American Mineralogist, Volume 77, pages I i,33-1143' 1992

Structural chemistry of Mn, Fe, Co, and Ni in manganese hydrous oxides:Part I. Information from XANES spectroscopy

Ar,.q.rN M,q,Ncrau*Laboratoire de Min6ralogie-Cristallographie, Universit6s Paris 6 et 7, CNRS UA09, Tour 16, 4 place Jussieu,

75252Pais Cedex 05, France

ANaror-n I. GonsmovInstitute of Ore Geology and Mineralogy (IGEM) of the USSR Academy of Science, 35 Staromonetny prospekt,

109017 Moscow, Russia

Vrcron A. DnrrsGeological lnstitute of the USSR Academy of Science, 7 Pyzhevsky prospekt, 1090 I 7 Moscow. Russia

AssrRAcr

The oxidation state and coordination number of Mn, Fe, and Co in hydrous manganese

oxides have been investigated by high resolution XANES spectroscopy. At the MnK edge,

spectral sensitivity is high enough to differentiate between Mn ions of different oxidation

states and site octupations. Application of this technique to various poorly crystallized

hydrous oxides, including samples of birnessite, vernadite, asbolane, and manganese goe-

thite, indicates that Mn atoms are generally tetravalent. If low-valence Mn ions are present,

they certainly make up less than 20 ato/o.In natural Fe-containing vernadite, Fe atoms are

sixfold cooriinated, on the basis of a detection limit for t4lFe3+ ions of about l0 at0l0. Co

has been found to be trivalent in both cobalt and cobalt nickel asbolane. MnK preedge

spectra have also been demonstrated to be sensitive to the local structure of tetravalent

manganate. Spectral intensity was found to depend on the ratio of edge- to corner-sharing

MnOu octahedra: the larger the tunnel size of the manganate, the lower the intensity of

the preedge. Application of this concept to the structure of 6-MnO, suggests that vernadite

contains ur *utty corner-sharing octahedra as todorokite. On the basis of XANES data,

6-MnO, does not possess a layered structure and should not be considered as long-range

disordered birnessite.

INrnonucrron The purpose of this work is to more accurately deter-

In mineral structures, Mn, Fe, and co atoms can occur -i1e^th9 oxidation state and site occupation of Mn' Fe'

in a number of oxidation states and sites. For;;,;;;;, and Co.in various hydrous manganese oxides by using

Mn2+, Mn3+, Fe2*, Fe3*, and co2* ions can b. i;fi; ll"y Xb.::tption near-edge structure (XANES) spectros-

both octahedral and tetrahedral coordinationr,-*tr"r"ur copy' This crystal chemical information provides con-

Mna+ and low-spin co3+ are resrricted ro ocrahed;i;rcs. :iti]lt: ll-l"terpreting structural data obtained for the

Anundersrandingoftheelecrronicstructure"f.;;;;1; :"-.9 Tif:*ls using XRD' selected area electron dif-

minerals can be important for constrainlne r,-"1;iui"i": fraction (SAED)' energy dispersive spectrometry @DS)'

terpretations from X-ray diffraction (XRD) d",".;;; il- and extended X-ray absorption fine structure (EXAFS)

formation is particularly useful for elucidating th;;;- :p::troscopy' These results are presented and discussed

ture of poorly crystallized hydrous .*10., # *ftlt in the.second part of this paper (Manceau et al'' 1992)'

diffraction intensity data fail to provide u.r.rlq.r"'r'or"u: ..Yy:^spectra are usually separated into two parts:

tion. For example, several models have been ;;;o;; 111:t":9c" and the main-edge regions (Brown et al''

for the structure of ferrihydrite. Some ..r"u..ti#tiu.rl 1988)' The former is located to the lower energy side of

found only sixfold-coordinated Fe uro-, Oo*.'?rro tnt,tt".-"*:sing absorption edge and corresponds to ls

Bradley, 1967), whereas others have reported ,rgtin*t, - 3d type.electronic transitions' The latter extends from

fi[ing of tetrahedral sires, 330/o by Eggleton ""i";;;;:

1.t* :l:toon volts above the preedge to about 50 ev

rick (1988) to 50o/o by Harrison et al. (1967). I";;i;;. It$llf the absorption edge and corresponds mainlv

insrance, chukhrov er al. (1988) interpreted xR;";;;; 9,T*t*:-:tcatteringresonances of photoelectrons eject-

from iron vernadire by assuming the presence;f"J;;; ld.yittr low kinetic energv' Apte and Mande (1982) and

one-third of (Fe,Mn) atoms in fourfold .oorai,r#ol'l""' "ttlt :l it,!1n80)

found that the enersv of the main-edge

and preedge peaks of manganese oxides varied linearly

3g041 Grenoble cedex, France. ergy shift with increasing valence results partly from in-

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I 134 MANCEAU ET AL.: MANGANESE HYDROUS OXIDES I

edly measured to monitor the stability of the energy cal-ibration. Mn and Fe preedge spectra were recorded insteps of 0.06 and 0. l3 eV, respectively.

Corrected preedge absorption structure was obtainedby subtracting the arc-tangent-fitted interpolated tail ofthe main edge from the observed spectra. The absorbancewas then normalized with respect to the absorption jumpof the main edge (Ap). This jump was determined byfitting first-order polynomials to the data 100 eV belowthe preedge and from 100 to 400 eV above. The differ-ence between these two lines (at the energy at which thepreedge starts) was used as the normalization factor forthe corrected preedge. The reliability of the experimentalmeasurements and our analytical treatment were con-firmed by recording several spectra of the same sampleduring different beam sessions.

Materials

A series of well-crystallized manganese, iron, and co-balt oxides, which served as reference compounds, andother Mn-, Fe-, and Co-bearing minerals were analyzed.The minerals are listed in Table I along with their for-mulae and selected crystallographic data.

Birnessite. A synthetic birnessite sample, NaoMn,oOrr.9H,O, synthetized by Giovanoli (Giovanoli eI al., 1969,1970a, 1970b; Giovanoli and Stahli, 1970) was exam-ined, in addition to two natural birnessite samples (Bl,Ml2) from (Fe,Mn) micronodules of the Clarion-Clip-perton province, Pacific Ocean (Chukhrov et al.,1978a;Shterenberg et al., 1985). Sample Bl contains Mn (MnO,: 67.66 wto/o) and, in addition, Mg, K, Na, and Ca (MgO:8.58 wto/0, K,O :2.43 wtol0, NarO: 1.90 wto/0, andCaO: 0.52wto/o). A noticeable feature of sample Ml2 isthe absence of Ca, Mg, K, and Na and the presence ofsmall amounts of Ni, Cu, and Co (NiO : 1.650/o, CuO :0.920/o, and CorO. :0.160/o).

Vernadite. A synthetic vernadite sample (d-MnOr) wasprepared by adding HrO, to a stirred I M KMnO, solu-tron at room temperature (Chukhrov et al., 1987). Theprecipitate was freeze dried, then washed with O.l MHNO3, distilled HrO and finally I M NaCl. A large col-lection of Fe-bearing natural vernadite samples previ-ously studied by Chukhrov et al. (1987 , 1988) and Man-ceau and Combes (1988) were examined further. Datafrom EDS indicated that the Mn/Fe ratio in these naturalparticles ranged from 3/l to l/1.

Mn-bearing goethite. A sample of manganese goethitefrom the Fe-Mn crusts of Krylov Underwater Mountain(Atlantic Ocean) has been described by Varentsov et al.(1989) as goethite groutite. Our EDS data showed themean composition of particles to be about 500/o MnO,and 500/o FerOr, but with a variable MnO, content rang-ing from 33 to 620/0.

Asbolane. Three asbolane samples were investigated.Two cobalt nickel asbolane samples from New CaledoniaNi ore deposits were previously studied by Manceau etal. (1987) using EXAFS spectroscopy. A cobalt asbolanesample from Saalfeld (Germany) was originally charac-

creased attraction between the nucleus and the ls coreelectrons and partly from the effect of final-state wavefunctions. Final states of 3d character (preedge region)are more tightly bound and hence are less sensitive tochanges in ionicity and in solid-state effects than the morediffuse, higher energy, final states of the main absorptionK edge (multiple-scattering region). Thus, the oxidationstate is more reliably determined from the energy posi-tion of the preedge peak than from that of the main ab-sorption edge spectrum. Belli et al. (1980) reported thatalthough molecular orbital (MO) transition assignmentsare qualitatively correct for interpreting main XANESfeatures, solid-state effects are also important. If con-firmed, such spectral sensitivity to the manganese oxidestructure would be of particular interest; it would allowMnK XANES to complement EXAFS spectroscopy forthe determination of the local structure of hydrous man_ganese oxides. FeK XANES spectra of minerals have beenmore extensively studied. Near-edge and main-edge ab_sorption features from FeK XANES spectra have beenshown to reflect the site geometry and valence of Fe inminerals (Calas and Petiau, 1983; Waychunas et al., l9g3).More recently, Manceau et al. (1990a) applied this spec-troscopy to test the possible presence oftetrahedrally co_ordinated Fe3+ in hydrous iron oxides. In this case, FeKpreedge features were shown to be particularly sensitiveto the t61Fe3+/l41Fe3+ ratio with a detection limit f61 rrtpsr+ions at less than l0 ato/o. In minerals, Co can be divalentor trivalent, tetrahedrally or octahedrally coordinated, andin a low-spin or high-spin configuration. Facing thesecontrasting electronic structures, XANES spectroscopyshould yield valuable information about the crvstalchemistry of Co in hydrous manganese oxides.

ExpnnruBNTAL DETATLSAnalytical techniques

XANES spectra were obtained at the EXAFS IV sta_tion at the LURE synchrotron radiation laboratory (Or_say, France). The electron energy ofthe DCI storage ringwas 1.85 GeV, and the current was between 200 and250mA. The incident beam was monochromatized using pairsof reflecting Si (5 I l ) crystals so that the Bragg, angle nearthe MnK edge was near 66.. At this angle, the energyspread due to vertical divergence of the beam was 0.45eV, and the intrinsic width of the reflection curve (Dar_win width) equaled 0.05 eV. These widths are signifi_cantly smaller than the Mn core-level width 0.16 eV)that results from core hole lifetime. The overill resolu_tion of XANES spectra was equal to the convolution ofthe instrument with the coreJevel widths and was as smallas 1.2-1.3 eV. With this high level of resolution, exper_imental spectral broadening was negligible. Harmonicsdue to higher order reflections were filtered out with bo_rosilicate glass mirrors (Sainctavit et al., lggg). The en_ergy calibration of MnKXANES spectra was made takinga maximum of the preedge peak for MnCrrOo at 653g.7eV. The preabsorption feature in MnCrrOo was repear_

MANCEAU ET AL.: MANGANESE HYDROUS OXIDES I I 135

Trele L Selected mineralogical and crystallographic information on samples

Mineral name ldeal comoositionOxidation state and site

occupation of cation Full point group

-0)C2,, Ca(2)Cr,(3)-(4)C,(5)-(6)

C"'{71Crr(8)c2n, c4h19)D."(10)D."(11)

4"(2)ol13)o"(13){14)

D",(15)Dr,(l6)D2h/d71D.118)-(19)D.120)o,l21l4e2)--(23)oh(l3)D6'124)

Note:1:ciovanoli (1980a); 2:Wadsley(1952),PaulingandKamb(1982); 3:PostandVeblen(1990); 4:Drits(1987); 5:Wadsley(1953)'P,o-stand Appteman (19S8): 6:Chukhrov et dt.'1tSZbO1, Gioianoli (19806); 7:Post and Bish (1988); 8:Wadslel (195.31 Turner and Post (1988);S: eydtrOm and eysirom (1950), Post et a|.41982), Miura (1986), Vicat et al. (1986); 10: Bystrom (1949); 11 : Baur(1976); 12 = Effenbergeretal.(1981i; 13: Hiil et at. (197i); 14': Bricker (1065); iS: Szftuta 6t d. (tgOg); i6: Szytula et al. (1970); 17 : Erying (19351; 18: Patrat et al. (1983);ig:io*e and Bradtey (1Si67); 20:Btaki et it. (tSeO); 21 :Hegg'(1935), Venivei (1935); 22:Arnold (1986); 23:Taylor and Heiding (1958);24: Delaplane et al. (1969), Deliens and Goethals (1973).

PhyllomanganatesBuserite (2-D)Lithiophorite (2-D)Birnessite (2-D)Asbolane (2-D)Chalcophanite (2-D')Vernadite (2-D')

TectomanganatesTodorokitePsilomelane (romanachite)HollanditeRamsdellitePyrolusite

OtherRhodochrositeManganese spinelHausmanniteFeitknechtite

GoethiteAkaganeiteLepidocrociteFeroxyhiteFerrihydriteHematiteMaghemitelron phosphate

ErythriteCobalt spinelHeterogenite

Manganese oxides

MnO,(Al,Li)MnO,(OH),Na*Mn,nO.r.9HrOMn(O,OH),(Ni,Co),(O,OH),' nH,OZnMn.O,.3HrO6-MnO,

(Na,Ca,K)o s,(Mn,Mg)6Olr. nHrO(Ba,HrO)rMn501o(Ba,Na,K)Mn"(O,OH),ua-MnO,MnO.

MnCO.MncrrooMn.O.B-MnOOH

llon oxidesa-FeOOHB-FeOOH"y-FeOOHd-FeOOH5Fe,O3.9H,Oa-FerO.1-Fe.O"FePO4

Co compoundsCo.(AsOoL.SHrOCoAl,OnCoOOH

t6l Mn4+16lMn1+

I6tMnl+

16lMn1+

16lMn1+

16lMna+

16lMnl+

t6tMn4+

16lMn4+

16lMn.+

l6lMn.+

16lMn2+

I4tMn2+

I6lMn3+ + I11Mn2+

t6lMn3+

t6l Fe3+totFe3*

r6tFe3+

rorFeF*rer FeF*L6l Fe3+I6lFe3+ + I4lFe3+

14tFe3+

torCcl2*rrrCd*rorCd*

terized by Chukhrov et al. (1982a) by X-ray and electrondiffraction, EDS and XPS. The cation composition of thelast has Mn. Co. and Ca in the ratio l:0.63:0. 12.

Rrsur,rsMnK preedge

Preedge spectra for some of the reference compoundsare presented in Figure la. A shift in the preedge towardhigher energy occurs with an increase in the oxidationstate of Mn in the mineral regardless of the coordinationnumber. Spectra for several manganese oxides are shownin Figure lb. There is a change in intensity and full-widthat half maximum (FWHM) for the preedge peaks thatdirectly reflects progressive changes in structure. As thesize ofthe tunnel ofoctahedra increases, the preedge in-tensity and FWHM decrease (compare Fig. 2 with Fig.1b). This rather unexpected relationship was verified byrecording these spectra several times at different beamsessions. Variations were not due to a preferential ori-entation of the powder in the polarized X-ray beam (i.e.,texture effect) as shown by recording spectra of severalsamples at the "magic angle" (Manceau et al., 1990b).Additionally, comparison of the spectra of two pyrolu-

site, four todorokite, and two psilomelane samples fromdifferent localities showed that the chemistry and originof the minerals had no influence on spectral shape. Weconclude that the intensity and shape ofa preedge spec-trum depend only on the structure of the manganese ox-ide mineral and that this information can be used on aphenomenologic basis to assess the local structure ofun-known structures. Preedge sp€ctra of unknown samplesare compared to those of the reference minerals in Figure3. The energy of the preedge peak is the same as those intetravalent manganates, and the intensity is always foundto lie between those ofbuserite and todorokite.

MnI( main edge

Main-edge spectra for some of the Mn reference com-pounds are presented in Figure 4a. The main edge shiftstoward higher energy with increasing formal oxidationstate of Mn ions. The steeply rising part of the XANESspectra ofmanganates and hydrous manganese oxides ap-proximately superimposes upon that of pyrolusite (Fig.4b, 4c). However, the spectral shape and, more specifi-cally, the edge-crest intensity, progressively increase frompyrolusite to buserite. This is opposite to the trend ob-served for the preedge spectra.

py ro l u s i t e

I 1 3 6

0 . 1 4

MANCEAU ET AL.: MANCANESE HYDROUS OXIDES I

IUozotrIc0

trtroz

uJozocroao

0 . 0 9

0 .04

6536 6539 6542 6545 6 5 4 8ENERGY (eV)

0 .09

o.o7

0 . 0 5

0 . 0 3

E2 o.or

6536 6539 6542 6545 6548ENERGY (eV)

Fel( XANES

The FeK preedge spectra for some t41Fe3+ and t61Fe3+reference minerals are presented in Figure 5a. Figure 5band 5c show the spectra ofvernadite compared to thosefor reference minerals and simulated compositions r4tFe3+ /(t6lFe3+ + t4rFe3+) using linear combinations of goethiteand FePOo spectra.

CoKXANES

The CoK main absorption edge spectra from someI6lCo2* (erythrite), tot6qr* (CoAlrOn), and t6rco3+ (Co-OOH) reference minerals are presented in Figure 6 alongwith that of the Saalfeld cobalt asbolane. The spectrumof asbolane is featureless and resembles those of erythriteand CoOOH. The main-edge energy for asbolane is thesame as that of CoOOH, but it is larger than that oferythrite.

DrscussroNInterpretation of spectra from reference compounds

The Mn preedge spectra (Fig. l) show a chemical shiftthat relates to the oxidation state of Mn in the mineral.A shift was previously noted by Belli et al. (1980), but

Fig. 2. Schematic structure of MnO, polymorphs. phyllo-manganates have no (2-D) or few (2-D,) corner linkages of Mnoctahedra. This family includes buserite (2-D), lithiophorite (2_D), asbolane (2-D), chalcophanite (2-D,), and birnessite (2-D or2-D'). Tectomanganates have many corner linkages and 3-D Oframework. They are built of single to multiple chains of edge-sharing Mn octahedra. These chains are crossJinked by corners,resulting in tunnel structures. This family includes todorokite,psilomelane (or romanachite), hollandite, ramsdellite, and py-rolusite.

our data show a significantly larger shift. This is dem-onstrated, for example, by the 1.5-eV diference betweenMn2+ and Mna* ions in comparison with a few tenths ofan electron volt reported by Belli and coworkers. We canspeculate that lower spectral resolution in the older ex-periments has reduced the precision of the determinationofpreedge energies.

The intensity of the preedge spectrum for MnCrrOo isfour times greater than that for MnCO, (Fig. la). Thedata agree with the general observation that in a tetra-hedral environment preedge spectra are four to seventimes more intense (see, e.g., Lytle et a1., 1988). The en-hanced intensity can be explained by dipole selection rules,although a weak contribution of quadrupole transitionshas been experimentally demonstrated (Driiger et al.,1988; Poumellec et al., l99l). Allowed dipole transitionsfrom the ls level must have a final wave function with psymmetry. Therefore, a major part of the preedge is usu-ally ascribed to electric dipole transitions to p-like stateshybridized with 3d states. The ligand-field induced p-dmixing is symmetry dependent, and Brouder (1990) hasrecently demonstrated that the transition strength is ruledby the full-point group symmetry of the crystal: it is neg-ligible or intense depending on the centrosymmetry of thefull point group. For example, the low preedge intensityof sixfold coordinated Mn2+ ions in MnCO. results fromthe existence of an inversion center inthe Dro full pointgroup (Table l).

Comparison of the preedge spectra from MnCrrOo(t4rMn2+) and MnCO, (r6iMn'z+) shows that for a givenoxidation state, the FWHM decreases with a decrease incoordination number. Similarly, comparison of spectrafrom MnCOr, p-MnOOH (t6rMn3+) and B-MnO, (t61Mn4+)indicates that for a fixed coordination number, the preedgeFWHM increases with the oxidation state. This increasein line width as coordination changes form four- to six-

bus€rite chalcophaniae todorckite

psilomelm hollandit€ ramsd€llite pymlusiae

p y r o l u s i t e

ramsde l l i te

ho l land i te

MANCEAU ET AL.:MANGANESE HYDROUS OXIDES I tl37

Lu

z(n(f(Jofo

Eoz

lrJozdl.EoU)o

Eoz

0 .07

0 . 0 5

0 .03

0 . 0 1

6539 6542 6545ENERGY (eV)

0 .07

0 .05

0 .03

0 . 0 1

- 0 . 0 16539 6542 6545

ENERGY (eV)6 5 4 8

fold, and from Me'+ to Me("+r)+, is consistent with thels-3d type of assignment because the line width reflectsthe ligand field splitting of the 3d orbitals. For example,t61Mn3+ is a Jahn-Teller ion and the splitting of the indi-vidual orbital states is large enough that the preedge spec-trum of p-MnOOH is separated into two well-resolvedabsorption bands.

In lepidocrocite, goethite, and akaganeite (Fig. 5a) ls- 3d absorption is weak, only about 3-40lo of the inten-sity of the absorption j ump. The slightly increased inten-sity of the preedge spectrum of hematite in comparisonwith those of other t61Fe3+ reference compounds is anom-alous, since the space point group of hematite (Dro) andthe Fe site symmetry 6m) are both centrosymmetric. In-creased intensity may result from long-range order effects,which are known to influence preedge characteristics(Driiger et al., 1988). In the case of FePOo (rorFe'*), a lackofinversion center at the Fe site is reflected by the five-fold higher intensity for the preedge spectrum.

Oxidation state and site occupation of Mn

All of the preedge spectra for hydrous manganese ox-ides (vernadite, D-MnOr, asbolane, birnessite) are foundat the same energy as those of manganese oxides (Fig. 3),

0 .07

0 .05

0 .03

0 . 0 1

6539 6542ENERGY (CV)

6 5 4 5 6 5 4 8

0 .07

0 .05

0 .03

0 . 0 1

- 0 . 0 16536 6539 6542 6545

ENERGY (eV)

suggesting that the range of valences for Mn atoms islimited. This interpretation is corroborated by main-edgeenergy data (Fig. 4b, 4c). The absence of compelling ev-idence for large amounts of Mn3* and Mn2+ in any ofthese manganese oxides agrees with recent studies. In-deed, even though it has long been assumed that Mnatoms in manganates were divalent and tetravalent, nu-merous XRD studies (Post et al., 1982: Giovanoli, 1985;Tamada and Yamamoto, 1986; Vicat et al., 1986; Postand Bish, 1988; Post and Appleman, 1988), XPS studies(Murray and Dillard, 1979; Chukhrov et al., 1982b; Dil-lard et al., 1982), and redox titration studies (Murray etal., 1984) have demonstrated that they are not. The scar-city of low-valence Mn ions is consistent with the struc-ture ofnearly all tectomanganates and phyllomanganates,where the unit-cell parameters of the plane of the octa-hedral chains, ribbons, and layers are similar. Indeed, ifmanganese oxides contained abundant divalent or triva-lent Mn atoms, these heterovalent cations would lead tosignificant variations in the unit-cell parameters.

The proportion of Mn3+ may vary from one structureand one sample to another, but the studies cited abovesuggest that it generally constitutes less than about 200/0.Given the relatively weak chemical shift between Mn3+

lr,Jozaotroad)

;Eoz

luozd)tr@

Eoz

6536 6 5 4 8

Fig. 3. MnK preedge spectra of samples and reference materials compared with spectra of buserite and todorokite. (a) Asbolane

from Germany and manganese goethite; (b) d-MnOr, (c) vernadite, (d) birnessite.

r,n)

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6530 6540 65s0 6560 6570 6580ENERGY (eV)

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doz.

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Eoz

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Eoz

LUoz.dl(EoU)co

Eoz.

lJlozf;r

oU)o

Eoz

MANCEAU ET AL.: MANGANESE HYDROUS OXIDES I

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0 . 0 8

0 . 0 4

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0 . 0 6

0 .03

7 1 0 8 7 1 1 0 7 1 1 2 7 1 1 4 7 1 1 6 7 1 1 8ENERGY (eV)

0 . 0 6

0 . o 3

7 1 0 8 7 1 1 0 7 1 1 2 7 1 1 4 7 1 1 6 7 1 1 8ENERGY (eV)

Fig. 5. FeK preedge spectra. (a) Reference minerals. Fromlowest intensiry to highest: lepidocrocite, akaganeite, goethite,hematite, maghemite (t6lFe3+ + t4rFe3+), and FePOo (qFer*). (b)Vernadite spectrum compared with selected reference spectra.(c) Dotted lines represent linear combinations of goethite andFePO. spectra. From lowest intensity to highest: 5olo [arFe3+, 100/0vtpgt+,20o/o t4lFe3r, and 300/0 r4tFe3+.

estimated the detection limit of tatMn2+ ions by XANESto be about l0 ato/o.

Mn atoms in manganese goethite are also in a tetra-valent state (Fig. 3a). This is not surprising because Mnminerals, associated with iron oxyhydroxides in the oxi-dized zone ofweathering profiles or on the sea floor, areexclusively Mna* oxides. Based on an analogy with thelarge site distortion of Cu2+ (Jahn-Teller ion) in coppermagnesium phyllosilicates (Decarreau, 1985; Decarreauet al., 1992), Mn3+ would be unlikely to enter the goethite

6590

6530 6540 6550 6560 6570 6s80 6590 6600ENERGY (eV)

6530 6s40 6550 6s60 6s70 6580 6590 6600ENERGY (eV)

Fig. 4. MnK main edges of manganese oxides. (a) Referencecompounds: MnCO, (r6lMn,+), MnCrrO. (l4lMnr+), p-MnOOH(r6rMn3+), pyrolusite (t61Mn4+). (b) Evolution of the shape of thels - 4p transition in the tectomanganate-phyllomanganate se-ries. (c) Comparison of D-MnO, spectrum to those of todorokiteand birnessite. Note similarity of d-MnO, and todorokite spec-lla.

and Mn4+ preedge spectra and the high variability of theirintensities among tetravalent manganates, our precisionis within this range. We successfully fitted the experi-mental spectrum for MnrOo spinel with a weighted sumof two spectra: Y:MnCrrOo (t4lMn2+) and %6-MnOOH(l6lMn3+) (Fig. 7c). On the basis of this result, we have

FePO4maghemi teh e m a t i t eg oe th i te

akagane i tel e p i d o c r o c i t e

buser i tevernad i tetodorok i te

p y r o l u s i t er a m s d e l l i t e

b m a g h e m i t e

A . . . . f e r o x y h ih e m a t i t e _ _ v e r n a d i t e

goeth i te . \

synth. b i rnessi teb i r ness i t e #M12

s i m u l a t i o n s_ _ v e r n a d i t e

g oe th i te

ulozotU)o

;Eoz

lu

zd)(f

ad)

Eoz

L!Ozotroao

Eoz

MANCEAU ET AL.: MANGANESE HYDROUS OXIDES I

77oO 7710 7720 7730 7740 7750 7760 7770ENERGY (eV)

Fig. 6. CoK main edge spectra of reference minerals. Ery-thrite ltot6ozt;, CoAlrOn (t4lco'?+), CoOOH (t6rcor+) comparedwith the spectrum of Co in asbolane. The amplitude was cali-brated by assuming the atomic part of the absorption jump as anormalization value.

structure in large amounts, thus limiting the extent of thegoethite-groutite solid solution. This explanation mayequally apply to the experiments of Stiers and Schwert-mann (1985) and Ebinger and Schulze (1989), who foundthat the proportion of Mn3* substituted for Fe in goethitewas less than l0 atolo and 14-34 ato/o, respectively.

Local structure of hydrous manganese oxides

Within the phyllomanganate-tectomanganate senes,there is a progressive change in the intensity and the shapeof preedge and main-edge spectra (Figs. lb, 4b). Inter-pretation of the trend is not straightforward and will beconsidered, with some details, below.

Preedge. One possibility is that an increase in peakintensity (from buserite to pyrolusite) correlates with anincrease in the Mna*/Mn3+ ratio. In order to test thishypothesis, l inear combinations of pyrolusite and

6-MnOOH spectra were computed (Fig. 7a). Althoughthe shape and intensity ofthis suite oftheoretical spectrareproduce the trends observed in the tectomanganate-phyllomanganate series (Fig. l), the simulations shift pro-gressively to lower energy as Mn3+ increases. The oppo-site situation is experimentally observed in referenceminerals and samples: the lower the amplitude, the high-er the energy ofthe left side ofthe preedge ofmanganates(Figs. lb, 3). Thus, the preedge spectra of MnO, poly-morphs cannot be fitted by a linear combination of Mno*-Mnr' reference speclra.

A second interpretation might be attempted using sym-metry effects. However, all of the manganese oxides arecentrosymmetric (Table l), which theoretically precludes

any intensity for the forbidden ls - 3d preedge absorp-tion. Thus, it is concluded that the preedge trend amongtetravalent manganates cannot be qualitatively interpret-ed within a simple quantum molecular-orbital frame-work.

A third possibility might explain the trend in terms ofchanges in average Mn-O bond distances. In V com-pounds (Wong et al., 1984) and in some transition metal

I 139

0 . 0 9

0 . o 7

0 . 0 5

0 . 0 3

0 . 0 1

6536 6539 6542 6545 6548

ENERGY (EV)

0.09

o.07

0.05

0.03

0 . 0 1

6 5 3 6 6539 6542 6545 6548

ENERGY (EV)

0 . 1 3

0 . 1 1

0 .09

0 .07

0 .05

0 . 0 3

0 . 0 1

6536 6539 6542 6545 6548

ENERGY (EV)

2/rt6tMrf + (g-MnOOH) with selected references.

complexes (Kutzler et al., 1980), variations in the oscil-lator strength ofthe preedge were attributed to a decreasein size of the coordination polyhedron. This empiricalrelationship between the size of the molecular cage, de-

fined by the nearest-neighbor ligands, and preedge inten-

sity was interpreted within a quantum-mechanical frame-

IUOzotroao

iEoz

pyro lus i te

. ' . s i m u l a t i o n s

1l\\

b / \ Pyro lus i te

. . . s i m u l a t i o n s/,,' _ 'i\

(it ii\- . , .1 i \ , \ r ' . . \

\:AN

. . . s i m u l a t i o n s

MnrOo

pl'/ffi1

I 140 MANCEAU ET AL.: MANGANESE HYDROUS OXIDES I

coordination shells (see, e.9., the review of Petiau et al.,1987). In the first of these theoretical approaches, themain absorption maximum is identified as a dipole ls -4p transition to the triply degenerate 41," molecular or-bital (Apte and Mande, 1980). Hence, splittings or broad-ening in the vicinity ofthe edge crest is evidence ofbro-ken degeneracy in the antibonding orbitals due toasymmetric metal-ligand bonding. The symmetry of finalstates can be probed by angular measurements (Hahn andHodgson, 1983; Fr6tigny et al., 1986) using single crys-tals, but in their absence, near main-edge spectral featuresare mainly interpreted qualitatively, by comparing knownwith unknown structures.

The main-edge spectrum of MnCO, is featureless, witha single, well-defined crest (Fig. 4a). This edge shape agreeswith the high symmetry of the Mn site Qm),withthe fullpoint group (Drr), and with six equivalent Mn-O distanc-es (2.19 A;. fne main-edge spectra of phyllomanganatesare also featureless (Fig. 4b), indicating a highly sym-metric Mn site. This interpretation corroborates that de-termined from their weak preedge intensity. In contrast,several features are observed on the edge crests ofpyro-lusite 1tel14tto*) and p-MnOOH, which, in the case ofI6lMn:+ ions, can be interpreted by the nondegeneracy ofp orbitals as a result of Jahn-Teller distortion. In pyro-lusite, p orbitals are also nondegenerate, since the Mnoctahedron is stretched along the z axis [d(Mn-O) : 1.88A x 4; d(Mn-O): 1.90 A x 2, site symmetry mmm;Baur, 19761. Although main-edge features are sensitiveto subtle changes in site geometry (Waychunas, 1987); itis surprising that this small variation in the Mn-O dis-tances leads to such large crest broadening. Apparently,a simple molecular orbital approach is not sufficient todescribe the edge features ofmanganese oxides; it is nec-essary to consider a larger molecular cluster that wouldinclude nearest cation shells.

Solid-state effects in the main-edge spectra of manga-nese oxides were first recognized by Belli et al. (1980).They interpreted spectral differences between MnO andB-MnO, (pyrolusite) in terms of differences in octahedrallinkages: each octahedron shares 12 edges in the formeroxide and two edges and eight corners in the latter. Ofparticular interest is the similarity of spectra for MnOand the phyllomanganates: all are characterized by anintense and featureless ls - 4p transition indicating acompletely unfilled final p state. These similarities inspectra reinforce the idea that in manganese oxides near-est cation shells are important for the final state wavefunction, as in both of these structures MnOu octahedraare linked only by edges. However, the most convincingsupport for the existence of solid-state effects in the vi-cinity of the edge crest comes from a comparison of spec-tra from TiO, (Waychunas, 1987) and MnOr. Rutile andpyrolusite, which are isostructural, possess similar spec-tra. The same is true for phyllomanganates and anatase,where chains of edge-shared TiO, octahedra are linkedthrough edges yielding a pseudolayered structure.

The edge crest increases in intensity and is less broad

work (Kutzler et al., 1980; Wong et al., 1984). From that,it was deduced that the smaller the cage, the higher theintensity of the ls - 3d transition. Mean Mn-O bond

and Veblen, 1990). However, in all likelihood, bond_length variation is not large enough to account complete-ly for the threefold increase in the intensity of thepyrolusite spectrum relative to that of buserite. In ouropinion, it should rather be regarded as a factor that mayreinforce some other unelucidated effect.

Although we are unable to provide an explanation basedon physical parameters for the trends within the manga_nate suite, we can state that peak intensity is roughlyproportional to the ratio of edge- to corner-sharing MnO.octahedra, i.e., the larger the tunnel size (Turner and Bu_seck, 1979), the lower the peak intensity and the smallerits width. The lowest magnitude is observed for sodiumbuserite, which has no corner-sharing octahedra (detailspresented in Manceau et al., 1992), and the highest in_tensity is obtained for pyrolusite, where each octahedronshares eight corners and two edges. Our structural inter_pretation is strengthened by the fact that the trend ob_served in the phyllomanganate to tectomanganate seriescan be reproduced by linear combinations ofbuserite andpyrolusite spectra (Figs. lb, 7b). Furthermore, the im_portance of long-range order effects on preedge featureshas a parallel in the case of hematite (Dr?iger et al., l ggg).Thus, preedge intensity offers a clue to local structure ofunknown manganates.

The preedge spectra ofhydrous manganese oxides canbe divided into three groups based on their height (Fig.3). The first group includes samples that have a weakpreedge peak, such as the asbolanes from Saalfeld andNew Caledonia and the manganese goethite. Their spec-tra show no evidence of vertex-sharing MnOu octahedra(Fig. 3a). The second group contains D-MnO". The simi-larity of this spectrum with that of todorokite (Fig. 3b)suggests that d-MnO, does not possess a layered structure.This interpretation is fully consistent with main-edgespectra and data from EXAFS and XRD and indicates a3-D O framework structure (Manceau et al.. 1992). Thethird group comprises natural vernadite and birnessite.These spectra are ofintermediate height, ranging betweenthose of the two preceding groups of samples, which sug-gests that vemadite and birnessite might have some cor_ner-sharing octahedra in their structure.

Main edge. The shape and intensity of the main-edgecrest of tetravalent manganates progressively change inthe series from pyrolusite-ramsdellite to buserite (Fig. ab).Spectral features observed on the main absorption edgeare usually interpreted using molecular orbital theory,sometimes in terms of density of states, or by multiplescatterings about the atomic cage constituted by nearest

from tectomanganates to phyllomanganates (Fig. 4b). Onthe basis of the preceding discussion, this trend may alsobe explained with a solid-state approach. A progressivechange in local structure is manifested by an increase inthe number of edge-sharing octahedra at the expense ofcorner-sharing octahedra. Thus, short-range mineralstructure can be deduced from main-edge spectra, just asfrom preedge spectra. Edge-crest spectral features provideinformation about the way octahedra are linked to eachother, so their analysis often reinforces evidence derivedfrom preedge data.

The similarity between the main-edge spectra of to-dorokite and D-MnO, affords an additional clue to thestructural difference between birnessite and 6-MnO, (Fig.4c). The MnK edges of vernadite (Fig. ab) and birnessite(Fig. 4c) are all similar to those of reference phylloman-ganates. They have a single, well-defined, and intense edgecrest suggesting similar short-range order. This findingappears at variance with preedge results, which suggesteda scarcity ofcorner-sharing octahedra in both vernaditeand birnessite. Given the evident low sensitivity ofXANES spectroscopy to short-range structure, one mayconclude that both preedge and main-edge results areconvergent, indicating that, if present, there are few Mn-O-Mn vertex linkages, or they are at least less abundantthan in todorokite.

Site occupation of Fe

Fe3* ions, unlike Mna+, can occupy both sixfold- andfourfold-coordinated sites in minerals. The low crystal-linity of ferric gels results in a large distribution of Fe3+sites that have slightly different types and arrangementsofneighboring ions. This local disorder renders classicalstructural and spectroscopic methods, such as Mossbauerspectroscopy (Murad and Schwertmann, 1980; Cardile,1988), essentially insensitive to the coordination envi-ronment of Fe. Fortunately, this is not a limitation forpreedge spectroscopy: it has been successfully used to de-termine the Fe sites in natural and synthetic hydrous fer-ric gels (Combes et al., 1989, 1990; Manceau et al., 1990a).

Preedge spectra of all vernadite samples are similar tothat of feroxyhite (Fig. 5b), whose structure is similar tothat of hematite (Patrat et al., 1983). Thus, we do notdetect fourfold-coordinated Fe in vernadite. The abilityof preedge spectroscopy to detect the presence 6f tal['s:+can be evaluated from the examination of the maghemite(7-FerO) spectrum. If we compare the full ls - 3d in-tensity of maghemite to a linear combination of spectrafor goethite and FePOo, a mean talFe3+ concentration of27o/o is determined (Fig. 5c). This estimate can be com-pared with structural data from the literature to assessthe site occupancy of Fe in an unknown oxyhydroxide.The 7-FerO, possesses a defect spinel structure, and therehas been some controversy over the distribution of va-cancies (see, e.9., Sinha and Sinha, 1957; Ferguson andHass, 1958; Annersten and Hafner, 1973). Depending onthe author and probably on the nature of the materialstudied, the distribution of vacancies ranges from a sta-

I l 4 l

tistical distribution over all tetrahedral and octahedralcation sites to a position exclusively at octahedral latticesites. The percentage of larFe may be considered to bebetween 33 and 37010, which is significantly greater thanour experimental value (27o/o).ln the worst case, our de-termination of the tetrahedral content derived from thepreedge spectroscopy might be underestimated by l0o/0.Based on the similarity of vernadite to feroxyhite spectraand this l0o/o detection limit, we conclude that there isno evidence for fourfold-coordinated Fe3+ ions in ver-nadite.

Electronic structure of Co in asbolane

The main-edge structure and energy suggest sixfold co-ordination and a trivalent state for Co in asbolane. Thisinterpretation agrees with the Co-(O,OH) distance deter-mined from our EXAFS data. This topic will be discussedfurther after presentation of more data in the second partof this work (Manceau et al., 1992).

CoNcr,usroNs

This study demonstrates the potential for XANESspectroscopy in studies of crystal chemical properties ofMn, Fe, and Co in minerals. Specifically, MnK preedgespectra have been shown to reflect changes in the valenceand coordination number of Mn atoms. Sensitivity is highenough to allow differentiation of Mn ions with differentoxidation states and site occupations as, for instance, thetwo oxidation states of Mn2* and Mn3+ in MnrOo, or thetwo Mn sites (t4rMn,t61Mn) in 1-MnrOr' Such distinctionis not currently possible with other spectroscopic tech-niques, such as X-ray photoelectron spectroscopy (XPS;Oku et al., 1975; Murray and Dillard, 1979; Dillard etal., 1982; Crowther et al., 1983; Murray et al., 1985),energy electron loss spectroscopy (EELS; Rask et al.,1987), and UV-visible reflectance spectroscopy (Strobelet al., 1987). Preedge spectra have also been shown to besensitive to the local structure of tetravalent manganeseoxides because the intensity MnOr spectra depends onthe ratio of edge- to corner-sharing MnOu octahedra. ThusMnK XANES spectroscopy serves as a structural tool tocomplement other techniques for studying short-rangestructure of highly disordered manganese oxides.

The analysis ofvarious hydrous oxides has shown thatMn is essentially tetravalent even in natural Mn-bearinggoethite. The detection limit of low-valence Mn ions isestimated to be about 20o/o. ln hydrous manganese ox-ides, Fe and Co are both trivalent and octahedrally co-ordinated. The detection limit of talFe3* by FeK preedgespectroscopy is about 100/0. The preedge and main-edgespectra of 6-MnO, are similar to those of todorokite, in-dicating that the two structures have a similar ratio ofedge- over corner-sharing octahedra. Our evidence con-tradicts the generally accepted belief that d-MnO, pos-sesses a layered, birnessiteJike structure. Additional proofof the distinct structure of 6-MnO, and birnessite will beprovided through EXAFS and XRD data presented inthe second part of this work (Manceau et al., 1992).

MANCEAU ET AL.: MANGANESE HYDROUS OXIDES I

l t 4 2 MANCEAU ET AL.: MANGANESE HYDROUS OXIDES I

AcxNowlnocMENTS

The authors thank R. Giovanoli for providing the synthetic birnessitesample, D. Bish for providing several hollandite, psilomelane, and to-dorokite samples, and the staffof the LURE synchrotron radiation facil-ity A critical reading of a preliminary version by G.A. Waychunas isacknowledged. Iast, and not least, the authors express their gratitude toSusan Stipp, who improved the English. This research was supported byCNRS/INSU through the DBT program, gant 90 DBT 1.03 (contributionn o . 4 l 3 )

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M.lxuscnrpr REcEIVED Aucusr 15, 1990Mexuscrurr AcCEPTED Jur-v 6, 1992

MANCEAU ET AL.: MANGANESE HYDROUS OXIDES I