SOME STABILITY RELATIONS in the SYSTEM Mn-Oz-Hzo at 25" and ONE ATMOSPHERE TOTAL PRESSURE Ownrc Bnrcrnn, H Art:Ard,U N'irtersity, Cambridge, M Assachwsetts

Home , Manganese oxide, Manganosite, Ramsdellite

THE AMERICAN MINERALOGIST, VOL. 50, SEPTEMBER, 1965

SOME STABILITY RELATIONS IN THE SYSTEM Mn-Oz-HzO AT 25" AND ONE ATMOSPHERE TOTAL PRESSURE Ownrc Bnrcrnn, H art:ard,U n'irtersity, Cambridge, M assachwsetts.

ABsrRAcr Stability relations in the system Mn-OrHzO rvere investigated at 25o C. and one atmosphere total pressure.Seven compounds, Mn(OH)2, MnsOr, y-Mn:Os, r-MnOOH, 6-MnOz, 7-MnO2, and hydrohausmannite s'ere synthesized under controlled conditions of Eh and pH. Hydrohausmannite was found to be a mixture of MnsO+ and/or l-Mn:Or and p-MnOOH rather than a single phase.The name feitknechtite is proposed for naturally occurring B-MnOOH. Free energy of formation data were obtained for the first six com- -132.2 -133.3 pounds listed. They are respectively: -147.14 kcal., -306.2 kcal , kcal., kcal., - 108.3 kcai, and - 109 1 kcal. Eh-pH diagrams were constructed to shorv stability relations among the phases in the system Mn-OrHzO. Good correspondencewas found between stability relations observed in the laboratory and those observed in natural occurrencesof manganeseoxides. A model is describedfor the supergeneoxidation of rhodochrosite.

INrnooucrroN Manganese,a transition metal, is capableof existing in a number of differentoxidation states (2f,3+,4+,6+,7+). The 2f and 4* oxidation states are the lowest and highest that have been observedin nature, although the ephemeralexistence of other statesis conceivable. Commonly, manganeseoxide minerals contain more than one oxidation state of manganese,and a continuousseries between 2* and 4* is observed in the average oxidation state of manganesein supergeneoxides (whether a 3f oxidationstate actually occursin the solid state or is real- ized through an equal distribution of 2* and 4f statesremains a moot question). The mineralogyof the oxides,hydroxides and hydrous oxidesof manganeseis varied and complex. Application ol n-ra)/diffraction techniques to the identificationof theseminerals has revealeda large group consisting of more than thirty mineral species(Hewett and Fleischer,1960). A great many of theseminerals are found in supergenedeposits and have beenformed by the supergeneoxidation of manganese-richprotore. The assemblageof manganeseoxide minerals in the system Mn-Oz-HzOob- servedin supergenedeposits shows little variation with the type or origin of the protore from which it was derived.Commonly, the protore consists of rhodochrosite, either of sedimentary or hydrothermal origin, although silicateprotore is not unknown. At a number of depositsthe protore consists of carbonatescontaining as little as 2-3 per cent manganese(reported as MnCO3). 1296 THE SYSTEM Mn-Oz-IIzO 1297

The closesimilarity in oxideassemblage among the supergenedeposits, regardlessof protore type, indicatesthat the primary factors controlling the formation of this assemblageare the physico-chemicalvariables of the supergeneenvironment; the temperatureat a value closeto the mean annual temperatureof the region, the pressureat approximatelyone atmosphere.Under these conditions, each oxide mineral is stable only within certainlimits of oxygenpartial pressure)depending upon the oxidation state, or states, of manganesethat it contains.The manganese oxides are sensitive to the intensity of the oxidizing conditions of their environment,and may be thought of as a sort of oxygenbarometer that reflects the oxidation gradient in supergeneprocesses. Because water is ubiquitous in the supergeneenvironment, and Eh (oxidation potential) and pH (negativelog of hydrogenion activity) are easilymeasured vari- ables in aqueous systems, it is convenient to expressstability relations amongsupergene minerals in terms of thesevariables (Garrels, 1960). Valuable information could be obtained concerning both the occurrence of manganeseoxides and conditions pertaining in supergenedeposits that contain manganeseoxide minerals if thermochemicaldata were availablefor the manganeseoxide species.Little data of this nature may be found in the literature, and the presentstudy was undertaken to provide such data. The scope of the work is limited to the system Mn-Oz-HzOat 25" and one atmospheretotal pressure(Table 1). Information derived from a study of this system provides data for a number of the important supergeneoxides of manganeseand servesas a point of departure for the investigation of those oxides containing additional componentsnot includedin this system.

ExpnnrlrBNIAL PRocEDURE The apparatus and proceduresdescribed below were used in most of the experimental work here reported. Where other apparatus or tech- niqueswere used a brief descriptionof them is included. The system under investigation was contained in a flat-bottom glass vessel of approximately 500 ml capacity. The rim of the vessel was ground plane and fitted with a plastic top through rvhich a number of holes were drilled. This arrangement permitted electrodes, buret gas inlet tube, and thermometer to be inserted into the system, and aliquots of solution to be removed. When necessary, a gas-tight seal was efiected by using stopcock grease between the piastic top and the ground-glass rim of the vessel. Temperature in the vessel was main- tained at 25o+0.50 by enclosingit in a water jacket through which water from a constant temperature bath llas circulated. Eh was measured using a Beckman model WM millivolt-meter. The Eh electrode system consisted of a bright platinum electrode in conjunction with a saturated calomel reference electrode; a second platinum electrode was used for a solution ground. pH was measured using a Beckman model W pH meter. The electrode system consisted of glass electrode (Beckman 8990-80) in conjunction with a saturated calomel reference electrode, a platinum 1298 OWEN BRICKDR

solution ground, and a thermal compensator. Eh and pH were recorded continuously for the duration of the run on a model Y153X89-C-11-111-26 Minneapolis-Honeywell re- corder, The oxidation potential of the system was regulated in various ways. In runs made at very low oxidation potentials hydrogen gas was initially used to reduce the system. Nitro- gen gas, purified of COz and O2 by passing it through ascarite and then through a V3+ solution, provided an inert atmosphere and prevented difiusion of atmospheric oxygen and carbon dioxide into the system. Either air, purged of CO2 by passing it through ascarite, or oxygen gas \vas used to pro-

Terr-r 1. Meuc.q.wrsn Coupoul+rs rN rnn Svsrnlr Mn-OrHrO at 25o C. aNn ONn AruospnrnB Tornr, Pnpssunn

Compound Mineral Name Composition

MnO Manganosite MnO Mn(OH)z Pyrochroite Mn(OH)z MnaOr Hausmannite MnaOr ?-MnzOal MnOOH a-MnOOH Groutite MnOOH 0-MnOOH Feitknechtite2 MnOOH 7-MnOOH Manganite MnOOH MnzOa Bixbyite MnrO: 6-MnOs Birnessite3 (Na, Ca)MnzOr42.8HrO 7-MnO2 Nsutite Mn1 *Mn*O2 r"(OH)r" B-MnOz Pyrolusite MnOz MnOz Ramsdellite MnOz

1 The terminology of Verwey and DeBoer (1936) was retained for this compound although in this study it was found to contain one water of hydration rather than being anhydrous. No natural occurrences of this compound have been described 2 Described in this study. The mineral hydrohausmannite rvas found to be a mixture of Mn:O+ and B-MnOOH rather than a single phase. 3 Formula given by Jones and Milne (1956) calculated from analysis of an impure sample. 6-MnO2, a compound containing only manganese, but giving a,n fi4ay pattern identical to birnessite has been prepared by a number of difierent investigators.

vide an oxidizing atmosphere. Oxidation was also achieved in some runs by using hydrogen peroxide. The highest oxidation potentials observed in these experiments rnere reached through disproportionation of manganese compounds such as Mn:Oa into Mn2+ and one of the non-stoichiometric forms of MnOz. These reactions will be discussed in more detail in a subsequent section. A11chemicals used in the experimental work were of reagent grade. Distilled water, purged of COz and 02, was used in the preparation of all solutions Before any reactions were initiated in the reaction vessel described above, the system was purged of oxygen and C02 lvith pure nitrogen gas. The pH electrode system was standardized against two bufier solutions before each run. fn most cases pH7 and pH4 buffers were used. pH9 buffer was also used occasionally. Standardization procedures alr,vaysprovided buffer checks within * 0 02 pH units of the pH value of the buffer solution. This is within the limit of error sDecified bv the manufacturer TIl E SYSTT;.M M n-Oz-HzO 1299

of these bufiers The pH electrode system was also checked after each run. rn a normal run lasting several days, no discernible drift occurred in the electrode standardization. The Eh electrode system was periodically checked against Zobell's solution (zobell, 1946). The potential reading observedwas always within severalmillivolts of the potential specified for Zobell's solution. The system was stirred constantly to prevent inhomogeneity. A magnetic stirrer was placed beneath the reaction vessel and a teflon-coated stirring bar placed in the vessel. This arrangement provided a vigorous stirring action. Aliquots of solution were periodically removed from the vessel and filtered through a 0.22p millipore filter. The oxidation products were identified by r-ray diffraction. In most runs only small quantities of solid were produced, and therefore it was necessary to use a powder camera rather than the faster difiractometer method. For routine identification a straumanis type 57.3 mm diameter powder camera was employed. Reported d spacings were measured on films taken with a Straumanis type 174.6 mm diameter camera. A Philips c-ray diffraction unit employing Mn filtered iron (K.:1.9373 A) radiation was used for all *-ray work here reported.

ExpBnrlrnNrAr, DETERMTNATTON ol rIrE Srasrtrrrns oF pHASES rN rHE SysrEM Mn-O2-H2O Ar 25oC. AND ONE AruospuBnp'Iorar pnpssunr Pyrochroite,Mn(OH)z The mineral pyrochroite, first describedby Igelstrcim (1864) has a Iayer structure of the brucite type, space grotp C3m with o:3.35 and c:4.69 A (Aminoff, 1919). Watanabe et at. (1960) obtained unit cell dimensionsoI a:3.323 c:4.738 A for pyrochroitefrom the Noda-Tama- gawa Mine, Japan. I. Preparation De Schulten (1887) prepared Mn(OH), by treating a manganous chloride solution with concentratedalkaii hydroxide under a hydrogen atmosphere.He obtained crystais up to 0.2 mm in diameter by redis- solving the precipitate in its mother liquor at elevatedtempertaure and coolingslowly. Simon and Frcihich(1937) and Moore, EIIis and Selwood (1950) used De Schulten'sprocedure to prepare Mn(OH):. Klingsberg (1958)prepared Mn(OH), by subjectingmanganese metal to 200 psi of water pressureat 363" C. for one day. He found that the transition MnO-Mn(OH)2 could be made to proceedreversibly at elevatedtem- perature by varying the water pressure,and has determined the Psr6 (psi)-T" C. curve for the reaction. fn this study Mn(OH)z was precipitated from dilute manganousnitrate, perchlorate,and sulfate solutions at 25o C. and one atmosphere total pressurein an oxygen-free environment by increasing the pH with dilute sodium hydroxide. No attempt was made other than aging the precipitate in the solution, to increaseits crystallinitv. It was found that 13OO OWENBRICKER the same quality r-ray diffraction patterns were obtained regardlessof whether the fresh precipitate was r-rayed immediately or aged for several days in the solution. The reflections on thesepatterns are strong and sharp, indicating that the fresh precipitate is well crystallized. Feit- knecht anclMarti (1945)obtained similar resultsin their investigationsof

'I'asr,n 2. X-I{av D,lra ron PvnocnnorrE AND Mn(OH)z

Pyrochroite Pyrochroite Noda- Nordmark, Sweden Tamagawa Mine Mn(OH)z Mn(OI{)z (This (Ramdohr, 1956) Iwate, Japan (Klingsberg, 1958) study) (Watanabe et al., 1960)

4.91 465 VS 4.743 100 4.728 100 4.72 4.52 A 285 m 2 897 10 2.870 t4 2.87 243 S 2.+60 40 2 -453 40 2.45 10 2.36 2.361 6 2.36 <1 1.81 S 1.829 20 | 825 26 t.826 6 I.OJ m | -661 10 1.658 o 1.658 4 156 m 1 568 5 t..)o/ 4 1.565 3 t.434 <1 1.38 m 1.384 1.381 1.382 2 | 374 z r.346 t ..tJo <1 1.22 m | 230 5 Broad 1.227 2 1.18 1.180 1.182

the aging of Mn(OH)z . X-ray data for pyrochroite and for Mn(OH)z prepared in this study are comparedin Table 2' No chemical analysis of Mn(OH)r was made owing to the difficulty in preventing oxidation during handling. Aliquots of the Mn(OH), suspension treated with o-tolidine and examined spectrophotometrically showedno signsof the presenceof oxidation stateshigher than Mn2+. Brickstrcimite,an orthorhombic modification of Mn(OH)z from Li.ng- ban, Sweden,has been reported by Aminoff (1919).Buerger (1937) has suggestedthat backstrdmite is actually pyrochroite pseudomorphous after manganite. Grigoriev (1934) claimed to have prepared this modifi- THE SYSTEM Mn-Oz-HzO 1301 cation at temperaturesabove 80o C in the presenceof a large excessof water. No evidencefor an orthorhombic modification of Mn(OH)2 was found in this study.

2. The equilibrium constantand free energyof Mn(OH)z The equilibrium constantof Mn(OH)z has beendetermined by a number of investigators (Table 3). Becauseof the diversity of values obtained in these investigations, the equilibrium constant was redeter- mined. Three setsof experimentswere made. In the first two sets,manganese

Taelt 3. Equrr,renrurrComsrlxr ano l-nnr Exrncv or Fonuarronor Mn(OH)z

Investigator Log K Fou'(os)rt

Herz (1900) -12.1 -146.0 kcal. Sackur and Firtzmann (1909) -13.4 - 147.8 kcal. Britton (1925) - 13.89 - 148.49kcal. Oka (1938) -12.9 -147 .l kcal. Fox, et ol , (1941) -12.8 - 146.9kcal. Nasanen (1942) -146 89 kcal. Kovalenko (1956) -t2.35 - 146.39kcal. This work (1963) -13.01+0.04 -147.34+0.05kcal.

I Calculated from the reported K values using the freeenergyvalues for Mnun2+and OHuo- listed by Latimer (1952). sulfate solutionsof known concentrationwere titrated with NaOH until approximately half of the Mn2+ was precipitated as Mn(OH), (the amount precipitatedbeing known exactly). The pH was measured,then known amounts of KCI or KNOe solution were added to increaseionic strength, and the pH recordedafter each addition. In the last set of experiments,the ionic strength was moderatedby adjusting the concentration of manganesesulfate rather than adding neutral salts. A nitrogen atmosphere was used in all of the experiments to prevent oxidation. The concentration of Mn2+ remaining in solution is known from the stoichiometry of the reaction, pH is a measureof as+, and K*:as+X a6s-i therefore Kw (1) aE+ where aeq- and as+ are the activities of hydrogen ion and hydroxyl ion and K* is the dissociation constant of water. The concentrations of Na+ and SOa2-in the system are also known, as are the concentrations of t302 OWEN BRICKI')R neutral salts added to the system. Using this data, a mixed activity- concentrationconstant K1, can be calculatedfor Mn(OH)z: Kr : a2ou-C,uo, Q) where Ca1,,z+is lhe concentration of manganous ion remaining in solution. The activity' of an ion is related to its concentration by:

j Ci: (3) 1i' where'yi is the activity,-coefficient of i, and equation (2) can be rewritten:

1ir: uro"-.uI"'* (4\ 7Mn21 The thermodl'namicstability constant,K, for Mn(OH)2 is given by:

f(: 2yoz+626s- (), ThereforeK is related to K1 by:

K: KTMn,+ (6)

Log Kr valueswere plotted againstthe squareroot of ionic strength and the curve extrapolatedto zero ionic strength (Fig. 1,2, 3).r The ionic strength is given b-v: r: r/2Lciz1 (7) whereCi is the rnolalconcentration of speciesi, andZi its charge.Lt zero ionicstrength 7M1r+: 1 and Kl: K. The Debye-Hiickel theory can be used to determinethe limiting slope in this extrapolation. From the Debye-Hiickel theory:

. 4j\\/I _ log ?fitn,,: (8) f l_ ag;T where A and B are constants, the values of which depend upon the nature of the solvent, the temperature and the pressure,and i is the effective diameter of the ion in solution. Rewriting equation (6) in Iog- arithmic form and substituting for Mn2+. 4A\n logK:log(t- - ^- (e) t 1 ao1/I

1 Tables listing results of all equilibration runs have been deposited as document No. 8509 with the American Documentation Institute, Auxiliary Publications Project, Photo- duplication Service, Library of Congress, Washington 25, D.C. Copies may be secured by citing the documentnumber, and remitting $2.50for photoprintsor $1.75for 35 mm micro- film. Advance payment is required. TIIE SYSTEM Mn-Oz-IlzO 1303

In dilute solutionsat 25oC. the term 1*iBVI approachesunity, the constant A is approximately0.5, and the equationreduces to: iogK:Io1Kt-2\,/l (10) Log Kr is a linear function of the square root of ionic strength. The Iimiting slopeis -2. In Fig. I, 2, and 3 both log Kr and log K values have been plotted against the squareroot of ionic strength. The log K values were deter-

-t3

logK' or logK RunI -ta Run2

Run3 o.2 1T

Frc. 1. Activity product of Mn(OH)r determined by extrapolating log Kr values to infi- nite dilution using the Debye-Hiickel limiting slope. Ruled figures are iog K values calculated from the experimental data using the Debye-Hiickel equation to obtain yu,z+. Ionic strength moderated by KCI. Size of circles indicates precision of measurement. mined by multiplying log K1 values by the activity coemcient,TMr2+, calculated from the Debye-Hiickel equation. In the experiment using sulfateion to moderateionic strength, the log K valueswere first calcu- Iated neglectingthe weak MnSOaocomplex (Bjerrum el al., 1958).These log K valuesall fall below -13.01, the extrapolatedvalue from the other experimental data. Recalculation of Iog K, taking into account the MnSOaocomplex, yields valuesin line with the chloride and nitrate ex- 'fhe periments. MnOH+ complex (Perrin 1962)was not found to be important over the concentrationrange of Mn2+ usedin theseexperiments- The log K values are constant within the limits of error involved in measurementof pH and Mn2+ concentrationwith the exceptionof a few values.The log K1 curveswere extrapolatedto zero ionic strength along 1304 OWEN BRICKER

logK' or log K

o.2 1/T

Fro. 2. Same as 1 but ionic strength moderated by KNO:. the limiting slope of - 2 in the regions of the square root of ionic strength <05. The Debye-Hiickel limiting law describesthe behavior of activity coefficientswith very good accuracy in this concentration range (Robin- son and Stokes,1959, pp. 230-231).Extrapolation of the curves to zeto ionic strength leads to a value of - 13.01+ 00.4for log Kuo(on)r,slightly smalier than the results of Oka (1938).The free energy of formation of Mn(OH), calculatedusing the log K value reported above,and the free energyvalues of Mn"n2+listed by Latimer (1952)is -147.34+.05 Kcal.

-t3

logK' or logK

o.?

Frc. 3. Same as I but ionic strength moderated by MnSOr. THE SYSTEM Mn-Oz-HzO 1305

This is 0.4 Kcal more negative than the value which Latimer calculated from the work of Fox, Swinehartand Garret (1941). Hausmannite, Mn3Oa Naturally occurringMnrOr was firsl noted by Werner in 1789.Haidin- ger (1827)named this material hausmannite.I{ausmannite has a tetragonally distorted spinel structure,l space grotp I4f amd, with a:5.76 and c:9.44 L (Aminoff, 1936).The unit cell containsMnqMnsOra. 7. Preparation MnrOacan quite easilybe preparedin pure form by any one of a number of methods.Most preparationprocedures utilize high temperaturein the presenceof air to effect the decompositionof manganoussalts and subsequentoxidation to Mn3Oa,or, alternatively, the decompositionand reduction of MnzOror MnO2 to Mn3Oa. Shomate(1943) obtained high purity- MnrOr by decompositionof elec- trolytic MnOr at 1050'C. Millar (1928),Southard and Moore (1942),and Moore, Ellis and Selwood (1950) prepared MnrOa by the thermal decompositionof manganoussulfate in air at about 1000oC. Klingsberg (1958) prepared MnrOr by hydrothermal techniques.Krull (1932) reduced MnOz to Mn:rOawith hydrogen gas at 200oC. MneOr can be prepared from any of the oxides,hvdrous oxidesor hydroxidesof manganese,as well as from a variety of manganoussalts (e.g. chloride,nitrate, suifate) merely by thermal decompositionin air at about 1000' C. MngO+can also be prepared by oxidation of an aqueoussuspension of manganoushydroxide (Feitknecht and Marti, 1945). In this study it was found that MnrOa could be prepared and pre- served indefiniteiy in an aqueousenvironment under controlled conditions of Eh and pH. Electron micrographsshowed that the product consisted of well formed crystals of pseudocubichabit (Fig. a). MnrO+ was preparedfrom solutionsof manganousnitrate, sulfate and perchlorate. First Mn(OH)2 was precipitated by alkalization of the manganoussolu- tion with sodium hydroxide; then the Mn(OH), suspensionlvas oxidized with air purged of COz or with oxygen gas. Slow oxidation of the Mn(OH)z suspensionled to the formation of a cinnamon-brown compound which had aL n-tay pattern identical to that of hausmannite (Table 4). Chemicaianalysis of this material from three separateprepa- rations yielded calculatedformulas of MnOr rza,MnOl aesand MnOr sga with an error of * 0.01 (analyzedby the method of Gattow, 1961).Water content was deterrnined on only the first of these samplesafter drying to

1 At temperatures above 1170oC. hausmannite inverts to a cubic structure (McMurdie et aL..1950). 1306 OWEN BRICKER constantweight at 110oC.1.37 per cent water was found. It is difficult to assessthe roie of water in very fine precipitates.There are examplesof finely divided materials which tenaciously hold traces of non-essential water to temperaturesof 500-600oC. Whether or not the water found in the MnrO+preparation is essentialor merely adsorbedor included water cannot be determinedwith certainty.

2. FreeEnergy of Formotion of MnzOq Maier (1936)calculated a value oI -319.82 kcal as the free energy of

:rj

Fro. 4. Electron micrograph of MnaOr showing crystals of pseudocubic habit. X80,000. formation of Mn3Oa,using Roth's (1929) value of heat of formation and a caiculatedentropy. Shomate(1943) reviewed the heat of formation data for Mn3Oa,all of which had been determined by combustion methods, and redeterminedthese data using heat of solution methods. Rossini el o/. (1950)Iist a value of -306.0 kcal as the free energy of formation of MnaO+.Mah (1960),using Shomate'sheat of solution data and a correc- tion factor given by Brewer (1953)calculated a value of -305.85 kcal as the free energy of formation of MneO+. In this study MqOa was preparedin an aqueousenvironment by the method describedabove, and the Eh and pH of the suspensionwas recorded during the oxidation reaction. Provided the Nernst relation is obe-u-ed,the potential of the sJistemis a function of pH and of the Mn2+ activit_r.For the reaction: 3 Mn2+*4HzO: MnaOr* t11+12e- (11) THE SYSTEM Mn-Oz-HzO 1307

Talr,n 4. X-Rev Dere lon HeusueNxrrn, MqOa, ANDy-Mn2OB

Hausmannite ' Ilfeld, Harz MnrOr Mn*O+ 7-Mn2O3 Germany (Moore et a1.,7950) (This work) (This work) (Ramdohr, 1956)

4.92 m 4.90 .2r 4.92 8 4.92 7 3.08 S 3.08 .42 3.08 8 3 .08 9 2.87 2.87 .16 2.87 2 2.88 2.75 S 2.76 60 2.76 9 2.76 9 2.48 VS 2.48 1.00 2.48 10 2.49 10 2.36 m 2.36 .25 2.36 2.36 3 2.03 m 2.03 .2r 2.03 6 2.04 6 1.82 1.828 5 1.83 1 1.79 m 1.795 4 .79 3 |.70 1.700 2 .70 2 r.64 1.64r 2 61 I 1.57 1.58 .32 r.J/-) 4 .57 1.54 1.55 .63 1.542 9 1.54 8 1.44 S r.4+ .26 r.440 3 l.M 3 1.38 1.381 1 1.38 1 r.34 1.346 2 1.34 3 t.28 m t.28 1.277 3 t28 2 1.23 w 1.229 2 1.23 t.19 vw 1.193 2 1.19 I .18 1.180 2 1.18 1.12 m 1.122 1 l.t2 1.10 1.099 I 1.10 1.08 m 1.081 2 1.08 1.06 1.061 1 1.06

1.03 m 1.030 L t.o2 m 1.019 o r.02 .98s m .985 z the potential is:

Eh : E. _ 0.236 pH _ 0.0891 log ayoz+ (12) The experimental design permits Eh and pH to be measured,concentration of Mn2+ is known and can be converted to activity by application of the Debye-H{ickel equation. Therefore, the standard potential of the reaction. Eo. can be calculated. E" : Eh + 0.236pH + 0.0891log ayor+ (13) From the standard potential, the free energy of the reaction can be calcu- Iated. AF"e: nE'$ (14) 1308 OWENBRICKER where n is the number of eiectronsinvolved in the reaction and $ is the faradal, constant.If the free energiesof all other speciesinvolved in the reactionare known, the freeenergy of MnrOacan be calculated. AFolrogoq: AF"n -f 3 AF"u",*f 4 AF'n'o (15) The Eh-pH curves of two oxidation experimentsare shown in Figs.5 and7. Figures6 and 8 are the correspondingplots of Eo as a function of the reciprocalof the squareroot of time. The theoreticalsiope for reaction (11) is -0.236 pH. The Eo curvesare constant within the limits of error of measurementof Eh and pH, the Eo valuesbeing 1.809v. and 1.825v. for runs one and two respectively. Free energy values for MnaO+ calculated from these Eo values and the free energiesof Mnoo2+and H2Oaq listed by Latimer (1952) are -306.5 kcal. and -305.8 kcal. These free energy values are in reasonableagreement r'vith the free energy values listed by Latimer and Mah which were calcuiatedfrom calorimetricdata. In a separateset of experimentsMn3Oa was preparedb1- thermal decompositionof l\tnSOa'HzOat 1000oC' This MnsOrwas then placedin a slightly acid environment (pH 5) and Eh and pH recorded.Mn2r concentration was determined colorimetrically.The Eo values were repro- ducibleto f0.01 v. and correspondedto the valuesobtained above. ^,'- MnzO: It was found that upon prolonged oxidation in an aqueousenvironment cinnamon-brown MneO+ became progressivell'darker in color. Electron micrographsof this material showedcrystals having a pseudocubic habit but generallyless well developedand smailer in size than MnaOa.X-ray patterns of samplestaken periodically during the oxidation disclosedonly the pattern of Mn3Oa,but chemicalanalysis f ielded ox.vgencontents as high as MnOr a+.Simon (1937)reported a preparation having the MngO+structure and the composition MnOraa'0.15 HrO. Moore, Ellis and Selwood(1950) found that a compound could be prepared which had the structure of MneO+but was very closeto MnOOH in composition.Verwev and de Boer (1936)prepared a compoundhaving the structure of MneOabut the compositionMnOr ro. They cailed this compound7-Mn:Oa. Drotschmann (1960)prepared a compoundhaving the hausmannitestructure and the compositionMnOr rs. He found, on the basis of magnetic susceptibilitYmeasurements, that this compound shouldcontain only the divalent and tetravalent statesof nranganese. An experimentwas made in which a suspensionof MnaO+was strongly oxidizedwith oxygengas. Eh and pH wererecorded during the oxidation' The r-ray pattern (Table 4) is identic:Llwith that of Mn3Oa,with the exceptionof weakeningand/or absenceof someof the high etnglereflec- THE SYSTEM Mn-OrHtO 1309

66.57 pH

Fro. 5. Eh-pH response of M&O4 suspension. au^z+:2.34X10-3. Size of circles indicates precision of measurement. 2.OO

t.90

F.o r.80

t.70

r.60 oo o.t t, o.2 o.3 r;t^

Fre. 6. Plot of Eo vs. Tyry-l/z for reaction 31412+fa[H2Q:MngOr*8H+*2e. Size of circles indicates Iimits of error of calculation. 1310 OWEN BRICKER

Ehu

Aq pH

Frc 7. Eh-pH responseof MnsOr suspension.&u'zi:4 68X10-3. Size of circles indicates orecision of measurement.

tions. Analysis of material prepared under similar conditions yielded the formula MnOraar+01 '0.49H2O, close to MnOOH. A plot of the Eh-pH curve (Fig.9) showsthat it has a slopeof -0.1i7 pH correspondingto the reaction:

Mn2+ * 2HzO: MnOOH * 3H+ * e- (16) The potential is given by: Eh : E' - 0.177pH - 0.0591og ayoz+ (r7) A plot of the Eo potential of the reaction againstthe reciprocalof the squareroot of time (Fig. 10) is constantwithin the Iimits of experimental error at +1.548 v. The calculatedfree energy is -132.2 kcal. X-ray investigations showed that samples of 7-MnrO3 kept in suspensionfor several months had partially inverted to 7-MnOOH, indicating that THE SYSTEM Mn-Oz-IIzO 1311

r.90

troLV

t.80

O.l _r* O.2 - tl .MIt* N.

Frc. 8. Plot of Eo vs. Turw-vz for reaction 3141:+f4HzO:MnaOa*8H+*2e. Size of circles indicates limits error of calculation.

?-Mn2O3is unstablewith respectto 7-MnOOH at 25" C. and one atmosphere total pressure. In the course of these experiments it was assumed that the Mn2+ concentration in the solution remained constant after precipitation of the solid. In many casesfine precipitates are known to absorb ions from solution. Examination of equation (12) indicates that any change in the

400 6.5

Frc. 9. Eh-pH responseof t-MnzO: suspension.a.mz+:1.48X10-3. Size of circles indicates orecision of measurement. LJIL OWEN BRICKER

Frc. 10. Plot of Eo vs. Tyyp-r/2 for reaction Mn2++2HrO:7-MnrO3(MnOOIl) *3H+*2e-. Size of circles indicates limits of error of calculation. manganousion concentrationwill be reflectedby a changein the Eh or pH or both. On thesegrounds it was thought desirableto examine the variation in manganousion concentration during the courseof the oxidrL- tion reaction. The experimental apparatus was set up exactly as in previousruns; however,aliquots of solutionwere withdrawn after precipitation of Mn(OH)r, beforeoxidation of the suspension,and periodically during the oxidation. Figure 11 showsthe results of this experiment.It can be seen that the mansanous ion concentration remains constant

700

o 50 E I a o -400 40 E g LrJ Lrl 9U

)i I t

I x

7 'c

to Tn,trru

Frc. 11. Response of Eh, pH, oxidizing equivalents, and manganous ion concentration during oxidation of Mn(OH), suspension. TllF. SYSTEM Mn-Oz-HzO 1313 within the limits of error of the analytical determination (Mn'+ was determined colorimetricallyafter oxidation to MnO+-). The pH of the system, nearly constant before oxidation, decreased during the oxidation. The Eh of the system rose from its previously steadyvalue. When the experimentwas terminated, the Eh and pH had almost leveled out. The oxidizing equivalent of the system was determined colorimetrically using o-tolidine. It was essentiallyzero before oxidation, rose sharply during oxidation, then slowly decreased.This determinationis a measureof the amount of manganese,with an oxidation state higher than 2{, presentin the solid phase.Complete homo- geneity of the system is therefore essentialfor an accurate determination. It was noted after a period of about thirty minutes that solid material was beginning to adhereto the electrodesand to the walls of the reaction vessel.Since the oxidized portion of the system is the solid phase, the oxidizing equivalent wiil appear to decrease as increasing amounts of solid adhere to the vessel,accounting for the apparent decreasein the oxidizing equivalent of the system after thirty minutes. The Mn2+ concentration calculated from the stoichiometry of the precipitation reaction correspondsto the analytically determined Nln2+ concentration.There is no measurableadsorption of Mn2+ by the solid phase.

Hyd.rohausmannite Feitknecht and lVIarti (1945)prepared a hydrous compoundthat had a manganeseto oxygen ratio between MnsOaand MnOOH, by oxidation of an aqueoussuspension of Mn(OH)2. The r-ray diffraction pattern of this material was identicai to that of hausmannitewith the exceptionof a strong extra reflection in the low angle region and a general weakening of the intensitiesof the remaining reflections.They believedthat this compound was a hydrated form of hausmannite,capable of somevariation in manganeseto oxygen ratio, and named it hydrohausmannite. Frondel (1953)described a mineral from Franklin, N. J., that had an r-ray diffraction pattern identical to that of the compoundprepared by Feitknecht and Marti. He indexed this pattern in terms of a tetragonal cell with a:5.79 and c:9.49 A. Using the chemical analysisof biick- striimite (identical to hydrohausmannite)cited by Aminoff (1919) and the specific gravity of hausmannite, he calculated the approximate unit cellcontent to be x:1.155 in the formula:

(unllou"XIu) lrtrttto,u*(otI)* Feithnecht, Brunner and Oswald (1962) found that the material originaliy describedby Feitknecht and Marti as hydrohausmannitewas I3I4 OWDNBRICKER

actually a mixture of two phases:B-MnOOH and MnrOa.Electron micrographsof their synthetic hydrohausmannitedisclose particles of two distinct morphologies:an equant morphology correspondingto Mn3Oaand a platy hexagonalmorphology correspondingto B-MnOOH. B-MnOOH was prepared separately to provide rc-ray ald chemical data. The r-ray pattern is listed in Table 5. A partial chemicalanalysis of this material discloseda manganeseto ox1'genratio of MnOr.rs.No total analyseswere made.

Fro. 12.Eiectron micrograph of synthetichydrohausmannite. Large platelets are B-MnOOH;equant material Mn:Or X150,000.

Table 5 contains r-ray data for hydrohausmannite. A pattern of p-MnOOH is included for comparison. If the a-ray reflections of B- MnOOH are subtracted from the hydrohausmannitepattern, only the reflectionsof hausmanniteremain.

l. Erperimental It was found that material having an r-ray pattern identical to natural hydrohausmannitecould be preparedfrom dilute Mn2+ solution ( =0.001 m) by alkalization with NaOH in the presenceof oxygen gas. Electron micrographsof this material disclosedtwo distinct morphologicaltypes; the one, platlets of hexagonalshape, the other equant in appearance (Fig. 12). The platy material is B-MnOOH, the equant material Mn3Oa andfor T-MnzO:.Material precepitatedfrom highly oxygenatedsolution THE SYSTEM Mn-Oz-HzO 1315

T,c.ero5. X-Rev Dnr,r. ron HvnnoHeusMANNrrE.qNo p-MnOOH

I{ydrohausmannite Franklin, N. J. Hydrohausmannite (Fondel, 1953) (Feitknecht and Marti, 1945)

d1 I'

4.95 2 4.84 5 4.65 10 4.56 10 3.10 3 3 .05 8 2.89 2 2.86 J 2.78 2.74 8 2.67 4 2 .50 6 2.47 8 2.38 4 2.36 6

2.05 2.02

t.u4 1.84 1.803 r.78 1.7r0 1.69 I.OJJ 1.63 I .586 1.56 t.549 1.53 r.M7 r.43 llydrohausmannite 0-MnOOI{ B-MnOOH (This Work) (Feitknecht) (This Work)

4.90 +.61 4.62 10 4.62 10 3.08 J 2.87 2 2.75 A 2.65 2 2.67 2.@ 2.48 4 2.37 4 2.37 2.36 2 broad 2.03 z r.u 2 I .84 1.96 1 broad r.79 1 r.70 1 r.57 i 1.56 1.55 1.54 n 1.50 1.44 .t 1.27 2 1.23 1

I Measured from a line diagram kindly provided by W, Feitknecht. 2 All of the reflections on this pattern are broadl the two so marked are very broad. 1316 OWEN BRICKER has a manganeseto oxygen ratio of l\{nOr.so,indicating that the equant material in the mixture must be y-N{nzOs(B-\InOOH has not been observedwith an oxvgento manganeseratio exceedingI,Inor ro).I{aterials precipitatedfrom lesshighly oxygenatedsolutions have oxygen to man- ganeseratios between\InOr 33and MnOr 5s.Precipitation of this mixture appearsto take place directly from solution. I'he low pH (=7) at which precipitation occurs,however, suggeststhat there is local supersatura- tion with respect to \{n(OH), around the drops of base at the instant thel' enter the solution. This causesprecipitation of Mn(OH), which is immediately oxidized to a mixture of llnsOa and B-l,InOOH. Slow oxida-

Frc. 13. Electron micrograph showing hexagonal platelets of B-MnOOH. X80,000. tion of X'{n(OH)zproduces only l{n3Oa as shown above. Apparently, the slow oxidation allows time for the complete transformation of the brucite-type layer structure of NIn(OH), into the tetragonally distorted spinel structure of N,InrOa.The structure of B-MnOOH appears, from morphologicand r-ray data, to be closelyrelated to the ItIn(OH)2 structure (Fig. 13). The lattice of B-I'InOOH is quite distorted as indicated by the broadnessof the r-ray reflections.The basal spacing has con- tracted from 4.72A in lt.rlOU)z to 4.62A itr B-lt.IOOH as a consequence of the increasein chargeof all of the manganeseions fton2l to 3* (or one-half of the manganeseions from 2* to 4f ) and a loss of protons from the structure. A completeseries of compoundsspanning the composition range between I{n(OH)2 and B-\IInOOH very likely exists,although preparation of the lesshighll' oxidizedmembers has not been attempted. This seriescan be representedby the formula:

Mn"(oH,)=(M"11-u'lltorl ,*o,*)=B-MnooH (19) TIIE SYSTEM Mn-Oz-HzO 1317 in which the divalent manganeseis oxidized to the trivalent state (or statisticaldistribution o{ 2* and 4f , giving an averageof 3*) and half of the protons migrate out of the structure. A mixture of p-\InOOH and l,InrO+ is alwa1'sformed upon rapid oxidation of and ItIn(OH), suspension.Supergene oxidation of pyrochroite at the Noda-Tamagawamine, Japan, also producesa mixture of ItInsOq 1,i

Frc. 14. Electron micrograph of MqOa partially oxidized to 7-MnOOH (needles) X80,000.

and l-\{nOOH consistentwith the laborator.vobservations' Upon stand- ing for long periods of time (severalmonths) in contact with oxygen, aqueoussuspensions of X,InaOaand B-1,{nOOHinvert to y-N{nOOH. y-l'InrOaalso inverts to 7-MnOOH upon standingfor long periodsin an aqueous environment (Fig. 14, 15). Preparation of a pure sample of 0-l,InOOH from aqueoussolution was not possiblein the range of conditions used in this work. Some \fnsOq or 7-n'In2O3was always formed along with the 6-MnOOH. In order to prepare a pure sample of 0- n'InOOH for r-rav work and chemicalanalvsis, it was necessaryto oxi- 1318 OWD,NBRICKNR

dize Mn(OH)2 in a stream of oxygen at low hurnidity (Feitknecht et al. 1962). The instability of B-MnOOH made it impossible to obtain free energydata by the method usedin this study. X-ray examination of hydrohausmannite from Lingban, Sweden, yielded data identical to that obtained for synthetic hydrohausmannite (Table 5). A sample of material from Lingban was ground in an agate mortar and examinedby electronmicroscopy. (Fig. 16).The shapeof the fragments produced by grinding depends to a large degree upon the

Frc. 15 Electron micrograph of r-Mn:Og (equant) and B-MnOOH (platy) partially altered to 7-MnOOH (needles).X80,000.

cleavageof the material involved. In this casethe morphologic types are much lessdistinct than in the synthetic material that was preparedin an environmentallowing free growth of externalcrystal faces.Nevertheless, the sametwo morphologictypes can be distinguishedas in the synthetic material. Hydrohausmanniteis a mixture of two phases,B-MnOOH and Mn3Oa,and is not a valid mineral species.The name feitknechtiteis here proposed for naturallv occurring B-MnOOH, after Dr. Walter Feit- knecht, Professorof Chemistry at the University of Bern, who first synthesized this compound and who has made many valuable contribu- tions to the chemistry of the manganeseoxides. Manganite Manganite was first reported by de Lisle in 1772.It has spacegroup B2t/d and unit cell dimensionso:8.88, b:5.25, c:5.7 1 A (Bu"tg"r, TIIE SYSTIIM lv[4-Q2 II2O 1319

1936). The unit cell contzrinsMnaOs(OH)s. Manganite is pseudoortho- rhombic with 0:90o. The structural analysisby Buerger is basedupon trivalent manganeseoccupving sites in a sheet-likeframework of oxygen and hydroxyl ions. Manganite has a superstructureof the marcasitetype

Frc 16 Electron micrograph of hydrohausmannite from Lingban, Sweden. Platelets (larger grains) are p-MnOOH; equant material MnrOr. X80,000.

which exists up to the decompositiontemperature of the compound (Dachs, 1962).Krishnan and Banerjee (1939) found that the magnetic anistropl-data for manganiteindicated non-equivalentoxidation states for the manganeseatoms. They suggestedthat half of the manganese atoms had an oxidation state of 2* and the other half 4f, giving an overallaverage of 3t for the compound.In an investigationof manganite using both r-ray and neutron diffraction methods, Dachs (1963) has verified the earlier crlstallographic work of Buerger, giving refined atomic parameters,and shown that the hydrogen ions are bound to cer- t320 OWEN BRICKL.R

tain oxygensin the structure.On the basisof his study he concludedthat the existenceof mixed oxidation states (2f and 4*) among the manganeseatoms in this compoundis highlf improbable. The writer favors the conclusionof Dachs. Manganite occursas a low temperaturehydrothermal mineral and is commonly found in depositsformed b.v meteoric waters (Pala"cheet al. 1955).It is the stable polymorph of n4nOOH at 25o C. and one atmosphere total pressure.

l. Preparation 7-MnOOH has been prepared by several investigators. Klingsberg (1958) prepared y-MnOOH hydrothermally. Feitknecht and Marti (1945)prepared 7-MnOOH from manganoussalt solutionsby precipitation with ammonia in the presenceof hydrogenperoxide. They obtained a product similar to manganite,but the r-ray pattern containedan additional weak low-angle reflection. Moore, Ellis and Selwood (1950 prepared ?-MnOOH by precipitation with ammonium hydroxide from a manganoussulfate solution in the presenceof hydrogen peroxide,fol- Iowed by drying at 60" C. under vacuum. In this study, 7-MnOOH was preparedby oxidizing a suspensionof Mn(OH)z with hydrogenperoxide. A comparisonof the r-ray data for this material and other synthetic and natural manganites is given in Table 6. Electron micrographs of this material show well formed crystais having a bladed habit (Fig. 17). It was found that under prolonged oxidation with air or oxygen, a suspensionof Mn(OH), oxidizesfirst to Mn3Oa,then to 7-Mn2O3,and finally 7-MnOOH beginsto form. If the initial oxidation is very rapid, someB- MnOOH is also formed. Completeconversion of MneOr to 7-MnOOH with air or oxygen alone could not be accomplishedover the duration of the experiments.Feit- knecht et al (1962)report that completeconversion of a mixture of MnaOa and B-MnOOH to 7-MnOOH occursafter four months of oxidation with oxygengas. A number of other methodsfor the preparationof 7-MnOOH can be found in the literature; however, the products obtained are not sufficientlycharacterized to substantiatethese methods.

2. Free Energy of Formation of 7-MnOOH Few data on the free energyof formation of 7-MnOOH are availablein the literature. Wadsley and Walkley (1951)give the half cell:

MnOzf H+ + e- : MnOOH; AF"o: - 25kcal. (20)

The free energyof this reaction was determinedfrom electrodepotential measurements.Calculation of the free energy of 7-MnOOH from this TIIE SYSTEM Mn-Oz-HzO 1321

Tesr-E 6. X-R,tv Dar.a. loR MANGANTTn.l,r.ro 7-MnOOII

Manganite Ilfeld, z-MnOOH 7-MnOOH z-MnOOH Harz Germany (Feitknecht and (Gattow, 1962) This Work (Ramdohr, 1956) Marti, 1945)

dr lr

3.41 VS 3.40 20 4.79 3.41 10 264 m 2.64 10 339 S 2.65 5 2.52 \/ 2.53 o 2 -62 m 2.52 1 2.41 m 2.42 1A 2.52 2.41 8 2.26 m 2.28 8 2.40 S 2.36 .2 218 m 2.20 8 2.27 m 2.28 3 m r.79 12 2.19 m 2.20 3 1.70 S t.7l 7 1.78 lV 1.781 2 r.66 t.67 t6 t.70 1.710 .s 1.63 r.04 6 t.67 S r.675 9 150 m 150 7 r.64 r.637 .1 r.43 m t.M 7 1.50 1.502 1 r32 m 1.43 1 At S 1.435 1.5 t.29 t.32 7 143 m 1.323 .5 t.28 1.30 2 1.262 .2 .26 .28 L |.214 .2 .24 .26 4 1.182 .1 .21 .24 2 1.158 .1 .lb m t.2l 4 1.139 .2 116 m 1.18 7 1.118 .1 1.13 S 1.16 6 1.099 .2 1 11 m 1.13 8 1.080 .r 1.10 m 1.11 o 1.015 .1 1.08 m 1.09 o 0.993 .1 1.03 m 1.08 6 1.01 m 1.03 7 0.990 m t.o2 J 1.01 7 .99 8

1 ,,d" values and intensities taken from a line diagram constructed using 0 angles. (Feit- knecht, pers. comm.) equation, using the free energy value of B-MnO, Iisted by Mah (1960) yields a value of -136.3 kcal. Drotschmann (1951) gives data which permit calculationof a value of - 136.6kcal. as the free energyof formation of y-MnOOH. Bode, Schmier and Berndt (1962) obtained value of -134.6 kcal. from electrodepotential measurements. In this study 7-MnOOH was prepared by the method described above, and the Eh and pH of the suspensionwere measured. Figures 18 OWEN BRICKER

Frc. 17. Electron micrograph of 7-MnOOH. Crystals are well formed and have bladed habit characteristic of manganite. X80,000.

and 20 show the Eh-pH responseof two 7-MnOOH preparations. An analysis of the product from the first experiment ind.icatedan oxidation grade of Mnor.srt or. Determination of water bv differenceand recalculationof the analysisin terms of one Mn _vieldsMnOl 67(OH)se. The E" values were calculatedfrom the Eh-pH data using the reaction:

Mn2+ * 2H2O:7-MnOOH* 3H+ f e- (21)

The potential is given by: Eh : E. _ 0.177pH _ 0.059log ayo,* (22)

Eh, pH and ax1^2+are known; thus: E" : Eh + 0.177pH f 0.059Iog ay.z+ (23)

Figures19 and 21 show Eo as a function of the reciprocalof the square root of time. The respectiveE" valuesare 1.490v. and 1.500v. The free energy of formation of 7-MnooH was calculatedfrom these E" values and the free energy values of HzO aq. and Mn2+ aq. Iisted by Latimer (res2). THE SI'STEM Mn-Oz-HQ t'Jz'J

.300 r- 6.8 6.9 7.O 7.1 7.2 7.3 pH

Frc. 18. Eh-pH response of 7-MnOOH suspension. 2'y"2+-1.26\10-2. Size of circles indicates precision of measurement.

From AF": tgB' (24) and

AF"-uooou : [po"oz+ 'q' * 2 AF'u"o *AFon (2s) "q. one obtains a value oI -133.2 kcal. for the free energy of formation of "y-MnOOH. This is about three kilocalories less negative than the values reported by Wadsley and Drotschmann and slightly more than one kilo- calorie lessnegative than the value of Bode et al. The free energy values reported by Wadsley and Drotschmann were calculated using data obtained from battery investigations in which con- centrated electrolyte solutions were employed. Estimation of activity coefficientsin this type of system is uncertain. The value of Bode et al. is basedon electrodemeasurements of the reaction: y-MnOOH:6-MnO:+H'+e- (26)

t.60

Fo r.50

t.40 o o., _lo o.2 Tuir.r

Fro. 19. Plot of Eo vs. TMrN-r/2 for reaction Mn2++2H2O:y-MnOOH*3H+*2e- Size of circles indicates limits of error of calculation. 1324 OWEN BRICKER

Frc. 20. Eh-pH responseof 7-MnOOH suspension.au^r+:2.34X10-3. Size of circles indicates precision of measurement.

These measurementswere carried out in an unbuffered saturated KCI solution, open to the atmosphere,with an initial pH of 6.4. The method used to determine the pH of the KCI solution was not specified.Meas- urement of pH in saturatedsalt solutionsis subject to large errors.The Eo potential of reaction (26) is a function of pH; thus the free energy value of y-MnOOH determinedby this method is questionable. Schmalz(1959) found that the free energy of hydration for the reaction: Fezos* Hzo:2 FeooFr (27) t,60 E9 r.50 r.40

n2 It v' c o.3 I nrr-r.r

Frc. 21. Plot of Eo vs. TMrN-r/2 for reaction Mn2++2HeO:.y-MnOOH*3H+te Size of circles indicates limits of error of calculation. THE SYSTEM Mn-Oz-HzO 1325 was on the order of 0.1 kcal. Assumingno free energyof hydration for the reaction Mn:Os* HzO:2 MnOOH (28) and using the free energy value of Mnros listed by Mah (1960) and the free energy value of HzOaq. listed by Latimer (1952), one obtains -133.27 kcal. as the free energyof formation of r-MnOOH. The experi- mentally determined free energy value of 'y-MnOOH (-133.3+0.2 kcal.) indicatesthat the hydration energyof reaction(23) is lessthan 0.2 kcal. This is the same order of magnitude as the hydration energy of the analogousiron system.

D-MnOz. McMurdie (1944) described an artificial manganesedioxide that gave an r-ray diffration pattern consisting of two lines. He believed it to be a distinct speciesand proposed the name 6-MnOz for this compound. Feit- knecht and Marti (1945) prepared compounds varying in composition between MnOr zaand MnOr s2by oxidation of ammoniacal aqueoussus- pensionsof manganous hydroxide with hydrogen peroxide, and between MnOl s2 and MnOr e6by reduction of KMnOa solutions with HCI or hydrogen peroxide. These compounds all gave similar r-ray patterns; however, some contained more reflections than others. Feitknecht and Marti called the compounds manganous manganites and considered them to be double-layer compounds with varying degreesof order. They suggestedthat these compounds could be represented by the formula 4MnOrMn(OH)r'2HsO. The *-ray patterns correspondedto McMur- die's 6-MnOz but for the most part contained additional lines. Cole et al. (1947) and Copeland et al. (1947) found that air oxidation of alkaline manganoussolutions yielded a product with an x-ray pattern identical to manganous manganite. The compounds prepared by Copeland et al. varied in oxidation grade betweenMnOr.so and MnOr.so.McMurdie and Golovato (1948)reported a synthetic 6-MnOzthat gave aL n-ray pattern containing two reflections in addition to the two they had originally reported in 1944. They also reported a naturally occurring 6-MnOz in manganeseore from Canada. Buser el al,. $95a) investigated the sub- stancesdescribed as 6)MnOz and manganous manganite. They obseved that the r-ray pattern of products with an oxidation grade above MnO1.e6 consisted of only two reflections corresponding to the pattern of Mc- Murdie's (1944) 6-MnOz.X-ray patterns of products with an oxidation grade below MnOr.go corresponded to the patterns of manganous manganite describedby Feitknecht and Marti. Although it was concluded that manganousmanganite and D-MnOzare the same phase, they recom- 1326 OWEN BRICKITR mended retaining both names,6-MnOz to describecompounds with an oxidation grade above MnOl e6,and manganousmanganite to describe thosewith an oxidation gradebelow MnOr so.The well-crystallizedcom- pound prepared by Buser et al. s',tpportedthe double layer structure proposed by Feitknecht and Marti and made it possible to index the *-ray pattern on the basisof a hexagonalunit cell with o:5.82 and c:14.62 A. The absenceof basal reflectionsfrom the r-ray patterns of the more hrghly oxidizedcompounds was attributed to a disorderingof the structure as Mn2+ in the interlayer was oxidized to Mn3+ or Mna+. Surface area measurementsby the B.E.T. method indicate surfaceareas on the order of 300 m'z/g for the 6-MnO: and 45 m2f gfor the manganous manganite (Buser and Graf, 1955).Conflicting results have beenreported by Glemserand Meisiek (1957)who prepareda D-MnOzwith the oxidation grade MnOl ggthat gave an r-ray pattern containing basal reflections (Table 7). Glemseret al. (1961)have discussedthe preparationand propertiesof d-MnOz.They found that 6-MnO: with an oxidation gradeup to MnOl eecould be preparedwithout the lossof the basalr-ray reflections, and divided the preparationsinto three subtypes (0, 6', 6") on the basis of the intensity and sharpnessof the r-ray reflections.Each subtype was preparedover the full range of composition.Previous surface area measurementsby McMurdie and Golovato (1948)of d-MnOzprepared by the same method that was subsequently used by Glemser et al., indicated surface areason the order of 40 m2/g. Apparentiy the presenceor absence of basal reflectionsis a function of particle size rather than oxidation grade.The particle sizeof 6-MnOr showingno basalr-ray reflectionsis so small that the particles can be consideredto be "two-dimensional" 'Ihe cr1'stalsattaining a thickness of only a few atomic layers. D-MnOz and manganousmanganite preparations that display the basalreflections have surface areas of nearly an order of magnitude less than those not showingbasal reflections. Differencesin the particle sizeof this compound are related to preparation procedures.On this basisit is suggestedthat the name manganous manganitebe dropped and thesecompounds be referredto as 6-MnOz.It is interestingto note that nearly identical *-ray patterns,with the exception of line width and line intensity, were obtained from preparationsof D-MnOrranging in compositionfrom MnOr z+to MnOr.ss. Buser and Griitter (1956)found that D-MnOzwas one of the primary manganesecompounds in deepsea manganese nodules. Jones and Milne (1956) described a new manganesemineral found in a fluvio-glacial gravel deposit near Birness,Scotland. It gave an *-ray pattern identical to D-MnOz and analvsis of an impure sample gave a formula close to IHE SYSTEM Mn-Oz-HzO t327

'l'anr,n 7. X-Rev Dlre lon BrnNnssrrE, MANcANous N{erce.xrrc eNl 6-X{nO:

Feitknecht and Marti (1945) Buser el aL (1954) 6-MnOz Glemser and Meisiek Manganous Manganous Manganite (1957) 6-MnO: Manganite

6.9 s 2.43 7.2 S 7.2 8 3.49 1.42 3.63 m 3.6-3.7 2 2.39 m 243 S 2 -42 7 1.67 m 1.42 S 2.12 2 1.49 1.62 2 | 39-1.40 o

Yo Hariya (1961) Jones and Milne (1956) Birnessite Birnessite This Work (1963)d-MnOz Todoroki Mine, Rirness, Scotland Hokkaido, Japan

7.27 S l.3l J/ 7.2 8 3 .60 4.69 11 475 9 244 m 3.69 o 2.44 10 1.412 m 5 2.12 4 broad J -Zl 4 r67 1 broad 1 An 2.45 .l 8 broad z.3t 7 |.27 1 2.09 5 1.41 8

I Measured from line diagram.

(Nuot Caos) MnzOu.2.8 HzO. They named this mineral birnessite. Frondel et al. (1960) described birnessite from Cummington, Mas- sachusetts,where it occursas a product of the weatheringand oxidation of manganese-richrocks. Hariya (1961)reported an occurrenceof birnessite in the Todoroki Mine at Hokkaido, Japan. Levinson (1962) found that birnessitewas a major constituent of a shipment of manganeseore from a mine near Zacatecas,Mexico. Zwicker (pe5s.comm.) reports that birnessiteis the first oxide formed in the oxidation of manganesecar- bonaterocks at Nsuta, Ghana.Birnessite is a low temperaturemanganese oxide formed by the supergeneweathering and oxidation of rocks containing manganese. 1328 OWEN BRICKER

L Free Energy of Formation of 6-MnOz. In this study D-MnO2was prepared by disproportionation of suspen- sions of MnsO+.When a suspensionof MnsOais acidified, the following reaction occurs: MnaOr*4H+: MnOz*2 Mn2+ 4-2HzO (29)

It was found that mild acidification produced D-MnOzand more intense acidifi.cation produced 7-MnOz. Electron micrographs of D-MnOz show material of platy appearancewith very poorly developed crystal faces. Two preparations of D-MnOzwere made and the Eh and pH of the sus- pensionsmeasured over a period of time. The results of these measurements are shown in Fig. 22 and 21. The slope of both Eh-pH curves is -0.118, which correspondesto the slopeof the reaction: Mn2+* 2HzO- MnO:* 4If+ +2e- (30) The potential is given by: Eh : E" - 0.118pH - 0.0295log a1lo'* (31)

Analyses of the preparations gave oxidation grades of MnOr zr for the first and MnOr er for the second. Both preparations contained water; 9.1/ and 6.5/6 respectively. It is apparent from the slope of the Eh-pH curve that reaction (30), or a similar reaction involving a hydrated product, is the potential determining reaction. Manganeseions with an oxidation state lessthan four, presentin the solid phase,apparently behave as a diluent and influence the electrodepotential only with respect to the Eo obtained. Eo values for both experiments were first calculated from equation (31) and plotted against the reciprocal of the square root of time (Figs. 23, 25). An attempt was then made to take into account the differencein Eo produced by the diluent effect, assumingideal solid solution behavior.A mole fraction term was addedto equation(31) giving the following equation :

aMD2+ Eh : E' - .118pH - .0295log (s2) mole fraction MnOz

Anew Eo was calculatedfrom equation (32). The E" of the MnOr.76w&S raised0.003 v. and the Eo of the MnOl s1w&s raised0.002 v. It can be seenthat the change in E" due to the diluent effect is within the limit of experimental error. The Eo, and thus the free energy, of the solid phase does not show a strong dependency upon the amount of lower valent manganeseit contains. The free energy of formation calculated from the data for MnOr.zoand MnOr.er is, within experimental error, the free energy of the end member of the 6-MnO2 series.The Eo values obtained from equation (32) are *t.290 v. and +1.292 v. for the MnOr.zeand TEE SYSTEM Mn-OrHzO t329

.900

Ehv

.800

3.6 pH 4.8

Frc.22.Eh-pH response of 6-MnO:suspension. auoz+:3.80X10-3. Size of circles indicatesprecision of measurement.

MnOl 31respectively. The corresponding free energy values calculated from equation (30) using the free energy values of Mn2+ aq. and H2O aq. listed by Latimer (1952) are -108.3f 0.3 kcal. for MnOr.zoand -108.2 kcal. for MnOr sr. There are no free energy data available in the literature for 6-MnOz. The presenceof significant amounts of cations other than manganeseis characteristic of the chemical analysesof birnessite reported in the litera-

Frc. 23. Plot of Eo vs. Tyyry-l/z for reaction Mn2++2HrO:6-MnOz*44+*2e-. Size .of circles indicates limits of error of calculation. 1330 OWEN BRICKER

Frc. 24. Eh-pH responseof 6-MnOz suspension.aM^2+:4.26X10-a. Size of circles indicates Drecision of measurement.

r.50

1.40

-o trv

r.30

t o.3 o.4 r-/t t- MIN.

Frc. 25. Plot of Eo vs. TMrN-1/2for reaction Mn2++2HeO:6-MnO:i4I{+t2e- Size of circles indicates limits of error of calculation. THn SYSTI|M Mn-Oz-IIzO 1331

ture. The type material describedby Jonesand Miine (1956) had clay minerals and other impurities admixed. The material FrondeT et ol. (1960) had availablefor analysiswas limited in quantitl' and the presenceof impurities was not discounted.It is reasonableto expect that a layer compound like birnessitecould accommodatea large amount of other cationsin its structure and the free energvof formation of the compound could be changedappreciably by their presence.A great deal more work needsto be done on the chemicalcomposition of birnessitebefore the extent and the effecton free energyof incorporationof other cations into the structure can be fully evaluated. This study shows that the presenceof other cationsis not essentialto the formation of D-MnOz,and the free energyreported is that of the pure end member. Brenet et al. (1963) have attempted to describe the thermodynamic propertiesof all of the non-stoichiometricmanganese dioxides in terms of the formula:

MnO*(OH)+-2" (33) According to their calculations, a manganese dioxide of composition MnO, .76would have a free energyof formation of - t22.5 kcal. They be- Iievethat all of the manganesein the non-stoichiometricdioxides is in the tetravalentstate and that reductionby oxalate(or other reducingagents) during chemical anal.vsisoccurs according to the process: M"'ui1- x/z) unl'Y'o- (o3-oJ; J":'i l:l'1..,rX,?ol,*o + t/ 4(2- x)o,(s) (34) In this reaction only the tetravalent manganesebonded to oxygen is reducedby oxalate (or other reducing agent), and the tetrarvalentman- ganesebonded to hydroxyl is reducedb1-water. In this manner they ex- plain why analytical data for the non-stoichiometricmangzrnese dioxides falsely indicate the presenceof manganesein an oxidation state Iower than 4*. The validity of their hypothesesrests upon whether or not manganeseactually is presentin thesecompounds in an oxidation state iower than 4f and, if not, whether reaction(34) occurs.An examination of the standard analytical proceduresfor the analysis of manganese dioxide showsthat a large excessof reducingagent is presentin the system during the reduction, and the excessis then back-titrated to determine the quantity usedin the reduction of the oxide.The potentialsfor the half cells:

HrO=^1/2 Oz,er* 2HF* 2e- (3s) and

H:C:Or.- 2 COz,etI 2H+ I 2e- (36) 1332 OWEN BRICKER arc +1.229 v. and -0.49 v. respectively.It would thus appear highly uniikely, even if the tetravalent manganesebonded to hydroxyl ion had a potential high enough to oxidize water, that it would preferentially oxidize water in the presenceof an excessof oxalate. It is apparent from equation (34) that a certain amount of oxygen, varying as a function of x in the formula MnO"(OH)a-2,, should be generatedwhen a non-stoichiometric oxide is reduced.No oxygen should be produced when stoichiometric MnOz is reduced.In order to test this hypothesis,a seriesof experiments was conducted in which 6-MnOz with an oxidation grade of MnOr.soand B-MnOz with an oxidation grade of MnOzoo were reduced with acidic Fe2+solution in a manometric system. Identical analytical results are obtained when Fe2+is employed as the reducing agent in placeof oxalate.The half cellpotential:

Fe2+_FeB+ + e- B" : f 0.77v. (37) is lessfavorable than the oxalate potential for the reduction reaction. An increasein pressurefor D-MnO2,amounting at maximum to 85/o ol the pressureincrease predicted from equation (34) was observed.A similar increasewas observed,however, for the B-MnOzwhich should have produced no oxygen according to equation (3a). The observedchange in pressure,whether or not due to generation of oxygen, is not a function of the composition of the oxide, nor could it account for the amount of deviation from stoichiometry observedin these oxides.After complete reductionof the oxideshad taken place,it was noted that the solutionsin which B-MnOr had been reducedwere always more highly colored than the solutions in which 6-NInOzhad been reduced, even though the sameamount of oxide was usedin eachcase. This is a qualitative indica- tion that more of the ferrous iron was oxidized by B-MnO, than by d-MnOz even though there was essentiallyno differencein the amount of gas producedby theseoxides. The experimentsdo not prove that manganesein an oxidation statelower than 4f is presentin the non-stoichiometric dioxides;however,they do cast seriousdoubts on the hypothesis of Brenet et al., which also fails to take into account the structural differ- encesamong the non-stoichiometric oxides. Such differencesalone would appear to preclude the possibility of continuous variation of thermodynamic properties among these compounds. The free energy value of 6-MnO: (-108.3 kcal.) determined in this study appears to be the best value available for this compound at the present time. 6-MnO2 Cor vor| in composition between MnOr.za and MnOr.gg,depending upon the conditionsof the environment in which it formed and the conditionsto which it was subjectedafter formation. The Eo of reaction (32), and thereforethe free energyof formation of d-MnOz, THE SYSTEM Mn-Oz-HQ 1333

does not appear to be a sensitive function of the degreeof compositional variation. Compositional variation in this compound is most likely due to the presenceof Mn2+ or Mn3+ ions with the concomitant substitution of OH- for 02- to maintain electro-neutrality. The range of composition of the compounds examined in this study is rather narrow. It would be desirableto investigatecompounds over the entire compositionalrange shown by this compound.

y-Mnoz Dubois (1936)prepared a non-stoichiometricmanganese dioxide with a manganese-oxygenratio of MnOr ssthat had a\ fi-ray pattern differing from that of pyrolusite. Glemser (1939) prepared an oxide similar to that of Dubois by three different methods: oxidation of a boiling MnSOr solution containing KNO3 with ammonium persulfate, oxidation of a boiling MnSOasolution containing KNO3 with potassium permanganate, and decomposition of permanganic acid at 45o C. His products all gave identical r-ray diffraction patterns although they varied considerably in manganese-oxygenratio (MnOr.ze, MnOr e+, MnOr.sa). Boiling these compoundsin nitric acid increasedthe ratio to MnOr.szin each case; however, no structural change was observed other than a sharpening of the reflectionsin the r-ray patterns. Glemserproposed the name 7-MnOz for this compound. McMurdie (1944) found that 7-MnOz was a major constituent of electrolytically produced MnOz. Cole et al. $9a7) dis- tinguished three modifications of 1-MnO2 on the basis of r-ray data. Gattow and Glemser(1961) have discussedthe preparation and properties of 7-MnOz. Natural occurrencesof 7-MnOs have been reported by Schossberger (1940), McMurdie and Galovato (1948), and more recently by Sorem and Cameron (1960) and Zwicker et ol. (1962). The name nsutite was proposed by Zwicker et al. (1962) for naturally occurring 7-MnO2 becauseof its abundancein the manganesedeposits at Nsuta, Ghana. All of the reported occurrencesof nsutite are consistent with a supergeneorigin of this oxide.

l. X-ray Data There have been at least 2l different r-ray diffraction patterns of y-MnOz published by difierent investigators (Gattow 1961). The r-ray data for some of the better characterizedproducts are included in Table 8. The *-ray pattern of the material prepared in this study is identical to the pattern of manganoan nsutite with the exception of the absenceof the weaker reflections. Preferred orientation and degree of crystallinity could account for the difference in the number of x-ray reflections ob- 1334 OWD.NBRICKER served by different investigators.It is apparent, holvever, that differencesin the spacingsof somereflections exist. Brenet et al. (1955)noticed that an expansionof the unit cell of y-MnOr occurred during the first stage of dischargeof a dry cell battery. Feitknecht et al. (1960)reduced 7-MnO2 with hydrazine over the compositionrange MnOr goto MnOr ae without a change of phase. They found that the unit celi volume in- creasedlinearly with decreasingx in MnO* and suggestedthe mechanism: MnOz n e- n H+3 MnOz (38) * * "(OH)" whereby a part of the tetravalent manganese is reduced to a lower oxidation state and a corresponding number of oxygen atoms in the structure

Tlele 8. X-Ra.v Dlr,l lon Nsurtrr, MancmloeN Nsurrm, AND 7-MnOr

Zwicker et al. 1962 Zwrcker et al,. Feitknecht Dubois(1963) Nsuta, Ghana 1963 (pers.communi MnOr ss cation) 7-MnO, Manganoan Nsut y-MnOi

401 M 4. S 4.+6 VW 4.36 VW 4.47 5 2.58 2.36 m 4.10 VS 3.96 VS 4.07 10 2.41 M 2.06 m 2.66 M 2.69 tr. 2.68 5 2.11 S 1.58 m 2.+5 M 2.59 VW 2.+5 9 1.63 VS 140 2.39 VW 2.43 S 2.36 5 |.43 M 1.38 236 M 2.3+ M 2.16 8 1.38 M 2.16 S 2.22 VVW Z.IJ 2 2.13 M 2.r3 S r.66 6 .924 VW 2.07 VW 1.43 I .880 WW t.892 WW 1.398 z oo/ S 1.831 tr 108 I .517 w 1.706 tr. t.426 w 1.638 S 1.394 M 1.615 W |.372 VW 1.488 WW 1 330 III 1425 W r.292 ww 1.362 VW 1.257 vw 1 306 VVW r.225 ww | 250 tr. 1.18.7 VW 1.212 tr. 1.136 ww 1.165 tr. 1.r07 tr. 1.116 tr. 1.082 II/ 1 067 VW

VVW:very, very weak,VlV:very weak,W:r'veak, M:medium, S:strong, VS : very strong. TIIL. SYSTEM Mn Oz-HzO 1335

are bondedto protonsforming h1'droxylgroups, which maintains electro- neutrality. The increasein cell volume is attributed to the larger size of lower valent manganeseions in the compoutd. Zwicker et al. (162) noted that the 7-MnO2 from Ghana occursin two modificationswhich they designatednsutite and manganoannsutite. Sorem and Cameron had describeda third type as well, characterizedby the r-ra_vspacing 1.65 A. Chemical analysesshow that manganoannsutite contains more lower-valent manganesethan nsutite, and r-ray data indicate larger interplanar spacingsin manganoannsutite. They suggestedthat the ob- serveddifferences in interplanarspacings are due to the larger amountsof lower-valent manganesein rnanganoannsutite. Glemser et al. (1961) preparedthree types of 7-MnO2 (7-MnO2,7-'MnO2,7-"MnO2), varying only in the relative intensitiesand widths of the r-ray reflectionsin two cases,and the absenceof one reflectionin the third. No systematicshift in the spacingsof the *-ray reflectionswas observedwith variations in composition.Two 7-MnO2 preparationswere made in this study; one had an oxidation grade of MnOr gr, the other MnOr gg.No differences were observedin the f-ray spacingsof thesepreparations.

2. Free Energy of Formation of 7-MnOz No free energy data have been published for 7-MnOz. Wadsle)' and Walkley (1949) have measuredthe Eo potentials of some7-N{nO2 elec- 'Ihe trodesthat varied in oxidation gradefrom MnOr s: to MnOr ee. mea- suredpotentials exhibited a large amount of scatter; however,a trend of decreasingpotential with decreasingx in MnO* was observed.AII of the measuredEo potentials were higher than the potential of the B-MnO, electrode.Bode el al. (1962) measuredthe potentials of 7-NInO2 electrodes over the composition range MnOz oo to MnOr ro and found a linear decreasein potential with decreasingx in MnO*. The design of their experimentdid not permit the calculationof Eo potentialsfrom the measuredelectrode potentials; thus no free energyvalues could be calculated. Both investigationsindicate that the free energy of formation of ?-MnOz varies as a function of the manganese-oxvgenratio, becoming more negative with decreasingx in MnO*. In this study 7-MnO2 was prepared by the disproportionationof an aqueoussuspension of MnrOawith acid. Electron micrographsshow well developed acicular crystals (Figure 26). The Eh-pH response of the y-MnOz preparation is shown in Fig. 27 and the correspondingE" potential in Fig. 28. In the secondexperiment a constantpotential of 1.172 v. was measuredat a pH of 1.51.The manganousion activity tv.rs10-1 el. This suspensionwas put into a bottle and sealedto determine whether aging efiects were significant. After a year the bottle was opened and the 1336 OWEN BRICKER

Frc.26. Electron micrograph of 7-MnO2 showingwell developed acicularcrystals. X80'000,

Eh and pH measured. The pH had not changed, the Eh had increased 0.005 v. to ll.l77 v., and the manganousion activity was unchanged. The r-ray pattern of this material was identical to the original pattern before aging. The slope of the Eh-pH curve is -0.118 as in the caseof 6-MnOz. Assumingthe reaction:

Mn2+ * 2HzO: MnOz * 4IJ+ + 2e' (3e) the potential is given by Eh : E' - 0.118 pH - 0.0295 log ayoz+ (40)

Ehv

.800

42 44 46 pH

Frc.27. Eh-p[I responseof 'y-MnOgsuspension' aM^2+:6.92X10-3. Size of circles indicates precision of measurement. TH li SYSTI:M M n-Oz-lIzO IJJ /

Frc.28. Plot of Eovs. Tyyy-1/2 for reactionMn2+f2HzO:7-MnOrf4H+f2e-. Sizeof circlesindicates limits of errorof caiculatiol.

The activity of Mn2+ can be calculatedfrom the manganousion con- centrationusing the Debye-Hiickel approximation;Eh and pH are measured; thus Eo can be calculated: E' : Eh + 0.118pH i 0.0295log ayor+ (41) A plot of Eo as a function of the reciprocalof the squareroot of time is shown in Fig. 28. The two preparationshad manganese-oxygenratios of MnOr.gr and MnOr gsrespectively. The correspondingEo values arc +1.273 v. and +1.272 v. Free energyvalues calculated from theseEo potentialsand the Iree energiesof Mn2* aq. and H2O aq. listed by Latimer (1952) are -109.12+0.3 kcal. and -109.07*0.3 kcal. respectively.Apparently the Iower-valent manganese,causing deviation from stoichiometry in 7- MnO2,behaves in an analogousmanner to that in 6-MnO:. The theoretical slopefor reaction (39) or for a similar reaction involving a hydrated phaseis observed.Treating the lower valent manganeseas a diluent and recalculating Eo using equation (32) does not change the Eo of MnOr gg but increasesthe Eo of MnOr n to 11.273 v. The changein Eo with composition over the range from MnOr sr to MnOr gsis within the error of measurementsmade using the techniquedescribed above. The freeenergy value of -109.1*0.3 kcal. is, within the limit of experimentalerror, the free energyof formation of the pure 7-MnO2 phase.It would be desirable to investigate?-MnOz over its entire range of compositionalvariation to ascertainthe relationshipbetween Eo and composition.It is conceivable that at somevalue of x in MnO* the free energycurve of 7-MnOz might crossthe free energycurve of B-MnO2,reversing the relative stabilitiesof these compounds. CoNcr-ustoNsAND GEor-ocrc ApptrcATroNS General.It has been found that a number of oxides and hydrous oxides of manganesecan be preparedfrom aqueousMn2+ solution.Techniques in- 1338 OWDN BRICKI],R volve either direct precipitation or oxidation of suspendedprecipitates by the methods describedabove. These compoundsare, in most cases, well crystallized as indicated by r-ray diffraction and electronmicroscopy. The measurementof pH and oxidation potential of aqueous suspensionsof the oxides. when used in coniunction with values for the

I N ol L-t <-l

Mna04

Mn(OH)2

Fro. 29. Paths of reaction in the system Mn-OrH:O aI 25" C. and one atmosphere total pressure. activity of manganousion obtained through application of the Debye- Hilckel approximation to manganousion concentrations,permits calculation of free energy values. The free energy values determined in this study are in good agreementwith publishedvalues in the few casesfor which suchdata are available. The paths of reaction in the Iaboratory synthesisof the manganese oxidesare shown in Fig. 29. In every case,with the possibleexception of the mixture of MnsO+and B-MnOOH formerly calledhydrohausmannite, Mn(OH)r was precipitated first. It appearsthat direct precipitation of TIID SYSTEM Mn-Oz-IIzO 1339

the Mn3O4-P-NInOOHmixture occurs;however,as drops of a strong base enter the lln2+ solution, local supersaturationwith respectto Mn(OH), could causeprecipitation of that compound.The Mn(OH)2 would then be immediately oxidized, giving the appearanceof direct precipitation of the more highly oxidized compounds.It was found that the most highlli oxidized compound that could be synthesized using air, oxygen gas or hvdrogen peroxide was 7-MnOOH. In order to prepare the non- stoichiometricforms of manganesedioxide, it was necessaryto acidify suspensionsof MnrO+or one of the MnOOH polymorphs.Upon acidification, disproportionationof thesecompounds into Mn2+ and non-stoichiometric MnO2 occurs.The disproportionationreaction is a mechanismfor penetratingthe HzOz-Orbarrier discussedby Sato (1960).Thus, it is apparent that all of thesereactions can occurin the supergeneenvironment in responseto changingconditions of oxidationpotential and pH. A seriesof Eh-pH diagramshas beenconstructed to show the stability relations among the previously describedphases. Although free energy data are availablefor manganositeand bixbvite, thesephases were not included becausethey are not stable in the supergeneenvironment. Groutite and ramsdellitecannot be includedowing to the lack of thermochemicaldata. The free energy of formation of B-MnO2listed by Mah (1960)and the free energiesof formation of HzO aq. and Mn2+ aq. listed by Latimer (1952)have been used in addition to the free energy values determined in this study. Pyrolusite (B-MnO) has been included be- causeit is apparentlythe most stableMnOz phasein the supergeneenvironment, althoughit resistssynthesis at 25" C. in the laboratory. In view of the decreasein potential of 7-MnO2 electrodeswith decreasingx in MnO* observed by several investigators,it is conceivablethat a re- versalof the stabilitiesof 7-MnO2 and B-MnO2might occur at somelow value of x. Further investigation in this area is necessary.Figure 30 shows the least stable assemblagethat would be expectedin the supergene environment. Figure 31 shows the most stable assemblageof manganeseoxides in the supergeneenvironment. Figure 32 shows sec- tions drawn at pH:11 indicating the changesin assemblagethat wouid be expectedwith time. The free energy changesof the phasetransitions areindicated. Figure 33 is an Eh-pH diagramconstructed from previously existing thermochemical data and is included for comparison to the diagramsconstructed from data determinedin this study. A number of thoroughly investigatedsupergene manganese oxide deposits have beenformed by the oxidation of carbonateprotore. Figures 34 and 35 are, respectively,the least and most stable assemblagesof manganeseoxides with rhodochrositein equilibrium with groundwater containing a total dissolvedcarbonate activity of 10-2.This value was 1340 OWEN BRICKER chosenbecause it representsa reasonableapproximation of the average total dissolved carbonate activity found in subsurface r,vatersfrom carbonate bedrock (White et at., t963). Figure 36 shows the stable assemblageof oxides and carbonate in equilibrium with the atmosphere (Pcor:10-s.r).Thefree energyvalues of COr.*2-,HCO;aq', COr(g), and

MnaOo

Mn(OH )z

Ftc. 30. Eh-pH diagram showing stability relations among some manganese oxides and pyrochroite at 25o C. and one atmosphere total pressure. Boundaries between solids and Mnuo2+at &lr.z*:10{. rhodochrositeused in the constructionof thesediagrams were obtained from Latimer (1952).

GeologicApptications. Predictions have beenmade on the basisof the ex- perimentaldata concerningthe manganeseoxide assemblagesin the system Mn-Or-HrO that should be found in supergenedeposits. Owing to the fact that a number of the supergeneoxides have only recently been THE SVSTEM Mn-Oz-HzO 134l described, detailed descriptions of occurrences of these minerals are Iimited. Six supergene manganese oxide deposits have been chosen in order to compare observed mineral assemblageswith those predicted from experimental data.

o.6

Ehv

o.4 % o.?

-o.?

-o.4

Fro. 31. Eh-pH diagram showing stability relations among some manganese oxides and py'rochroite at 25" C. and one atmosphere total pressure, Boundaries between solids and Mn.oz+ 41 alror+: 10{,

At Minas Gerais, Brazil, protore consisting of manganese carbonate with some manganesesilicates has been oxidized to depths of several hundred feet by supergeneprocesses (Horen, 1953). Hausmannite is found in the protore-oxide contact zone and small amounts in the protore directly beneath the contact. In the oxide zone hausmannite is absent but pyrolusite, manganite, cryptomelane and one or more minerals that Horen grouped under the term "wad" are found. U. Marvin (pers. comm. found "y-MnO2 type" oxides in this deposit. Zwicker (pers. 1342 OWEN BRICK]JR comm.) identified birnessite, manganoan nsutite, and nsutite in the oxide zone adjacent to the protore. It is probable that these are the minerals Iloren called "wad." The hausmannite (which may be primarl-, or may reflect the first stage of oxidation of the carbonate) and the non-stoichiometric dioxides are found near the carbonate-oxide contactl pyrolusite and cryptomelane are found in the oxide zone farther from the carbonate-oxide contact.

+6 O2

- -' /r^ - lqz

+,2

Ehv

M naOa

t!4n(OH)2 Mn(OH)z

TtME+

Drownot PH= 1l

l'rc. 32. Changes in mineral assembiageas a result of aging. Changes in free energies between polymorphs are indicated.

Baud (1956)in describingthe mernganesedeposits at Moanda, French EqualorialAfrica, writes:

"La min6ralisation se prisente uniquement d proximit6 immddiate de la surface et elle affecte 1'aspectd'une carapace61>ousant toutes les formes topographiques du terrain actuel. Gr6.ceaux nombreux ouvrages de prospection fonc6s notamment dans ia partie Nord du plateau Bangombd, il est possible de relever des coupes qui montrent la regularitd et la continuitd remarquables de cette carapace en dehors de certaines pentes trop brutales." The protore at this deposit is bedded sedimentary rhodochrosite. Examination of the oxide-carbonatecontact revealedthat manganiteis in contact i,viththe protore (Jaffe and Zwicker, unpubl). The manganite forms a lar-eron the order of one millimeter in thicknessand is overlain TIID, SYSTL,M Mn-Oz-HzO 1343

by pyrolusite (Fig. 37). Microscopic examination of the contact zone shows that the manganite and immediately adjacent pyrolusite have a common crvstallographic orientation and that manganite is being pseu- domorphously replaced by pyrolusite with progressive oxidation (Fig.

o.4

o.2 Mn*'oq Ehv Mn2O3 o

-o.4 N I rN

- -o.8 o 7 pH

Frc 33. Eh-pH diagram showing stability relations among some manganese oxides and pyrochroite at 25" C. and one atmosphere total pressure. Boundaries between solids and dissolvedspecies at a dissolvedspecies:10{. Contour of aMnz+at 10 a.

38). The equilibrium partial pressuresof oxygen for each zone are indicated on the plates.The maximum thicknessof oxidesat this locality is approximatelyseven meters with the averagebeing three to four meters. The assemblageof .mineralsfound here is the most stable assemblage that one would expectto find in the supergeneenvironment, on the basis of experimental data. Oxidation of the rhodochrosite has proceeded nearly to completion,and only a small amount of protore is left. AtPhillipsburg, Montana ht'drothermally deposited rhodochrosite t3M OWEN BRICKER has undergone supergene oxidation into manganese oxides. The assemblage of minerals found in this deposit is manganite, birnessite, nsutite, cryptomelane and minor pyrolusite. Todorokite has also been described from this locality (Larson, 1962). L similar assemblagehas

d-. \ /a^''oe \ \ \

M n"oq

Mn(OH )2

Frc. 34. Eh-pH diagram showing stability relations among some manganese oxides and manganese carbonate at 25o C. and one atmosphere total pressure. Boundaries between solids and Mnun2+ drawn at aMn2+:10-$. Activity of total dissolved carbonate is 10-2'

been described from Butte, Montana where supergeneoxidation of hydrothermal rhodochrositehas also occurred (Allsman, 1956).At Butte, nsutite appears to be the most abundant manganeseoxide mineral. AIls- man originally identified this mineral as ramsdellite, but subsequent investigation by Fleischer, Richmond and Evans (1962) have shown it to be nsutite. Pyrolusite is the next most abundant oxide, with cryptomelane being least abundant. The pyrolusite is found in open cavities and intimately mixed with nsutite. Cryptomelane is found in greatest THE SYSTEM Mn-OrHzO r.t4J abundance where the potassium content of the circulating water is high. The mineralogy of the manganese deposits near Piedras Negras, Mexico has been studied by Zwicker (pers. comm.).t At this locality argillaceouslimestones and limey shales of low Mn content (<10%

o.6

Ehu

o.?

-o?

-o4 ?468rOt?t4 Dlt

Frc. 35. Eh-pH diagram showing stability relations among some manganese oxides and manganese carbonate at 25o C. and one atmosphere total pressure Boundaries between solids and Mnnn2+ drawn at aMn2+:10-6. Activity of total dissolved carbonate is 10-2.

MnCOa) have been weathered to depths of several inches to several feet leaving residual manganeseoxides. The apparent oxidation sequenceis: protore (dominantly carbonatescontaining Mn2+)->birnessite---+oxidized form of birnessite---+manganoannsutite-+nsutite. This sequenceis identical to the sequenceobserved in the laboratory disproportionation of XlnrOaor of the MnOOH polvmorphs as a result of decreasingpH.

1 Seealso Levinson (1962). 1346 OWEN BRICKER

A similar sequence has been observed in the manganese deposit at Nsuta, Ghana (Sorem and Cameron, 1960; Zwicker, unpubl.). The protore at Nsuta is sedimentarl' rhodochrosite. Supergene oxidation has proceeded to considerable depths forming a rich deposit of manganese )fi

\ trh.. p-Mn02

o.4

M n**oq

46 pH

Frc. 36. Eh-pH diagram showing stability relations among some manganese oxides and manganese carbonate at 25o C. and one atmosphere total pressure. Boundaries between solids and Mn.n2+ drawn at aMn2+:10i. Pcozis 10-3 5 atm. oxides.The mineralogyis the sameas that of the Mexican depositexcept that pvrolusite and cryptomelaneare present at Nsuta, and birnessite, found onlv near the carbonate-oxidecontact, is much less abundant. Pyrolusite is found at the surfaceof the deposit.At shallow depths it is Iessabundant and is intimately mixed with nsutite. The manganesedeposit at Noda-Tamagawa,Iwate, Japan contains the largest known body of pyrochroite (Watanabe et al., 1960). Over 50,000 tons of pyrochroite have been shipped from this localitv for THE SVSTEI4Mn-Oz-HzO r347

Frc. 37. Photograph of hand specimen from Moanda, French Equatorial Africa, showing contact between manganese oxides and sedimentary rhodochrosite (X2.3). Relict carbonate bedding persists into overlying oxide layer; contact represents downward limit of supergene oxidation. Partial pressures of oxygen at which phase transitions occur are indicated on plate Courtesy of H Jaffe, Union Carbide Nuclear Company. metallurgicalpurposes since 1950.The pyrochroite is a hydration product of manganositewhich apparently was formed by the metamorphism of sedimentary rhodochrosite. Hydration of manganosite took place either during the terminal stagesof metamorphism, or at alater time as a

Rll6s.oci.iBsStTg

Frc. 38. Polished section of contact between rhodochrosite and manganese oxide. Rhodochrosite (dark gray) is oxidizing to manganite (medium gray) which is pseudo- morphously replaced by pyrolusite (light gray). Partial pressures oxygen at phase transitions are indicated on plate. Courtesy of H. Jaffe, Union Carbide Nuclear Company. OWEN BRICKER result of descendingsupergene solutions. The upper portion of the deposit has been oxidized by supergenesolutions and consistsof pyrolusite, cryptomelane,ramsdellite,l and psilomelane.The first oxidationproducts of the pyrochroite are hausmanniteand h1'drohausmannite(MqOa and p-MnOOH). Unfortunately, no data are available concerning the zone betweenthe pyrochroite and the supergeneoxide zones. It is likely that a thorough examination of this deposit would reveal the completeoxida- tion sequencefrom pyrochroite to pyrolusite. In each of these depositsthe sequenceof oxidesobserved in passing vertically from the surfaceto the protore is similar. The compoundscon- taining tetravalent manganese are found close to the surface or along channels which allow supergene waters or air free access to greater depths.Compounds containing only divalent manganeseare found in the protore. Thus, a gradient in the averageoxidation state of manganesein the oxides exists and is a function of the intensity of the oxidizing environment. The most important mechanismsinvolved in the oxidation processare the circulationof air through moist ore and protore and downward percolation of air-saturated meteoric water through the ore and protore. At localitieswhere a high oxidation potential existsdown to the protore contact, that is, either where the descendingwaters are repien- ished at a rate faster than they are depletedof their oxidizing capability, or circulation of air is rapid with respect to depletion of its oxl.gen content, a high averageoxidation state of manganesewill be found through- out the oxide zone.Where descendingwater is depleted of its oxidizing capability with depth or circulation of air is limited, a sequenceof min- eralswill be observed,decreasing in averageoxidation state with increasing depth or with decreasingaccessibility to the oxidizing water or air. Both of thesemechanisms contribute to the oxidation of rhodochrosite deposits;however, an assessmentof their relative importance indicates that circulating air must play the major role in the oxidation process. Typical ore from Nsuta, Ghana consistsof nsutite, with small amountsof cryptomelaneand pyrolusite. The moles of oxygen necessaryto oxidize one mole of rhodochrosite to each of these oxides are given in Table 9. The value of X in the formula of nsutite is seldomhigher than 0.05.Thus it can be seen that each of these reactions requires nearly ! mole of oxygen for each mole of rhodochrosite oxidized. The amount of ox.vgen necessaryto produce one ton of this ore from rhodochrositeis approximately 4,000moles, or 140lbs. If air is the only sourceof oxvgen,670 lbs. would be required for each ton of ore produced.Under supergenecondi-

I This paper was written before nsutite had been described, and the substance identified as ramsdeliite might well be nsutite, as the ;r-ray patterns of these compounds are similar. THE SYSTLM Mn-Oz-HzO

'I'.q,nr,r 9. OxycBN Requrnrwxrs ron SouB Srrrcrrn Rra.crroNs

Reaction Oxygen Required

1 1. MnCOs -t MnOz * COz 1-a" Ozf moleMnCO: 1Oz:

I t.) 1a 2. 8 MnCO:* KrCOrf : KMnaOra-1, COz -of" OzfmoleMnCOr t 7Oz f I 3. MnCO:+ -x)Or*2xHzO ft - .l moleozlmoleMnCo3 ,(t f : u"i]-u"f;og-:*(oH)z* a 69, f xHr

tions, 670 lbs. of air occupy about 9,000cubic feet.Thus the oxidation of one ton of protore would require the amount of oxygen contained in 9,000 cubic feet of air. One ton of protore has a volume of about 10 cubic feet. Assuming a porosity of 18/6, it could contain 1.8 cubic feet of air. The complete oxidation of a given volume of protore would require about 5,000 completechanges of air. Assuming two changesof air per year, it would take about 2,500years to oxidizeone ton of protore,or, in terms of depth, an oxidation rate of approximately one foot per thousand years. Consideration of the mechanism of oxidation by air saturated water shows that approximately 12,000,000liters of water would be required to supply the oxygen necessaryfor complete oxidation of one ton of protore. About 420,000 cubic feet of air-saturated water would have to flow through each ten cubic feet of protore in the oxidation process.Again assuminga porosity ol 18/6 and ideal permeability, each given volume of protore would have to be subjectedto 230,000complete changes of water' Rainfall data are given in Table 10. If aerated water is the sole agent active in the oxidation process,it would require about 23,000 years to oxidize one ton of protore, assumingall of the rainfall in an area receiving 40'-60' per year passedthrough the soil into the protore. Much rainfall

T.lsr.r 10. RerNner,r.Dere lon SoMr Snr-ncrsoLoclr-rtrBs

Locality Approximate Annual Rainfalll

Nsuta, Ghana 40n-60' Moanda, French Equatorial Africa 60'-80' Phillipsburg, Montana lo'-20' Minas Gerais,Brazil 60'-80'

I Goode's World Atlas, 1lth ed., 1960. OWEN BRICKLR is lost in runofi and evaporationand oxygenlosses occur when the rn'ater 'fhe percolatesthrough the soil zone. time involved would thereforebe somewhat longer. In excessof 2,000,000years would be required to oxidize a rhodochrositedeposit to a depth of 200 feet. A prohibitive period of time would be necessaryto producea depositsuch as the Nsuta ore body if the only oxidation mechanism were aerated water. The dominant mechanismin the formation of oxide ore bodies of this type must be the circulationof air. It wasnot possibleto synthesizemanganosite, bixbyite, groutite,ramsdellite, or pyrolusite in the range of experimental conditions investigated. It is not surprising that manganositeand bixbyite could not be synthesizedunder supergeneconditions because both of these minerals are found only in "dr1"' ltitn temperaturedeposits. 11 the presenceof water, manganositeis unstable with respectto pyrochroite and bixbyite is unstable with respect to manganite. Groutite and ramsdellite are found only in supergenedeposits and pyrolusite is a common mineral in supergenedeposits. Why these minera"lsare not amenableto svnthesis under simulated supergeneconditions remains a mvstery. Attempts to synthesizegroutite and ramsdelliteunder hydrothermal conditionshave also failed (Klingsberg, 1958). Bode and Schmier (1962) claim to have synthesizedramsdellite by oxidation of 7-Mn02, but their product is not well characterizednor are the conditions of synthesis stated. The thermal transformation of ramsdellite to pvrolusite (Ramsdell, 1932; Fieischeret al.,1962); of groutite to ramsdellite(Klingsberg, 1958); and of D-MnOzto y-MnO: to pyrolusite (Glemseret al., 196I; Gattow and Glemser,1961) have been well demonstrated.Both 6-MnO: and 7-MnOr undergothermal transformationinto a-MnOz in the presenceof high po- tassiumconcentrations (Faulring et al., t960). In view of the association of groutite, ramsdellite,and pyrolusite with supergenedeposits, and the easeof thermal transformation among them, it is somewhat surprising that attempted synthesisat 25oC. and one atmospherepressure have not succeeded.The temperaturesnecessary to effectthe thermal transforma- tions describedabove are not achievedin the supergeneenvironment. Aging probably plays an important role in the formation of theseminerals in nature.

AcxNowrnrGMENTS The author is indebted to Dr. Robert M. Garrels,of the Dep:rrtment of GeologicalSciences at Harvard University, for his adviceand aid both during the research and in the preparation of the manuscript. Dr. Clifford Frondel, of the Department of GeologicalSciences, Harvard University, kindlv provided mineral specimensfor r-ray standards.Dr. Walter Zwicker, then of the LTnionCarbide Corporation, was particu- THI:, SYSTEM Mn-Oz-HzO 1351 larly helpful in both the theoretical and technical phasesof the study. The stimulating discussionswith Mr. Howard Jaffe of the Union Car- bide Corporation and his excellentadvice on manv problems that arose in the courseof the study are gratefully acknowledged.Mr. W. Roettgers of the Union Carbide Corporation prepared the electron micrographs. Dr. Werner Stumm and Dr. JamesMorgan, Division of Engineeringand Applied Physics, Harvard University, were verv helpful with some analytical problemsthat arosein the research.Financial support for the investigationwas kindly providedby the Union CarbideOre Company.

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L[anu.script receired, December 11, 1964; accepted.Jor publication, May 28, 1965.