Structural Chemistry of Mn, Fe, Co, and Ni in Manganese Hydrous Oxides: Part II

American Mineralogist, Volume 77, pages 1144-1157, 1992

Structural chemistry of Mn, Fe, Co, and Ni in manganese hydrous oxides: Part II. Information from EXAFS spectroscopy and electron and X-ray diffraction

ALAIN MANCEAU* Laboratoire de Mineralogie-Cristallographie, Universites Paris 6 et 7, CNRS UA09, Tour 16, 4 place Jussieu, 75252 Paris Cede x 05, France ANA TOLl I I. GORSHKOV Institute of Ore Geology and Mineralogy (IGEM) of the USSR Academy of Science, 35 Staromonetny prospekt, 109017 Moscow, Russia VICTOR A. DRITS Geological Institute of the USSR Academy of Science, 7 Pyzhevsky prospekt, 109017 Moscow, Russia

ABSTRACT The short- and long-range structural order of various hydrous oxides were investigated using extended X-ray absorption fine structure (EXAFS), X-ray and electron diffraction (XRD and SAED) and in situ chemical analysis (EDS). Materials examined include synthetic sodium birnessite, natural magnesium birnessite, natural nickel copper birnessite, synthetic vernadite (o-Mn02), a series of natural iron vernadite samples, natural manganese goethite, two natural cobalt nickel asbolane samples, and natural cobalt asbolane. The structure of birnessite is similar to that of chalcophanite, but there are no corner- sharing Mn4+-Mn4+ octahedra. Sodium and magnesium birnessite differ primarily in the stacking mode of their Mn octahedral sheets and their interlayer structure. Similarities and differences between these two minerals have been considered in terms of anion close- packed models. In contrast to birnessite, synthetic vernadite (o-Mn02) has edge- and corner-sharing Mn4+ octahedra and a three-dimensional anionic framework; o-Mn02 does not appear to be a c-disordered birnessite. In iron vernadite and manganese goethite, Fe3+ and Mn4+ ions are segregated in coherent scattering domains. In both minerals, Mn atoms form phyllomanganate-like domains; in iron vernadite, Fe domains have a feroxyhite-like local structure. Asbolane has been found to have a mixed-layer structure. Cobalt nickel asbolane has layers of Mn02, Ni(OR)2' and possibly CoOOR or CO(OH)3 alternating regularly along the c axis. In cobalt asbolane, Mn2+ tetrahedral layers are presumably regularly interstratified with layers of Mn02 and (CoOOH). This study provides new examples of hybrid structures among low-temperature materials and confirms their heterogeneous nature on a very fine scale.

INTRODUCTION nates have a lower degree of crystallinity, so few of their Characterization of many hydrous manganese oxides structures have been refined. Some studies of phylloman- by structural methods is difficult and frequently incom- ganates include birnessite and buserite: Giovanoli et ai. plete because of their poor crystallinity. Structural meth- (1970a, 1970b), Chukhrov et ai. (1985b, 1987a), Strobel ods can be divided into two groups: the diffraction tech- et ai. (1987), and Post and Veblen (1990); vernadite: niques are sensitive to medium- and long-range order, Chukhrov et ai. (1988); asbolane: Chukhrov et ai. (1980, and the spectroscopic techniques are sensitive to elec- 1987b), Drits (1987), Manceau et ai. (1987); and mixed- tronic or local structure. Our present knowledge of hy- layer buserite-asbolane: Chukhrov et ai. (1983, 1987b). drous manganese oxide structure is based mostly on the The main difficulties in the study of poorly crystallized former. The tectomanganates have a relatively good crys- hydrous manganese oxides are that (1) their X-ray dif- tallinity and have been well characterized. Significant fraction (XRD) patterns usually contain only a few broad, studies of the past 15 years include Chukhrov et ai. (1978a, weak reflections, and (2) several minerals (buserite, as- 1979, 1981, 1985a, 1986), Turner and Buseck (1981), bolane, mixed-layer asbolane-buserite) give nearly the Post et ai. (1982), Miura (1986), Vicat et ai. (1986), and same XRD patterns (Drits et aI., 1985). Recently, how- Post and Bish (1988). In contrast, most phyllomanga- ever, some progress has been made in simulating diffraction effects of defect structure models (Drits and Tchou- Present address: Universite Joseph Fourier, LGIT, BP 53X, bar, 1990; Chukhrov et aI., 1989), and in applying the 38041* Grenoble Cedex, France. Rietveld method for structure refinement of some of the 0003-004X/92/1112-1144$02.00 1144 MANCEAU ET AL.: MANGANESE HYDROUS OXIDES II 1145 better crystallized samples (Post and Veblen, 1990). Se- lected area electron diffraction (SAED) has also proven useful for the structural analysis of hydrous manganese oxides. Two characteristics that distinguish this method from XRD are its higher sensitivity to long-range order (Drits, 1987) and the possibility of focusing the electron beam on a small volume, thus allowing identification of discrete phases in a polymineral matrix. For instance, the combination of XRD and SAED completed with in situ chemical analysis (energy dispersive spectroscopy, EDS) has permitted the determination of the structure of metal- containing asbolanes and mixed-layer buserite-asbolanes. However, even at the micrometer scale, diffraction methods yield average structures at best. These are often not o 2 4 6 the same as actual structure for at least two reasons: (1) R (A) natural manganese oxides often contain large amounts of Fe, and the similarity in scattering power of Mn and Fe Fig. 1. MnK RDF of reference compounds, o-Mn02 and bir- prevents their distinction by diffraction techniques; (2) nessite. Solid line = room temperature (R1), dotted line = lower temperature. (a) From lowest upward: buserite (R1), o-Mn02 minor and trace elements in manganese oxides cause lo- (RT, 4.5 K), todorokite, ramsdellite, and chalcophanite (RT, 30 cal zones of different structure. K). Notice the similarity of RDF of todorokite and o-Mn02. Several spectroscopic methods have been used to refine Arrow indicates Mn06 corner-linked contributions. (b) From the structural chemistry of manganese oxides. Some of lowest upward: sample M12 (RT, 77 K), sample B1 (RT, 77 K), these, such as extended X-ray absorption fine structure synthetic birnessite (Na4MnI4027 "9H20) (RT, 4.5 K). RDF have (EXAFS) and infrared spectroscopy (IR), are sensitive to not been corrected for phase shifts. local or near-local structure (Potter and Rossman, 1979; Manceau et aI., 1987; Manceau and Combes, 1988; Stouff and Boulegue, 1988; Manceau, 1989; Manceau et aI., reflecting Si (311) crystals. Harmonics due to high order 1989, 1990a). The main advantages of X-ray absorption reflections were eliminated with borosilicate glass mirrors spectroscopy (XAS = EXAFS + XANES) are threefold: (Sainctavit et aI., 1988). EXAFS data were reduced using (1) almost all elements are spectroscopically active; (2) the plane-wave approximation with a standard procedure the structural information comes from more than the (Teo, 1986). To maximize the detection limit of atomic nearest coordination sphere of the X-ray absorber; and contributions (namely comer-sharing octahedra), (1) we (3) the spectra from specific elements can usually be ob- obtained spectra at room temperature and at low tem- tained separately. These aspects make XAS a valuable perature (77 K or 4.5 K) (Fig. la), and (2) we attempted tool for determining a particular atom's local environ- to reduce spurious peaks in the MnK RDF (radial distri- ment and for probing its electronic structure. The main bution function) caused by cutoff effects during the Fou- limitation of XAS is its lack of spatial resolution in com- rier transform. These Fourier transform artifacts were parison to SAED; that limits XAS use to single phases, minimized by using the Kaiser window with a T param- i.e., where the X-ray absorber phase is unique. eter of 3. In such conditions, the detection limit for cor- This paper extends the work begun in the XANES study ner-sharing Me atoms (Me = Mn, Zn . . .) is lower than of hydrous manganese oxides that was presented by Man- one Me per Mn atom (Manceau and Combes, 1988). ceau et ai. (1992). In this second part, we have combined X-ray and electron diffraction and energy dispersive data from EXAFS, XANES, XRD, SAED, and EDS. Our analysis. XRD patterns of powdered samples were re- aims are (1) to clarify the structure of natural and syn- corded with a DRON-UM or with a Siemens diffractom- thetic birnessite, (2) to examine the possible structural eter using CuKa radiation and a graphite monochroma- relationship between vernadite and birnessite, (3) to de- tor. SAED patterns and EDS spectra were obtained using termine the structural environments of Mn and Fe in a JEM-I00C microscope equipped with a Kevex spec- natural iron vernadite and manganese goethite, and (4) trometer with a voltage of 100 kV and a nominal current. to better define the structure of asbolanes. Samples were dispersed in aqueous solution with an ul- trasonic disperser. Thin colloid films covered with C were used as support, and grids were mounted on a tilting sam- EXPERIMENTAL DETAILS ple holder. Techniques for treating SAED patterns have Techniques been described by Drits (1987). EXAFS spectroscopy. We used the EXAFS IV station at the LURE synchrotron radiation laboratory (Orsay, Materials France). The positron energy of the storage ring DCI was Reference minerals. A series of well-crystallized man- 1.85 GeV, and the current was between 200 and 250 mA. ganese oxides (Manceau and Combes, 1988), and other The incident beam was monochromatized with pairs of Mn-, Fe-, and Co-bearing minerals were analyzed to serve 1146 MANCEAU ET AL.: MANGANESE HYDROUS OXIDES II

TABLE1. Name and ideal composition of references and samples which has Zn atoms lying above and below vacant Mn octahedra), have a few (Fig. 2; Part I). Refer - Mineral Ideal composition ence Birnessite Manganese oxides Phyllomanganates The XRD pattern of our synthetic sample is quite sim- Buserite (2-D) Mn02 1 ilar to that of synthetic sodium birnessite studied by Post Birnessite (2-D) Na4Mn14027' 9H2O 2 Asbolane (2-D) Mn(O,OHh(Ni,Co)X

Fig. 2. SAED patterns and EDS spectra for Fe-bearing vernadite particles. (a), (b) Representative of most of the particles where there is a higher concentration of Mn than Fe. SAED patterns contain only two hk bands; (c), (d) particles where Fe is more abundant than Mn. SAED pattern resembles that of feroxyhite. tudes and phase shifts functions, so EXAFS cannot dif- impurity was detected; a lack of even trace quantities of ferentiate between the two; thus, a good fit would also groutite is proved by the absence of reflections near 2.81, have been obtained by assuming Mn instead of Fe nearest 2.38, and 2.30 A (see arrows). Figure 6a is a TEM image neighbors. However, the presence of Fe in the vicinity of of some sample particles; EDS analyses show that they Fe is supported by the fact that Mn and Fe atoms possess all contain similar amounts of Mn and Fe, although the a different local structure. concentration of Fe is usually slightly greater (Fig. 6c). In spite of a high Mn content, SAED patterns are charac- Mn-bearing goethite terized by the same reflections as those expected for goe- The XRD pattern for manganese goethite shows broad- thite, which confirm the monomineralic nature of this ened hkl reflections that have approximately the same d manganese goethite at the scale sampled by diffraction values as those of goethite (Fig. 5). No manganese oxide techniques (Fig. 6b). 1148 MANCEAU ET AL.: MANGANESE HYDROUS OXIDES II

Fe K-edge (VERNADITE) Manganese goethite >- r- 6"M en z w zr-

20 30 40 50 60 70 80 2 e degrees (Cu Ka radiation)

Fig. 5. XRD pattern for manganese goethite. No Mn02 im- 1248 purity is detected. Arrows correspond to the (130), (111), and /". 7~ (140) reflections of groutite. 0 2 4 6 0 2 4 6 R (A) R (A) particles of samples asb and Mn-ox contain two incom- Fig. 3. EXAFS data for iron vernadite samples. (a) MnK RDF. mensurate hexagonal networks of hkO reflections and a unique series of basal 001 reflections (Fig. 8). The struc- Solid line = RT, dotted line = 77 K. Data were analyzed using tures of these particles can be described by two sublattices a Kaiser window with T = 3.0. (b) Compare shape of FeK RDF to that of feroxyhite. Data were analyzed using Hanning + flat having different hexagonal unit-cell parameters in the ab window (Manceau and Combes, 1988). RDF have not been cor- plane (aI = bI = 2.84 A for sub lattice I, and all = bll = rected for phase shifts. 3.01 A for sublattice II) but a common periodicity per- pendicular to them along c (c = 9.40 A). Our SAED re- The FeK RDF for this sample (Fig. 7a) is identical to flection intensity distributions are the same as those for that of goethite (Manceau and Combes, 1988). The MnK the cobalt nickel asbolane and nickel asbolane particles RDF contains well-defined second and third peaks whose described by Chukhrov et al. (1980). In each reciprocal intensities are comparable to those of todorokite (Fig. 7). sub lattice the 100 reflection is about half as intense as The similarity in the local Mn environment in goethite that of 110 (Fig. 8), but for each given hkO reflection, and in todorokite has been verified by Fourier-filtering reciprocal sublattice I reflections are stronger than recip- Mn-O and Mn-Mn contributions. The Mn-(O,OH) dis- rocal sublattice II reflections (e.g., compare 111 and 1111 tance is equal to 1.90-1.92 A, a value consistent with a in Fig. 8a, 8c). EDS spectra show that Ni concentration tetravalent state for Mn ions. varies in the particles of cobalt nickel asbolane. Our ob- servations show that with smaller Ni content, the hkO Asbolane reflection intensities for sublattice II are weaker (Fig. 8). Previous results showed that none of the asbolane sam- The particles containing only Mn and Co (Fig. 8e, 8f) ples were completely monomineralic (see description in show 001 reflections with approximately the same peak Part I). Sample asb and the cobalt asbolane from Ger- position and intensity distribution as those of Co,Ni par- many also contain small amounts of cobalt buserite and ticles, but they show only one hexagonal network of hkO calcium buserite, respectively. Periodicity along the c axis reflections corresponding to sublattice I. for asbolane and buserite are not the same; they are 9.5 The SAED pattern of the cobalt asbolane was described and 7 A, respectively (under vacuum conditions). Sample by Chukhrov et al. (1982). In spite of an absence of Ni, Mn-ox contains lithiophorite as an impurity. the particles displayed patterns with two incommensu- SAED patterns for most of the Co,Ni-bearing asbolane rate hexagonal networks of hkO reflections that they attributed to two sublattices (I and II), with al = 2.84 A

0.02 0.015 and all = 3.15 A. The 100, 110, and 200 reflection inten- - Exp. spectrum sities belonging to sub lattice I were the same as those for 0.01 Fit 0.01 ~\ cobalt nickel asbolane particles [1(110) 21(100)] where- 0.005 \ = i 0\ :-< as 100 and 110 reflections for sublattice II had approxi- 'J -0.005 mately the same intensity. -0.01 Fe K-Edge -0.01 ( Co and Mn RDF for the asbolane samples are plotted V VERNADITE) -0.015 in Figure 9a. The strong second peak attests to the pres- 4 6 8 10 12 4 6 8 10 12 WAVEVECTOR k(A: 1) WAVEVECTOR k(A 1) ence of a Me shell at 2.9 A. The appearance of a weak third peak in the CoK RDF for sample asb was shown Fig. 4. Fourier-filtered nearest Fe-Fe contributions to by analysis in the k space to result from a few corner- EXAFS. (a) Experimental data for goethite, feroxyhite, and vernadite sample 126B. (b) Fit for vernadite sample 124B. Notice sharing (Co,Mn) octahedra. Such linkages are undetect- that the wave beating at about 9-10 A-I is reproduced wen by able at the MnK edge by both EXAFS and preedge spec- assuming two Fe shells at 2.89 and 3.03 A, corresponding to face troscopy (Part I). One possible explanation for this effect and edge linkage, respectively. is that the impurity in the cobalt buserite has a 2-D' struc- MANCEAU ET AL.: MANGANESE HYDROUS OXIDES II 1149

a Mn K-edge Fe K-edge Manganese goethite

0 2 4 6 R (A)

Manganese goethite todorokite

o 2 4 6 R (A) Fig. 7. EXAFS data for manganese goethite. (a) Comparison of Mn and FeK RDF. (b) Similarity of the MnK RDF of todorokite and manganese goethite. Notice their third peaks (arrow). RDF have not been corrected for phase shifts.

k range (2kilR ~ 7r) rendering the overall wave envelope particularly sensitive to the presence of a small amount Fig. 6. (a) A TEM image of Mn-bearing goethite particles. (b) A SAED pattern of one of the particles; it resembles that of of [4]C03+.A set of simulations shows that the detection pure goethite. (c) A representative EDS spectrum. limit of fourfold-coordinated Co is less that 15-20%. NiK EXAFS spectra of the two cobalt nickel asbolane samples are almost identical to that of Ni(OH)2 (Manceau et aI., ture. Analysis in the k space of the Co-O contributions 1987). for asbolane shows in phase backscattered waves (Fig. DISCUSSION 9b). The coincidence of zero crossings proves that the Co distance is preserved from one sample to another. Man- Birnessite ceau et al. (1987) reported 1.92 A for this Co-O bond Two-dimensional vs. 2-D' structure. According to Gio- length. In order to test spectroscopic sensitivity to [4]C03+, vanoli and coauthors (Giovanoli et aI., 1969, 1970a, a simulated Co-O contribution at 1.77 A has also been 1970b; Giovanoli and Stahli, 1970), synthetic sodium plotted in Figure 9b. This wave is out of phase in the high birnessite has a two-layer, orthorhombic unit cell. Its ------

1150 MANCEAU ET AL.: MANGANESE HYDROUS OXIDES II MANCEAU ET AL.: MANGANESE HYDROUS OXIDES II 1151 structure was thought to be related to that of chalcophan- a ite. Both structures have edge- and corner-sharing octa- - Co K-edge . .. Mn K-edge hedra, but in the former, Mn was proposed to substitute for Zn in the interlayer region. With this 2-D' model, a (ASBOLANE) third RDF peak typical of corner-sharing octahedra is expected. Because of Mn substitution for Zn, interlayer Mn atoms would absorb X-rays at the MnK edge, result- New Caledonia ing in a stronger peak than that which occurs for chalcophanite (Fig. 1a). The detection limit of Mn neighbors being better than one atom at low temperature (Manceau and Combes, 1988), the absence of Mn4+ -Mn4+ pairings at 3.4-3.5 A (Fig. 1b) indicates that no Mn4+ ions occur at the Zn position. However, the presence of some low- valency Mn atoms (e.g., Mn2+) cannot be excluded from this study, as it would lead to larger Mn-Mn distances. o 2 4 6 The presence of Na, which has a small backscattering R (A) power, in the Zn position appears unlikely, since the adsorption of monovalent ions is usually nonspecific (Kin- 0.05 /-. / l\\ Asbolane Ural niburgh and Jackson, 1981), i.e., Na+ is adsorbed at min- \.1 .i \ Asbolane N-C eral surfaces by electrostatic attraction in the electrical 0.03 I double layer without establishing a chemical bond with \ Simulation 0x \ \ the surface. I' \ 0.01 " On the basis of the calculation of XRD patterns for ! \ different structural models, Chukhrov et al. (1985b) con- ~ cluded that his birnessite sample (B 1) had a 2-D' struc- -0.01 ture. However, with this method, it was impossible to determine the distribution of isomorphic Mn and Mg at- -0.03 oms among layer and interlayer octahedral sites. By com- Co K-edge parison with the chalcophanite structure, Chukhrov et al. (1985b) proposed that interlayer sites above and below -0.05 3 6 9 12 octahedral layer vacancies would more likely be filled with WAVEVECTOR k(A-1) Mn than with Mg atoms. Now, in light of the preedge results (Part I) and the EXAFS results (no Mn corner Fig. 9. EXAFS data for asbolane. (a) Comparison of nor- sharing in sample Bl), we suggest that Mn4+ atoms are malized Mn and CoKRDF. Notice the presence of a small peak solely located within octahedral layers, whereas Mg at- on the right side of the CoK RDF (arrow) indicating the presence of corner-sharing octahedra. (b) Comparison of Fourier-filtered oms and possibly Mn2+ IMn3+ occupy interlayer octahe- Co-(O,OH) contributions of New Caledonian and German as- dral sites. Because divalent cations have higher specific bolane and simulation assuming 4 0 neighbors at 1.77 A. Notice adsorption capacity than monovalent cations, Mg and, to the phase shift. a lesser extent, Ca seem to be better candidates than Na+ for filling octahedral sites above and below vacancies in Mn02 layers. may have Mn-Me corner-sharing octahedra that are de- The very small peak on the right side of the second tectable by EXAFS except when the metal site is filled RDF peak for sample M12 results from few Mn4+, Co, with light cations like Mg or Ca. Where interlayer Na is Ni, or Cu neighbors at 3.4-3.5 A (Fig. 1b). The presence present, strong corner sharing is unlikely. of Mn-Me corner-sharing octahedra at this distance can Layer stacking. One of the interesting problems in de- be accounted for by the absence of Na, Ca, and Mg cat- termining the structural chemistry of birnessite is deter- ions and the presence of 3d elements. Thus, one may mination of the factors responsible for different layer conclude that both natural and synthetic birnessite struc- stackings. One might suppose that the nature of ex- tures consist of octahedral layers containing Mn4+, but changeable cations might have an effect. To test this hy- they have different layer stackings. These structures, then, pothesis, let us consider birnessite structures in terms of

Fig. 8. SAED patterns and EDS analyses for cobalt nickel asbolane particles (sample asb). For the particles having relatively high content of Ni, SAED patterns consist of two incommensurate hk reflection rings. The smaller the Ni content, the lower the intensity of the inner hk reflections. The SAED pattern for particles that contain only Mn and Co displays only one hk reflection ring. In all cases, 1(10); ~ 1(11);, where I(hk) is the intensity of the hk reflection, and i = I, II is for one of the two incommensurate hk reflection rings. 1150 MANCEAU ET AL.: MANGANESE HYDROUS OXIDES II ---

1152 MANCEAU ET AL.: MANGANESE HYDROUS OXIDES II

close-packed models. For natural one-layer hexagonal cations would decrease, and electrostatic interactions be- birnessite, the structure can be written symbolically as tween the remaining cations would increase. This pre- dicted change of superperiodicity has actually been ob- AbCb,A' b' AbC (Chukhrov et aI., 1985b) served by Post and Veblen (1990). C' The question is now the following: are there vacant where A and C represent the layer anion positions, A' sites in the layers of sodium birnessite? If there are no and C', the H20 molecule sites, and band b', the cations vacant sites, the two-dimensional superperiodicity would in octahedral sites within layers and interlayers, respec- depend only on the degree of negative charge in the layer tively. The location of divalent cations (Mg, Ca, Ni, or and on the valency of the exchangeable cations. If layers Cu) above and below vacant octahedral sites permits full do contain vacant octahedral sites, divalent cations would local charge compensation because the charge of Mn4+ be located above and below those vacancies irrespective ions is balanced by two divalent cations. In addition H20 of the layer charge. In the latter situation, superperiod- molecules are arranged in the interlayer in such a way icity displayed in SAED would reflect the actual distri- that, first, they form Mg octahedra with 0 located on one bution of vacant sites within layers. According to Post side of the interlayer (CA' and C' A stacking) and, second, and Veblen (1990), exchange ofNa for Mg promotes the they form empty prisms with 0 on the opposite side (A' A reorganization of layer stacking and the arrangement of or CC' stacking). The H20 molecules and 0 atoms that Mg ions and H20 molecules such that Mg ions nearly form empty prisms in interlayers can then be connected locate above and below layer octahedral sites. Such an by H bonds. Thus, the arrangement of divalent cations arrangement of interlayer Mg is possible only if the un- and H20 in natural birnessite compensates for local charge derlying octahedral sites are empty. A second argument and enhances the cohesion of layers. favoring the existence of layer octahedral vacancies comes In synthetic birnessite, Na alone cannot provide local from a description of the mechanism of Cr(lll) oxidation charge compensation. Therefore, the presence of Na in to Cr(VI) by sodium birnessite. In Manceau and Charlet birnessite is expected to change the interlayer structure (1992) it is shown that aqueous free Cr3+ ions diffuse to in such a way that favorable conditions are created for and fill in layer octahedral vacancies where the Cr3+ to delocalized charge compensation and stabilization of lay- Mn4+ electron transfer takes place. ers. We will show that the sodium birnessite structure fulfills this expectation. By using the refinement achieved Vernadite by the Rietveld method (Post and Veblen, 1990), the ide- Synthetic vernadite (~-Mn02). Vernadite has a rela- alized one-layer monoclinic structure can be transformed tively large number of corner-sharing Mn06 octahedra, into a three-layer rhombohedral close-packed model, re- as is shown by the shape of its RDF and by the XANES sulting in the following sequence: AbCC'CaBB'BcAA' AbC data (Part I). It is not yet clear whether the vernadite . . . where A, B, C ,represent sites for layer 0 atoms; A', structure is closer to that of tectomanganate or phyllo- B', C' are sites for H20 molecules and N a cations of the manganate of the 2-D' type. In the first hypothesis, ver- interlayers; and a, b, c are layer octahedral sites. We see nadite would be built of multiple octahedral chains joined from the symbolic notation that H20 molecules form along their lengths by sharing vertices. In the second, it empty prisms with 0 atoms from two adjacent layers would have a layered structure with a few Mn atoms ran- (AA' A, BB'B, and CC'C stackings). In the absence of local domly removed from the layer plane to lie above or be- charge compensation, the arrangement of the interlayer low vacant sites, as in the chalcophanite structure. In our species favors the formation of the H bonding between opinion, vernadite should be regarded as a solid solution H20 molecules and 0 atoms. H bonding partly explains extending from phyllomanganate to tectomanganate. It is the difference between the observed structure and the ide- more conveniently pictured as a 3-D (O,OH) framework, alized interlayer structure described above. It may also where cubic and hexagonal close-packing arrangements explain the distortion observed in the hexagonal sym- alternate at random and where octahedral sites are ran- metry of the layers (the value of a/y13 in the monoclinic domly filled, but where two adjacent Mn(O,OH)6 octa- structure of sodium birnessite is much different than the hedra cannot share faces. Such a model can be viewed as value of the b parameter). Thus, H bonding is thought to a mosaic of single and multiple octahedral chains of vari- be responsible for decreasing the space-group symmetry able length ranging from 1 to n octahedra, and the chains from 3R to C2/m or lower. are joined at comers. Because an equal average number Our model suggests that the two-dimensional super- of cations are located between every two adjacent anionic periodicity observed in SAED patterns from sodium bir- layers, neither XRD nor SAED patterns of o-Mn02 show nessite in the past (Giovanoli et aI., 1970a; Chukhrov et basal reflections near 7, 3.5, or 2.35 A. Thus, o-Mn02 aI., 1989; Post and Veblen, 1990) and in the present study cannot be regarded as a c-disordered birnessite as pro- results from a periodic distribution of Na atoms in the posed by Giovanoli (1980a). Birnessite has either a 2-D interlayer region, which is thought to result form electro- or 2-D' structure, depending on the existence of octahe- static interaction. Under these conditions, the replace- dra that share comers, whereas 0-Mn02' like todorokite, ment of divalent cations by monovalent Na would lead has a 3-D anionic framework structure. As a result of to a new superperiodicity; i.e., the number of interlayer birnessite's layered structure, its diffraction pattern should MANCEAU ET AL.: MANGANESE HYDROUS OXIDES II 1153 contain basal reflections, even in the extreme case where goethite with a unique three-dimensional periodicity. With the coherent scattering domains contain only a few layers, these limitations, determination of the structural location i.e., it is c disordered (Reynolds, 1980; Drits and Tchou- and distribution pattern of Mn4+ ions becomes a partic- bar, 1990). The XRD curve of an extremely disordered ularly interesting problem. birnessite sample would then contain only two-dimen- The mismatch in cell parameters between the isostruc- sional hk bands and basal reflections whose positions tural minerals ramsdelli te and goethite is less than 10% would depend on the mean thickness of the coherent scat- (Bums and Bums, 1975, 1977). One could then reason- tering domain (Drits and Tchoubar, 1990). ably imagine an intimate intergrowth of these two phases. Iron vernadite. Simulation of the XRD pattern of hy- However, the Mn RDF of the manganese goethite sample drous oxides led Chukhrov et ai. (1988) and Drits et ai. more closely resembles those of todorokite and buserite (in preparation, a) to conclude that the absence of hkl than that of ramsdellite (Figs. la, 7b). This means that, reflections (I =1=0) is probably associated with (1) the mix- unlike Fe, Mn octahedra mostly share edges with neigh- ing of cubic and hexagonal anionic packings, (2) the small boring octahedra. Thus, we reject the hypothesis of a close size of coherent scattering domains, and (3) channels hav- ramsdellite-goethite association at the scale of the unit ing different size and distribution patterns. The absence cell. The difference of the local environments of Mn and of basal reflections results from similar or identical cat- Fe, the relatively large number of edge linkages, and the ions within each pair of anionic layers. The observation weak preedge spectra that is characteristic of a 2-D struc- by SAED offeroxyhite-like patterns simply indicates that ture (Fig. 3 in Part I) indicate that Mn atoms are segre- hexagonal anionic packing prevails in Fe-rich particles. gated within a discrete phyllomanganate-like local struc- Data from XAS show that Fe and Mn atoms have differ- ture. ent local structures and thus are segregated in domains. The question remains: do Mn clusters occur as a sep- At the nanometer scale and on an average basis through- arate phase or do they have a definite structural relation- out the bulk, Fe octahedra belonging to Fe domains have ship with the goethite lattice? A physical mixture of iron the same interpolyhedral connections as in feroxyhite, goethite and manganese phyllomanganate within the i.e., face, edge, and comer sharing. Instead, Mn domains sample would have been detected during electron and possess a 2-D' local structure. SAED and EDS results X-ray diffraction studies. The apparent discrepancy be- indicate that Mn- and Fe-containing domains are not tween information provided by EXAFS and the preedge space-separated phases, but they coexist within a unique spectroscopies (Part I) helps to determine how Mn clus- coherent scattering domain. ters relate structurally to the rest of the mineral. EXAFS Bums and Bums (1975, 1977) and Ostwald (1984) sug- shows that Mn06 octahedra share as many comers as in gested that natural vernadite possesses a hybrid structure todorokite (Fig. 7b), and from XANES we know that Mn in which Mn and Fe atoms are segregated within irregu- clusters possess a roughly buserite-like local structure (Fig. larly intergrown monoatomic clusters. Mossbauer spec- 3 of Part I). This paradox is solved if one considers that troscopy (Murad and Schwertmann, 1988) and EXAFS the third MnK RDF peak is in part due to the existence (Chukhrov et aI., 1988; Manceau and Combes, 1988) of MnOs-O-Fe(O,OH)s oxygen bridges. Although these demonstrated Fe and Mn segregation, but the intergrowth results do not exclude the existence of edge linkages be- mechanism remains to be proved. EXAFS spectroscopy tween Mn and Fe octahedra, one may postulate that some is unable to demonstrate whether epitaxial intergrowth domains exist where octahedral Mn and Fe chains are occurs with either corner- or edge-sharing linkage or both, joined only by comers. This local structure would resem- since it cannot distinguish Fe from Mn neighbors. How- ble that already known in nsutite (-y-Mn02), where py- ever, chemical identity can be indirectly determined rolusite and ramsdellite domains are joined by comers. through the short-range order sensitivity of the preedge Thus, the structural model for manganese goethite may structure (cf. Part I). The low intensity of the preedges of be described as follows (Fig. lOa). The coherent scattering ferruginous vernadite suggests a layerwise structure for domains are determined by three-dimensional anionic Mn domains (Fig. 3c in Part I). In addition, third peaks frameworks that have a strong tendency to be hexagonally are visible on MnK RDF (Fig. 3a), indicating comer shar- close-packed, resulting in hkl reflections in SAED and ing within Mn domains. These comer linkages are logi- XRD patterns. Each coherent scattering domain includes cally assigned to MnOs-O-Fe(O,OH)s oxygen bridges. Mn and Fe clusters linked through MnOs-O-Fe(O,OH)s bridges. The cationic arrangement of Fe clusters is the Mn-bearing goethite same as for goethite, whereas Mn is distributed layerwise Comparison of the Mn and Fe RDF for goethite dem- in Mn domains. The absence of basal reflections is ac- onstrates that Mn and Fe have different local structures; counted for by approximately the same number of cat- XANES (Part I) and EXAFS data show that Mn atoms ions between each 0 layer pair. The average structure can are tetravalent. Thus the difference in oxidation state be- be defined in terms of a goethite-like cell because of the tween Mn and Fe is the main deterrent to their random relatively regular distribution of filled and vacant octa- mixing among available octahedral sites. On the other hedral sites. However, the vacant sites of the iron goethite hand, X-ray and electron diffraction data indicate that unit cell must be replaced in the manganese goethite unit this mineral's structure is similar to that of homogeneous cell by some Mn, the quantity of which depends on the 1154 MANCEAU ET AL.: MANGANESE HYDROUS OXIDES II

CO(OR)3. The Co and Mn layers have the same size and are commensurate in the plane. Sublattice II corresponds to Ni(OR)2 layer fragments that are free of both Co and Mn, explaining the progressive weakening of SAED reflections as Ni content decreases. In cobalt asbolane, the appearance of two hkO reflection networks in combination with a unique series of 001 reflections with d(OOI) = 9.4 A corresponds to two regularly alternating incommensurate layers. The distribu- a tion of hkO reflection intensities for sublattice II is quite OO.OH . Mn e Fe different from that of cobalt nickel asbolane. Reflection intensity calculations led Chukhrov et al. (1982) to con- Unit cell of goethite Unit cell of Mn-containing goethite clude that this mineral contains alternating tetrahedral -@ ffi @ @---i - fU. -- ---0 ------0------.. -- and octahedral layers, but the similar scattering powers r i for Co and Mn prevent an unambiguous assignment of !~ ~ ~ CYJj 0 cations to particular sites. SAED, XANES, and EXAFS : i i · · 0 i ______j data make it clear that there are no tetrahedral CoOOR L ~_ {~_ _.. J L ~ -. ------0------0 ------. - - layers in cobalt asbolane. The a value of sublattice II (all

b Fe (0.8) aL Mn (0.2) = 3.15 A) is consistent with [4JMn2+layers and cannot be b Mn (0.2) o . attributed to any known [6JC03+and [6JMn4+layer struc- Fig. 10. A structural model for the Mn-bearing goethite. (a) ture. The presence of [4JMn2+layers is then supported but Hexagonal anionic closed packing where domains of goethite not proved by our results. The relative concentration of and phyllomanganate are intergrown. (b) Distribution of cations Mn2+ in the bulk sample is below our XANES detection over available octahedral sites in the unit cell of goethite and in limit, and a lack of spacial resolution with EXAFS pre- the average unit cell of manganese goethite. vents observation of such a structure. Therefore, we pro- pose a structure consisting of octahedral Mn02 layers al- ratio between Fe and Mn atoms. Figure lOb shows a ternating regularly with structural fragments of Mn(OR)2; comparison of unit cells for pure goethite and a mineral the position of Co atoms remains unknown. containing 67 at% Fe and 33 at% Mn. Atom segregation Asbolane One of the most interesting results of this study is the The two incommensurate reciprocal sublattices de- revelation of domain structure in natural hydrous oxides scribed in the result section arise from the regular alter- that contain some quantity of "foreign" cations. Do- nation of two layers. Their unit-cell parameters vary be- mains having a different chemical composition or oxi- cause of their different chemical compositions or dation state of cations may have different structural re- structures. The ratio I( 11O)/I( 100) has approximately the lationships. The limiting case is represented by asbolane, same value for reflections belonging to both sub lattices, where the domains are completely isolated. In cobalt indicating that these layers have an identical structural nickel asbolane," Mn02-Ni(OR)2 and possibly CoOOR motif. According to calculations made by Chukhrov et layers coexist within the same crystal. Each layer is an al. (1980), this intensity ratio is typical of octahedral lay- independent structural fragment that alternates regularly ers. in the c direction. Such a mixed-layer structure can be We can combine data from all of the techniques thought of as a polysome (Veblen, 1991), where slabs are (XANES, EXAFS, SAED, and EDS) to determine the sheet oxides of Mn and Ni. A second example is the pure structure of asbolane. From XANES, we learned that Co manganese mixed-layer asbolane recently recognized by atoms are trivalent and sixfold coordinated, which agrees Butuzova et al."(1990). Here, Mn is presumably present with a Co-(O,OR) distance of 1.92 A (Manceau et aI., in three incommensurate layers as Mn4+, Mn3+, and Mn2+. 1987). This distance suggests a low-spin configuration, as Likewise, the cobalt asbolane from Germany is thought high-spin [6JC03+ions would result in a distance close to to have alternating layers of Mn02 and Mn(OH)2. 1.99 A (Chenavas et aI., 1971). EXAFS data indicate that The second class of domain structure is represented by Mn and Co atoms possess an identical 2-D local struc- iron vernadite and manganese goethite, where domains ture, and it is impossible to use this method to determine of different structure and cation composition are epitax- whether Co substitutes for Mn in Mn02 layers or forms ially inter- or overgrown within the same anion frame- separate CoOOR layers. Nevertheless, in the SAED pat- work. In both minerals, Mn4+ ions have a strong tenden- tern of Ni-free particles from the cobalt nickel asbolane cy to combine by edge sharing of octahedra, but their we see 001 reflections with d(OOI) = 9.4 A. This indicates main structural differences comprise the following: (1) the that two layers with different scattering powers alternate anion framework of manganese goethite is a hexagonal regularly and leads to the conclusion that one layer type array, but in iron vernadite coherent scattering domains corresponds to Mn02 and the other to CoOOR or possess both hexagonal and cubic anionic packings; (2) MANCEAU ET AL.: MANGANESE HYDROUS OXIDES II 1155 the Fe domains in the two minerals have a different cat- et aI., 1993; Part I). New structural models that satisfy ion distribution over available octahedral sites, resulting both XRD and XAS results have been recently proposed in a feroxyhite-like local structure in iron vernadite and for Fe hydrous oxides (Manceau and Drits, in prepara- a goethite-like local structure in manganese goethite. tion; Drits et aI., in preparation, a, b) but remain to be established for Mn compounds. Structure of two-line hydrous oxides Knowledge of the local structure of hydrous oxides is CONCLUSIONS needed to understand the role of short-range cation or- 1. Natural magnesium birnessite has a one-layer hex- dering in the formation of long-range ordering. Indeed, agonal unit cell; synthetic sodium birnessite has a mono- in their highest disordered state, pure Mn and pure iron clinic subcell with structural similarity to chalcophanite hydrous oxides usually display only two weak, broad hk but lacking comer-sharing Mn4+-Mn4+ octahedra. Struc- bands (or "two lines") on X-ray and electron diffraction ture of the interlayer and layer stacking are controlled by patterns. EXAFS data suggest at least five distinct local the nature of the interlayer cation. The structures of mag- structures that can correspond to such a diffraction pat- nesi um and sodium birnessite can be described using an- tern; the actual local structure depends on how the gel ionic close-packed layer models. formed. Ferric gels that are freshly precipitated after com- 2. Synthetic vernadite (0-Mn02) is not c-disordered plete hydrolysis and oxidation of ferrous sulfate or chlo- birnessite. Its structure probably consists of a mosaic of ride solutions at neutral pH show a lepidocrocite-like single and multiple octahedral chains having variable ')'-FeOOH local structure (Combes et aI., 1986). How- length and width. The chains are joined by comers in a ever, gels synthesized from a nitrate or chloride ferric 3-D anionic framework. That there are the same mean solution show a goethite-like (a-FeOOH) or akaganeite- number of cations in each two adjacent anionic layers like ({3-FeOOH) local structure, respectively (Combes et explains the absence of 001 reflections. The absence of hkl aI., 1989). When a goethite-like gel is heated (to 92 °C) reflections results from structural disorder in the distri- for a few hours, it transforms into a feroxyhite-like, two- bution of chain sizes, the small coherent scattering do- line gel, and subsequently into hematite (Combes et aI., mains, and the mixture of anionic cubic and hexagonal 1990). This last local structure has been observed in all packing. of the natural two-line Fe gels studied up to now (the so- 3. The X-ray-amorphous hydrous iron manganese ox- called protoferrihydrite; Combes, 1988). It is interesting ide phase called iron vemadite that occurs on the ocean that the interpolyhedrallinkages that are typical of well- floor has a hybrid structure with coexisting Mn4+ and crystallized Fe minerals are already present in their pre- Fe3+ domains. In one domain, Mn atoms are distributed mineral gel. Thus, the structural formation of ferric ox- layerwise, as in phyllomanganate, whereas in the other, yhydroxides (a-, {3-, ')'-FeOOH) is undoubtedly decided Fe octahedra are linked as in feroxyhite. very early in the polymerization process. On the other 4. Natural manganese goethite, previously described hand, the structure of two-line Mn4+ hydrous gels (0- as goethite-groutite, consists of Mn4+ and Fe3+ domains Mn02) seems not to be strictly related to that of the sub- that have cation distributions similar to phyllomanganate sequent, well-crystallized polymorph. Mn gels most prob- and goethite, respectively. ably consist of a 3-D framework of randomly distributed 5. Cobalt nickel asbolane is formed by layers ofMn02, edge- and corner-sharing Mn02 octahedra; that might ex- Ni(OH)2' and possible CoOOH or CO(OH)3, alternating plain why they seem able to transform into a large variety regularly along the c axis. In cobalt asbolane, Mn2+ tet- of structures. rahedral layers are probably regularly mixed with layers It is evident from this discussion that diffraction effects ofMn02. However, additional experimental evidence for from extremely disordered Mn4+ and Fe3+ hydrous ox- this structure is needed. ides have low sensitivity to changes in local structures. This study reemphasizes the complex structure of some The similarity of XRD patterns for all two-line com- low-temperature minerals. Structural variability and im- pounds results from the fact that their coherent scattering perfection occur in the stacking of layered mineral struc- domains have extremely small values (a few tens of tures such as birnessite. Much about their structure re- thousands of cubic Angstroms) and usually are composed mains unknown, and one may foresee that birnessite with of mixed cubic and hexagonal anionic packings, where different degrees of periodicity in its two-dimensional layer each pair of anionic layers contains, on average, the same stackings as well as with structural imperfections in the number of cations. Structural interpretations of these fea- interlayer region will be found in natural environments. tureless diffraction patterns are hardly feasible and at least A second class of imperfection concerns the hybrid struc- not always unique. The idea of adding about 33% tetra- ture of low-temperature minerals at a very fine scale. Two hedrally coordinated Mn and Fe to fit XRD profiles types of structural situations have so far been identified: (Chukhrov et aI., 1988; Eggleton and Fitzpatrick, 1988, (1) a simple deviation from the random distribution of 1990) is then to be treated with caution; the concept is atoms in a solid solution series (Manceau, 1990), and (2) not supported by Mossbauer spectroscopy (Cardile, 1988) the existence of intimately mixed discrete phases. On one and contradicts XANES and EXAFS results at both the hand, these phases can entertain no structural relation- Mn and FeK edges (Manceau et aI., 1990b; Waychunas ship (except for the presence of H bonding) such as is 1156 MANCEAU ET AL.: MANGANESE HYDROUS OXIDES II observed for asbolanes. On the other, they can be epitax- Geology Review, 1983, 25, 838-847 (translated from Izvestia Akade- ially intergrown, as is true for iron vernadite, manganese mie Nauk, SSSR, Seriya Geologicheskaya, 5, 91-99). Chukhrov, F. V., Gorshkov,A.I., Drits, V.A., and Dikov, ¥.P. (1985a) goethite, and iron diaspore (Hazemann et aI., 1992). All Structural varieties oftodorokite. International Geology Review, 1985, these types of crystal defects result from nonequilibrium 27, 1481-1491 (translated from Izvestia Akademie Nauk, SSSR, Seriya crystallization in the thermodynamic and kinetic condi- Geologicheskaya, 11, 61-72). tions that prevail at the Earth's surface. For instance, ep- Chukhrov, F.V., Sakharov, B.A., Gorshkov, A.I., Drits, V.A., and Dikov, itaxial overgrowths are thought to result from a hetero- ¥.P. (1985b) Crystal structure of birnessite from the Pacific Ocean. International Geology Review, 1985,27,1082-1088 (translated from geneous nucleation in conditions of undersaturation Izvestia Akademie Nauk, SSSR, Seriya Geologicheskaya, 8, 66-73). (Charlet and Manceau, 1992). Deciphering the actual Chukhrov, F. V., Gorshkov, A.I., and Drits, V.A. (1986) Crystallochem- structure of these and other complex minerals is an im- ical systematics of Mn-minerals with tunnel structures. Izvestia Aka- portant first step toward an advance understanding of demie Nauk, SSSR, Seriya Geologicheskaya, 10, 3-18. -(1987a) News in crystallochemistry of manganese hydroxides. their conditions and mechanisms of formation. Zapiski Vsesoyusnogo Mineralogicheskaya Obchestva, 116, 210-222. Chukhrov, F.V., Sakharov, B.A., Gorshkov, A.I., Drits, V.A., and Bari- nov, N.N. (1987b) New mixed-layer phase from oceanic Fe-Mn nod- ACKNOWLEDGMENTS ules. Lithology and Row Materials, 5, 112-120. Chukhrov, F.V., Drits, V.A., Gorshkov, A.I., Sakharov, B.A., and Dikov, The authors thank R. Giovanoli for providing the sodium birnessite sample and the staff of LURE for the synchrotron facility. A critical read- ¥.P. (1987c) Structural models ofvernadites. Izvestia Akademie Nauk, SSSR, Seriya Geologicheskaya, 12, 198-204. ing of a preliminary version by G.A. Waychunas is acknowledged. The Chukhrov, F.V., Manceau, A., Sakharov, B.A., Combes, J.-M., Gorsh- authors express their gratitude to Susan Stipp, who helped improve the kov, A.I., Salyn, A.L., and Drits, V.A. (1988) Crystal chemistry of English. This research was supported by CNRS/INSU through the DBT program, grant 90 DBT 1.03 (contribution no. 414). oceanic Fe-vernadites. Mineralogicheskii Zhumal, 10, 78-92. Chukhrov, F.V., Gorshkov, A.I., and Drits, V.A. (1989) Supergenic manganese hydrous oxides, 208 p. Nauk, Moscow. Combes, J.-M. (1988) Evolution de la structure locale des polymeres et REFERENCES CITED gels ferriques lors de la cristallisation des oxydes de fer. Application au Blake, R.L., Hessevick, R.E., Zoltai, T., and Finger, L.'W. (1966) Refine- piegeage de l'uranium. These de Doctorat, Universite Paris 6, Paris. ment of the hematite structure. American Mineralogist, 51, 123-129. Combes, J.-M., Manceau, A., and Calas, G. (1986) Study of the local Bums, R.G., and Burns, V.M. 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