BALASHOVA V.V. (1973) Ferrihydrite.lzvestiya Akad. Nauk. SSSR, Ser. Geol. 4,23-33. (TransI. in lnt. Geol. Rev., 16, 1131-1143). CHUKHROV F.V., MANCEAU A., SAKHAROV B.A., COMBES J.M., GORSHKOV A.I., SALYN A.L. & DRITS V.A. (1988) Crystal chemistry of oceanic ferric vernadites. Mineralogicheskii J. 10, 78-92. CHUKHROV F.V., ZVYAGIN B.B., GORSHKOV A.I., YERMlLOVA L.P., KOROVUSHKIN V.V., RUDNITSKAYA Y.S. & YAKUBOVSKAYA N. Yu. (1976) Feroksigit-novaya modifikatiya FeOOH (Feroxyhite, a new modification of FeOOH). AN SSSR Izvestiya, ser. Geol. 5-24. (Trans. Int. Geol. Rev. 19, 873-890). COMBES J.M., MANCEAU A. & CALAS G. (1986) Study of the local structure in poorly-ordered precursors of iron oxihydroxides. J. Physique CS, 697-701. COMBES J.M., MANCEAU A. & CALAS G. (1990) Formation of ferric oxides from aqueous solutions: a polyhedral approach by X-ray absorption spectroscopy. II. Hematite formation from ferric gels. Geochim. Cosmochim. Acta 54, 1083-1091. COMBES J.M., MANCEAU A., CALAS G. & BOTTERO J.Y. (1989) Formation offerric oxides from aqueous solutions: a polyhedral approach by X-ray absorption spectroscopy. I. Hydrolysis and formation of ferric gels. Geochim. Cosmochim. Acta 53, 583-594. CORNELL R.M. & GIOVANOLI R. (1990) Transformation of akaganeite into goethitc and hematite in alkaline media. Clays Clay Miner. 38, 469-476. DRITS V .A., SAKHAROV B.A. & MANCEAU A. (1992a) Structure of feroxyhite as determined by simulation of X-ray diffraction curves. Clay Miner. 28, 209-222. DRITS V.A., SAKHAROV B.A., SALYN A.L. & MANCEAU A. (1992b) Structural model forferrihydrite. Clay Miner. 28, 185-207. EGGLETON R.A. & FITZPATRICK R.W. (l988) New data and a revised structural model for ferrihydrite. Clays Clay Miner. 36, 111-124. FEITKNECHT W. & MICHAELIS W. (1962) Über die Hydrolyse von Eisen(III)-perchlorat-Loseungen. He/v. Chim. Acta 26, 212-224. FEITKNECHT W., GIOVANOL! R., MICHAELIS W. & MULl.ER M. (1973) Die Hydrolyse der Lüsungen von Eisen (III)chlorid. Helv. Chim. Acta 56, 21\47-2856. FISCHER W.R. & SCHWERTMANN U. (1975) The formation of hematite l'rom amorphous iron(IIl) hydroxide. Clays Clay Miner. 23,33-37. HAZEMANN J.L., MANCEAU A., SAINCTAVIT PH. & MALGRANGE C. (1992) Structure of the œFcxAll_xOOH solid solution. I. Evidence by polarized EXAFS for an epitaxial growth of hematite-like clusters in diaspore. Phys. Chem. Miner. 19, 25-38. JENNE E.A. (1968) Controls on Mn, Fe, Co, Ni, Cu, and Zn concentrations in soils and wateT: the significant role of hydrous Mn and Fe oxides. Adv. Chem. Sel'. 73,337-387. MANCEAU A. & COMBES J.M. (1988) Structure of Mn and Fe oxides and oxyhydroxides: a topological approach by EXAFS. Phys. Chem. Miner. 15, 21\3-295. MANCEAU A., COMBES J.M. & CALAS G. (1990) New data and a revised model forferrihydrite: a comment on a paper by R.A. Eggleton and R.W. Fitzpatrick. Clays Clay Miner. 38,331-334. MANCEAU A., GORSHKOV A.I. & DRlTS V.A. (1992a) Structural chemistry of Mn, Fe, Co, and Ni in Mn hydrous oxides. II. Information from EXAFS spectroscopy, electron and X-ray diffraction. Am. Miner. 77, 1144-1157. MANCEAU A., GORSHKOV A.I. & DRITS V.A. (1992b) Structural Chemistry of Mn, Fe, Co, and Ni in Mn hydrous oxides. I. Information from XANES spectroscopy. Am. Miner. 77, 1133-1143. McKALE A.G., VEAL B.W., PAULIKAS A.P., CHAN S.K. & KNAPP G.S. (1988) Improved ab initio calculations of amplitude and phase functions for extended X-ray absorption fine structure spectroscopy. 1. Am. Chem. Soc. 110, 3763-3768.

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