Pseudomorphic replacement of manganese oxides by iron oxide minerals

Geoderma, 42 (1988) 199-211 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

199

Pseudomorphic Replacement of Manganese Oxides by Iron Oxide Minerals D.C. GOLDEN 1, C.C. CHEN 2, J.B. DIXON 1 and Y. TOKASHIKI3

1Department of Soil and Crop Sciences, Texas A & M University, College Station, TX 77843 (U.S.A.) 2EMRO, ITRI, 195-64, Sec. 4, Chung-Hsing Rd., Chu Tung, Hsin Chu, Taiwan 31015 (R.O.C.) :JDepartment of Agricultural Chemistry, University of the Ryukyus, Nishihara, Okinawa (Japan) (Received April 13, 1987; accepted after revision February 11, 1988)

ABSTRACT Golden, D.C., Chen, C.C., Dixon, J.B. and Tokashiki, Y., 1988. Pseudomorphic replacement of manganese oxides by iron oxide minerals. Geoderma, 42: 199-211. Manganese oxide minerals containing structural Mn 4+ ions can act as oxidizing agents towards ferrous ions in solution. This oxidation causes the precipitation of Fe (III)-oxides in association with manganese oxide minerals, a probable mechanism for the natural formation of iron oxides in concretions and nodules in the subsurface horizons of soil, where transport of reduced iron takes place. The synthetic manganese oxide minerals todorokite and birnessite when exposed to ferrous ions caused a pseudomorphic precipitation of iron oxide minerals. The pseudomorphism was observed to be a macroscopic phenomenon; the iron oxide precipitate consisted of fine crystallites of feroxyhyte, lepidocrocite, or akaganeite in a loose mesh having the form of the original manganese oxide mineral. The anions present and the original structure of the manganese oxide mineral influenced the nature of the iron oxide formed. Reactions of ferrous sulfate (0.05 M) with Na-birnessite, todorokite or soil minerals containing birnessite and lithiophorite yielded sulfateadsorbed-feroxyhyte as the major product. Reactions of ferrous perchlorate (0.05 M) with Nabirnessite yielded lepidocrocite and with todorokite yielded lepidocrocite and akaganeite. Reactions of ferrous chloride with birnessite or todorokite yielded akaganeite as the major phase. All iron oxides formed were poorly crystalline and contained adsorbed water.

INTRODUCTION

Soil Mn-oxides are generally found in association with iron oxides. They are found commonly as nodules, stains on ped surfaces, or rarely as pans. W h e n examined as thin sections with an electron microprobe, alternating zones of Fe- and Mn-enrichments could be observed in such nodules. Formation of Feand Mn-oxide associations has been attributed to repeated changes in redox conditions (Eh and p H ). There are very few data available on the mineralogy

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of Mn- and Fe-oxides found in Fe, Mn-nodules. The principal authigenic iron oxide found in manganese nodules has been goethite, less common were lepidocrocite, feroxyhyte, hematite, and ferrihydrite (Chukhrov and Gorshkov, 1981 ). With the exception of goethite and hematite, other iron oxide minerals (e.g. ferrihydrite ) and manganese minerals (birnessite, cryptomelane etc. ) are extremely difficult to identify due to poor crystallinity and low concentrations. Improved procedures (Ross et al., 1976; Uzochukwu and Dixon, 1986; Tokashiki et al., 1986) have made it possible to identify and quantify some of these minerals. The association of Fe-oxide and Mn-oxide should be linked to their genesis. In a situation where redox fluctuates, the oxidation of Fe 2+ or Mn 2+ to form Mn-oxides or Fe-oxides is determined by the Eh and pH of the system. Fe-oxides form at a faster rate in the presence of Mn-oxides (McKenzie, 1987 ). This "catalysis" appears to be the key to precipitation of Fe-oxides in Mnnodules. The purpose of this research was to investigate the formation of iron oxide minerals by the oxidation of Fe 2+ ions in the presence of natural and synthetic Mn-oxide minerals. The products of the reactions were identified and the kinetics of the reaction, effects of mineralogy of manganese, and the presence of anions were also investigated. These model reactions will be discussed in relation to observations on natural minerals.

MATERIALS AND METHODS

Birnessite (Na4Mn140279H20) was synthesized by the method of St~ihli (1968) and todorokite [ (Mgo.vTNao.o3)(Mgo.lsMn2+o.~oMn4+5.22)] 0123.07H20 was synthesized by the method of Golden et al. (1986) by autoclaving Mgexchanged birnessite at 155 ° C in a teflon-lined stainless steel container. Three natural clay samples containing manganese oxide minerals (Table I ) were also used. TABLE I Mineralogical composition of natural clay samples containing Mn-oxide minerals Sample #

Particle size

Location

Mineralogy*

Mn-7-BtC Mn-6-BtC Mn - l l - 2

< 2 ttm < 2 ttm <0.2 pm

Okinawa, Japan Okinawa, Japan Okinawa, Japan

Lp .2 >> Bs > K Lp=Bs>K Bs> Lp>> K

*~Lp ---lithiophorite; Bs -- birnessite; K -- kaolinite. *2Well crystallized.

201

Experiment I Kinetics of dissolution of Mn-oxides were examined by placing 50 mg of a Mn-oxide mineral (birnessite) in 200 ml of 0.05 M Fe 2+ [FeSO4 and Fe (C104) 2] solution adjusted to pH 4.9 + 0.1 at 22°C in an atmosphere of He and stirred with a magnetic stirrer. Small aliquots (2 ml) drawn at different intervals of time were freed from suspended matter by centrifugation. The diluted supernatant was analyzed for Mn and Fe by atomic absorption spectroscopy.

Experiment 2 Reagent grade ferrous chloride, ferrous sulfate, and ferrous perchlorate were used to prepare solutions of 0.05 M Fe 2+ ions. The manganese minerals (synthetic birnessite, todorokite, and natural clay samples containing Mn-oxides ) were deposited on glass slides and carefully dipped in the above solutions adjusted to pH 4.9 + 0.1 for periods ranging from 13 to 41 h. Iron-oxide precipitation was observed visually and the complete disappearance of the dark color of Mn-oxide was assumed to be the end of the reaction. The supernatant :solution was siphoned out carefully and the slide was transferred to a beaker of distilled water and left for 6 h. The process was repeated until all the soluble salts were removed. Then the slides were dried and irradiated with CuKa radiation using a Philips Norelco X-ray diffractometer equipped with a graphite monochromator. Chemical analysis, scanning and transmission electron microscopy, and infrared (IR) spectroscopy were performed on material taken from the slide.

Experiment 3 For the TEM investigations, micrographs were obtained of in-situ manganese oxide replacement. The Mn-oxide minerals were deposited on a special coded holey carbon grid and the same grid was floated in an inverted position on a drop of Fe e+ solution for a 12-h'period for the pseudomorphic Fe-oxide to form and then floated on a distilled water surface to wash out the excess salt (Mn 2+ and SO42- ). Once dry, the sample was examined under TEM. The ironoxides resulting from the kinetics studies also were deposited on a holey carbon grid and examined by TEM. IR spectra were obtained of 0.3-0.6 mg of sample dispersed in 300 mg of anhydrous KBr and formed into pellets at a pressure of 1.38-105 kPa for 5 rain. A Perkin Elmer 283 IR spectrometer equipped with a model 3500 data station was employed. The original birnessite sample and the pseudomorphic iron oxide samples were observed by scanning electron microscopy (SEM). Air-dried samples were mounted on aluminum stubs and examined with a JEOL JSM25 microscope

202

after coating with carbon. Transmission electron microscopy (TEM) was performed with a Zeiss 10C electron microscope operated at 60 kV. RESULTS

Experiment I The dissolution of birnessite was rapid and the amount of manganese released reached 27 mg (total Mn in 50 mg birnessite) in about two days for the ferrous sulfate system (Fig. 1 ). For the Fe (C104)2 system the rate of reaction was much slower and the total dissolution of birnessite took about 10 days. Experiments 2 and 3 The initial iron-oxide precipitate from the reaction of birnessite and Fe 2+ is pseudomorphic after birnessite (Fig. 2). The scanning electron micrographs ( SEM ) show the general similarity of this iron-oxide mineral to the platy morphology of birnessite; transmission electron micrographs (Fig. 3a and b) show more detail. The pseudomorphic iron-oxide is a composite of fine elongated particles that were sufficiently intermeshed to form a plate with an open fabric which resisted disruption by sonication. The X-ray diffraction pattern of the Fe-oxide had only two broad lines (Fig. 4, FeSO4/b) which could be assigned to ferrihydrite or feroxyhyte. The fibrous morphology strongly suggests it to be more like feroxyhyte (Chukhrov et al., 1976; Carlson and Schwertmann, 1980) rather than ferrihydrite which has a granular morphology (Schwertmann and Taylor, 1977). When synthetic todorokite reacted with ferrous sulfate, the iron-oxide prod1 O0

FeS04

~ 60

:~40 2Ot 0 0

I 50

I 100 Time,

I 150

I 200

I 250

hrs.

Fig. 1. Rate of M n ~+ released during the reaction of Fe 2+ with Mn-oxides in experiment 1.

203

i!!~i

Fig. 2. Scanning electron micrographs (SEM) of Na-birnessite (B) and pseudomorphic ironoxide precipitated by reaction of the former with FeS04 (F) from experiment 2.

uct still retained the overall morphology oftodorokite (Fig. 3d). The iron mineralogy and appearance, however, seemed to be similar to that obtained by reaction of FeSO4 with birnessite. The feroxyhyte resulting from the above reactions was transformed to goethite when allowed to incubate for about two weeks in the same solution or in distilled water at 22 °C (data not shown). Iron-oxide with fibrous morphology similar to that of the ferrous sulfatebirnessite reaction product was observed in the case of natural manganeseoxide samples from soil clays after reaction with FeSO4 solution (data not shown). The major silicate mineral found in all cases was kaolinite, whereas birnessite and lithiophorite were the manganese minerals. The IR spectra (Fig. 6) and the XRD patterns (not shown) of these three natural clay samples before and after reaction with FeSO4 were dominated by layer silicates. The difference IR spectrum (obtained by subtracting the IR spectrum of the DCB treated sample from that of the untreated sample) had peaks at 590, 650, 970, 1040, and 1130 cm -1 (spectrum C, Fig. 6), indicating the presence of a unidentate sulfate ion (Table II) (Nakamoto, 1963). The peak at 900 cm-1 (Fig. 6 ) could arise from partial dissolution of A1 from the layer silicate structure as this peak generally corresponds to octahedral A12-OH vibration. The 425 and 490 cm-1 peaks may be due to [FeO~] octahedral vibrations in poorly crystallized Fe-oxides. When birnessite reacted with ferrous perchlorate, poorly crystalline lepi-

204

docrocite was formed as the reaction product. It was similar in morphology (Fig. 3e) to the crumpled lamellar crystals of lepidocrocite reported by Mackenzie and Meldau (1959). Also, the final product was pseudomorphic after birnessite as in the case of the FeSOt-birnessite system. Only the broad XRD peaks at 0.332, 0.254, 0.193, and 0.152 nm (Fig. 4, Fe(C1Ot)ffb) were visible in this case. The 0.254-nm peak, however, was slightly larger than one would expect for lepidocrocite, indicating probable presence of some ferrihydrite. The presence of lepidocrocite was confirmed by IR peaks at 1018 and 350 cm-1. The T E M of the product from the Fe (CIO4)2 and todorokite reaction (Fig. 3f), however, showed the formation of particles which were morphologically different from those produced by the reaction of birnessite and Fe (ClOt)2 (Fig. 3e ), suggesting an iron mineral other than lepidocrocite. The fringes observed in Fig. 3f are about 30-40 .~ in width and continuous throughout the length of the pseudomorphic particle. Akaganeite as described by Watson et al. (1962)

205

Fig. 3. Transmission electron micrographs (TEM) of: a. birnessite; b. iron-oxideresulting from the reaction of the birnessite particle in 3a and FeSO4 from experiment 3; c. todorokite formed from birnessite; d. iron-oxide resulting from the reaction of FeSO4 and todorokite; e. iron-oxide resulting from the reaction of Fe (C104)e and birnessite; f. iron-oxideresulting from the reaction of Fe (CIO4)2and todorokite;g. iron oxide resultingfrom the reaction of FeCl2and birnessite; and h. iron-oxide resultingfrom the reaction of FeC12and todorokite (c-h from experiment 2 ). consists of individual somatoids made of rod-shaped crystals about 30 A thick separated by spaces of about 30 ~,. The fringes observed in Fig. 3f, however, relate to the recent interpretation of the structure of akaganeite by Holm (1985), where unit cells of a hollandite type ( 2 × 2 tunnels containing C1-ions) form a superstructure with larger tunnels which are 3 unit cells wide and 3 unit cells tall which have walls 1 unit cell thick. The IR peak at 425 cm-1 supports the identification of akaganeite in a very poorly crystalline form as suggested by T E M (Fig. 3f). Although akaganeite would be very unlikely in a system free of Cl-, the presence of 0.01% of C1- as an impurity in the Fe (C104) 2

206

0.332 Treatments 0 163

0.254

I

I°

-"'~-~:~": '~'~='='::--" I - -

Fe(O,o4)2,b,t='" :_Jt_:.:::...:.,z ,

....

='~-,~':"t " ~

3390"503749 nm

~

I- ' - - - ' v

,,"qN~*~

[

'

"~

,]

20

Fig. 4. XRD patterns of iron-oxides obtained by reaction of Mn-minerals with ferrous ions in solution.

used in this reaction may have provided the necessary amount. The FeC12Mn-oxide system gave clearly identifiable XRD patterns for akaganeite (Fig. 4, FeC12/b and FeC1Jt). The XRD pattern for the FeC12-todorokite reaction product was more crystalline than that for the FeCl2-birnessite system in that most of the expected lines for akageneite were present and were narrower (Fig. 4, FeC12/t). In all the systems where iron-oxide was formed from todorokite, a peak at 0.339 nm represented some unreacted manganite (7-MnOOH) which is an impurity in the original todorokite sample. The manganite crystals did not react completely during the time provided as these crystals were rather large and had relatively smaller surface areas (Golden et al., 1986 ). The TEM investigation of the FeC12-todorokite reaction product (Fig. 3h) shows wellformed akaganeite crystals in which continuous fringes (30-40 ~, repeat distance) in contrast to the FeC12-birnessite reaction product (Fig. 3g) and are evident in the pseudomorphic crystal replacing the original todorokite. Also, smaller akaganeite outgrowths could be observed which were twinned at 120 °. Characteristic infrared (IR) absorbances and X-ray diffraction peaks were obtained for the iron oxide products lepidocrocite and akaganeite. Some evidence for adsorbed perchlorate was apparent in the Fe (C104)2-manganeseoxide systems; the peaks near 1120 c m - ] [Fe (CIO4)2/b, Fe (C104)fft: Fig. 5] due to perchlorate ion diminished upon dialysis for 24 h. The IR spectrum of FeC12-manganese oxide system showed no evidence for adsorbed anions. The iron oxide resulting from the FeC12 and todorokite reaction gave a IR pattern identical to that for akaganeite (Marshall and Rutherford, 1971; Gonzalez-

207 i

i i Treatment

~

i

i

i

i

I

I

i \

/1010

/935

~ 48°1

l IO4O 1800

1400

1000

600

Wave number cm - 1

200

1200

1000

800

59o

l~jl

600

400

Wave number cm - 1

Fig. 5. Infrared spectra of iron-oxide minerals obtained by reaction of manganese-oxideminerals with ferrous ions (S = sulfate, PC = perchlorate, A = akaganeite, and t = todorokite). Fig. 6. Infrared spectra of natural Mn-mineralstransformedto iron-oxidesby Fe2+ ions (A), after dithionite-citrate-bicarbonatetreatment (B), and the differenceA - B (C). The spectra A and B were normalizedusing the kaolinite peak at 535 cm- 1, prior to substraction. Calbert et al., 1981). The two unidentified peaks at 1018 and 350 c m - i for Fe ( C104 ) 2-Mn-oxide systems and FeC12-birnessite systems probably were due to lepidocrocite which was present in the reaction products from Fe (CIO4)2 and birnessite, Fe (C104)2 and todorokite, and FeC12 and birnessite [Fe ( C 1 Q ) f f b, Fe (C104)2/t, and FeC1Jb: Fig. 5 ]. Chemical composition of the iron oxides (%Fe) formed by reaction of Fe 2+ ions with Mn-minerals (Table III) were consistent within the type of ironoxide mineralogy observed. The percent Fe, however, is lower t h a n for the stoichiometry of FeOOH due to adsorbed water inherent in such poorly crystalline precipitates and also due to chloride in the structure in the case of akaganeite. The Mn contents of the reaction products of todorokite, although small, were consistently higher t h a n t h a t of birnessite, within a given ferrous salt t r e a t m e n t due to a trace of manganite impurity in todorokite which did

208 TABLE II Infrared peak positions for S O l - ligands with decrease in symmetry (after Nakamoto, 1963) Compound

Symmetry vl Ve ...................................................

Free SO~-

Td

-

Unidentate SO][Co(NH3)sSO4]Br

C3v

970 (m) 438 (m) 1032 to 1044 (s) 995 (m) 462 (m) 1050 to 1060

Bidentate SO~-

C2v

[ (NH3) 4Co( NH2/~Co (NH3) 4] [ NO3 ] 3 SO4 Feroxyhyte-SO*~

-

v3 v4 (cm -1 ) ........................... 1104 (vs) *~

613 (vs)

1117 645 to 1143 (s) (s) 1170 641 610 and 1105

975 (m) 425 1045 1125 690 to 450 (m)

604 (s) 571

600

*lm = medium, s = strong, vs = very strong. *2A unidentate SO42- (?). TABLE III Chemical analysis of pseudomorphic ferric oxide precipitates obtained by reaction of manganese oxides with Fe e+ salts Reactants

Analysis of iron-oxide (%) Fe

Mn

FeS04 + t FeSO4 + b Fe (CIO4) 2+ t

34.58 36.27 40.33

0.30 0.00 0.83

Fe (C104) 2+ b FeC12 + t FeCl2 + b

44.36 50.82 51.62

0.31 1.07 0.38

Mineral formed

Feroxyhyte Feroxyhyte Lepidocrocite, akaganeite Lepidocrocite Akaganeite Akaganeite, lepidocrocite

t = todorokite; b = birnessite.

not completely react within the given reaction period due to its large particle size. A p p a r e n t l y t h e r e w a s l i t t l e o r n o M n s u b s t i t u t i o n i n t h e i r o n o x i d e m i n erals formed.

209 DISCUSSION The synthetic and natural manganese oxide minerals with structural Mn 4+ ions can act as oxidizing agents towards Fe 2+ in solution (Postma, 1985 ). Under suitable pH's ( - 5 ), the Fe 3+ will hydrolyze, polymerize and precipitate as iron oxides which are pseudomorphic after the original manganese oxide mineral. This process could be represented by the following reactions: Fe 2+ ( a q ) - , F e 3+ (aq) + e -

(1)

Fe 3+ (aq) + 3 H 2 0 - ~ F e O O H + H 2 0 + 3 H +

(2)

MnO2 (s) + 4 H ÷ + 2e- --~Mn2+ + 2 H 2 0

(3)

The reason for pseudomorphism appears to be the oxidation of Fe 2÷ at the site of the Mn 4+ ion and Fe 3+ ion being hydrolyzed and polymerized immediately following oxidation so t h a t the immobile Fe-oxide phase effectively replaces the former position of the Mn-oxide. Apparently the rate of hydrolysis of Fe 3+ (eq. 2 ) determines the extent to which the pseudomorphic form of Mnoxide is preserved. For good pseudomorphism the kinetics of hydrolysis of Fe 3+ must be fast or else the second phase would form separately due to migration of Fe 3+ from the site of oxidation. The anions in the solution had some influence upon the mineralogy of the iron-oxide formed in that SO~- gave rise to feroxyhyte and then to goethite, perchlorate (with C1- as an impurity) to lepidocrocite or akaganeite, and CIto akaganeite. Formation of either ferrihydrite or feroxyhyte has been reported in a stream receiving acid mine drainage rich in SO42- ions (Brady et al., 1986 ). Similar minerals were reported in ochreous deposits in drain pipes and ditches by Siisser and Schwertmann at neutral pH (1983). Chukhrov et al. (1976) also reported ready transformation of feroxyhyte to goethite. Inasmuch as both feroxyhyte and ferrihydrite are intermediates rather t h a n final products they are not expected to be found extensively in nature except in a Fe 2+ solution that is currently undergoing oxidation or when it is stabilized by impurities such as silica as in the case of ferrihydrite. Goethite is known to be associated with manganese oxide minerals in natural manganese nodules (Tokashiki et al., 1986; Chukhrov et al., 1978) but the occurrence of ferrihydrite and feroxyhyte has been reported less frequently (Chukhrov et al., 1978) than that of goethite and hematite. Chloride is known to occupy tunnel positions in akaganeite and akaganeite has been reported to form by oxidation of Fe 2+ in seawater (Holm et al., 1983 ). Several factors seem to influence the formation of akaganeite by the oxidation of Fe 2+ ions. Presence of chloride in the medium is essential as chloride ion occupies a position within the akaganeite tunnel and possibly stabilizes it (Holm et al., 1983). Secondly, the structure of initial Mn-oxide which oxidizes the Fe 2+ also seem to have some influence in determining the final Fe-oxide phase.

210

The effect of both factors, presence of C1- ' and the initial structure of the Mnoxide, could be observed in the two cases where Fe ( C104 )e (with C1- impurity ) and FeC12 reacted with todorokite to form akaganeite (Fig. 3, f and h). In both of the above cases, fringes corresponding to the akaganeite tunnels (30-40 wide) were observed parallel to the direction of the original todorokite fibers indicating some structural control by todorokite upon the akaganeite pseudomorph resulting from Fe e +-todorokite reaction. Sulfate adsorption was observed in the case of iron oxides derived from ferrous sulfate solutions. Sulfate ions adsorbed on poorly crystalline iron-oxide appeared to be monodentate ligands. The ferrihydrite and lepidocrocite both had lower than ideal stoichiometric iron contents. This is probably due to the high adsorbed or structural water content of poorly crystallized iron-oxides. A similar excess water content has been reported for poorly crystalline aluminous goethite (Fey and Dixon, 1981). A nonstoichiometric chlorine content has been reported for akaganeite (Holm et al., 1983 ); therefore, the percentage composition of our akaganeite sample may not necessarily be constant. Little or no Mn substitution in the iron-oxide structure was suggested by the chemical analysis of the iron-oxides resulting from the above reactions. The presence of goethite in association with Mn-oxides may form via the intermediate ferroxyhyte or lepidocrocite. Formation of akaganeite was related to the chloride anion and should not be anticipated in nature except in highly saline soils and in marine or similar environments. ACKNOWLEDGEMENT

The authors wish to thank Ms. Nancy Lee and for her assistance in preparing the manuscript.

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