Magnetite Forrnation by the Reduction of Hematite with Iron Under Hydrothermal Conditions

American Mineralogist, Volume6l pages927-932, 1976

Magnetiteforrnation by thereduction of hematitewith iron underhydrothermal conditions

Ar-lN Mnrrnnws Departmentof Geology,Hebrew Uniuersity Jerusalem,Israel

Abstract

The formation of magnetiteby the reductionof hematitewith iron in the presenceof aqueoussolution at 350-570"C, 1-2 kbar pressure,takes placeby two reactions:(l) the oxidation of iron metal with water, and (2) the reductionof hematitewith hydrogen.The overall oxidation-reductionreaction is acceleratedby increasedtemperature, pressure, and solutionacidity. The oxidation of iron occursby a replacementand armoring mechanism, whereashematite reduction occurs by reductivedissolution of hematiteand direct precipitation of magnetitefrom the bulk solution.The initial rapid rate of the hematitereduction may renderunreliable an oxygenisotope geothermometer calibrated using the reaction.

Introduction gen with compact stacking are parallel for both havinglittle or no The reductionof hematiteto magnetiteat elevated phases,with iron atomsapparently temperaturesand pressuresin the presenceof water effecton relativeorientations. presentobservations can be representedby the equation: The purposeof this note is to on the mechanismsof magnetiteformation by the 3 FerO, * H, = 2 FesOo+ HrO (l) reductionof hematitewith iron under hydrothermal kbar pressure,and to Magnetitecan alsobe formedat high temperaturesin conditionsat 350'-570oC,l-2 mechanisms the presenceof water by the reduction of hematite indicatepossible consequences of these geothermome- with iron metal.The reactioncan be expressedby the for a magnetite-wateroxygen isotope (Bertenrath The iron- equation: ter calibration et al., 1973). magnetite-wustitetriple point is at 575oC(Huebner, Fe*4FerOr=3FerOn (2) l97l), and the experimentalupper temperature limit was chosen so as to eliminate the possibility of (l Littleis knownabout the mechanisms of reactions ) wustiteformation. and (2) under hydrothermalconditions. However, severalstudies have beenmade on the reductionof Experimentalmethods and observations hematiteat or nearone atmospherepressure, using a varietyof reducingconditions: CO / CO H 2,H 2/H*O, The solid reactantswere weighed out accordingto and FelHzO(McKewen, 1960; Kawaski ", et al.,1962; the stoichiometryof equation(2). The solidsused Endomet al., 1964;Heizmann and Baro, 1966;Nabi were:iron (metal)powder (Fisons Ltd.); iron metal and Lu, 1968;Hara et al., 1969).These studies have shavings,approximately 1mm long (BritishChemical shown that magnetiteinitially forms as a surface StandardsNo. 26013)and iron (III) oxide> 99.995 layer on the hematitereactant, and then gradually percent(Koch Light Ltd.). Solutionswere either wa- replacesthe hematite.The kinetics of reaction are ter or dilute hydrochloricacid. Experimentalcharges controlled by diffusion of gasesto and from the consistedof 50-60 mg solid and 800-1000mg solu- hematite-magnetiteinterface, through the coating tion sealedinside 0.006" thick walled gold capsules. magnetitelayer. Topotaxic influences on the mecha- All experimentswere performed inside stainless steel nism of the reductionof singlecrystal hematite have cold-sealbombs, using water as the pressuremedium. also been determined(Heizmann and Baro, 1966; Reactionproducts were examinedby X-ray powder Moineauand Baro, 1967;Baro and Heizmann,1969; diffraction using iron filtered CoKa radiation. This Moineauand Baro, l97l). The epitaxialrelationship method could not satisfactorily detect the small betweenthe two mineralsis suchthat planesof oxy- amountsof iron in products,but visualexamination

927 928 ALAN MATTHEI4/S with a moderate-powerbinocular microscopesuf- Tlsrs l. Experimentalresults ficedfor this purpose.The molar proportionsof magnetite to hematitewere estimatedfrom the relative Toc t.hrs P.kbSolid Soluti ons Products Yiel d reactant s % intensitiesof the magnetite3ll reflectionsand the hematite 355 166 I Fe shavings 0.01M.HCl Magnetite 104reflections. For this purposea calibra- Hematite Hematite tion curvewas prepared by X-ray diffractionof mix- Iron 355 166 I " " 0.00lM. HC l Magnetite tures containing known amounts of magnetiteand Hematite hematite. Iron Scanning electron microscopy was em- 360 97 I Fe powder 0.01M.HCI Magnetite ployed in determining the nature of the reaction Hematite Hematite Iron mechanisms. 360 5i0 I " " Magnetite Experimentalresults are summarized,in Table l Hemati te '1""0.00lM.HClMagnetite Iron The estimatesof magnetite/hematite proportions are 360 510 Hematite givenas percentmagnetite yield, as deierminedfrom I ron ' the stoichiometryof equation(2). Temperaturesare 365 455 2 " Magnetite Iron (s)* ' quoted +5oC, pressurea200 bars; and the stated 360 0.25 2 " Water Magnetite reaction Hematite timesare from whenthe bombsequilibrated Iron at temperature(run-up timeswere approximately 45 355 455 2 ' Maqnetite (s) 'I Ir6n min.).The followingobservations can be madewith JOU O3U " Magnetite 75 respectto the data presentedin (l) Hematite the table. The I ron reactionof iron powder with hematiteis accelerated 365 1940 I .5 " ltlaonetite 100 by increasingtemperature, pressure and solution 465 t',l6 Fe shavings llater llagnetite 65 acidity. (2) Extensivemagnetite formation (>60Vo) Hematite Hematite Iron occurs in reactionsof short duration (0.25-0.5 460 48 Fe powder 0.0lM.HCl Magnetite 100 hours).Iron metalconstitutes only I I percentof the Hematite 'l00 450 136 0.00lM.HCl Magnetite '100 total iron of the reactants,thus the bulk of the high 4ss 358 0.0001M.HCIMagnetite 460 0.5 Fe powder l,later Magnetite 60 magnetiteyields must come from the reduction of Hematite lilitt* hematite.Additionally, Matthews(1974) has shown 450 48 ,, Magneti te that only slightoxidation of iron metalpowder (< Hemati te Iron 207o)occurs under similarexperimental conditions. 455 358 Magnetite Substantialmagnetite Iron (s) formation must occur during 455 515 Magnetite the run-upperiod. lr0n (s,l In the reactionsin which iron metal shavingsand 555 166 Fe shavings l.later Magnetite hematitewere the solid reactants,the HenEtite Iron shavingscould 545 0.5 Fe powder Water Magneti te be recoveredfrom the experimentalproducts, coated Hematite Hematite with a surfacelayer of magnetite,but otherwiseun- Iron 570 267 l'4agnetite '100100 reacted.The productsremaining after the extraction 540 357 Magnetite 30f, cJ{ Magnetjte 100 of the iron shavingscontained both magnetiteand hematiteat 350oand 465C, but at 555"Conly magnetite was detectable.Similar observationswere *(s) indjcates that only traces of iron are present in the reaction Droduct. madein severalexperiments in which iron powder was reactedwith hematite;whereas all hematitehad undergoneconversion to magnetite,traces of un- Magnetite crystallizesin the isometricsystem, and reactediron remainedin the product. most commonlyshows octahedral and dodecahedral SEM micrographsillustrating the mechanismsin- form. Thesehabits can be recognizedin someof the volved in the iron-hematite reaction are given in idiomorphiccrystals shown in Figure2. FiguresI and2. Two distinctgenerations of magnetite crystals can be observed:(a) irregular, tightly Discussion packed, magnetite crystals, surrounding the iron The experimentalobservations indicate that mag- metalgrains (Fig. I ), (b) well formed,loosely packed, netite formation proceeds non-stoichiometrically. magnetitecrystals, often revealingextremely well de- Reductionof hematiteto magnetiteoccurs more rap- velopedcrystalline habits (Fig. 2). No obviousorien- idly than the oxidation of iron. Completereaction is tatedrelationships exist between the type (b) crystals. achievedin conditionsof enhancedT. P. and solution MAGNETITE FORMATION UNDER HY DROTHERMAL CONDITIONS 929

system.Eugster and Wones(1962) note that the effectsof hydrogendiffusion from a pressuremedium bufferedby bomb walls are only noticeableat tem- peraturesgreater than 700oC.Specifically, Huebner (1971)states that thehematite-magnetitefOr) buffer is stablefor 2-3 daysat 750"C.At the lower temper- aturesof this study,hydrogen diffusion rates must be significantlyslower than at 700oC,particularly so sincereasonably thick-walled gold tubing was used. The non-stoichiometryis evidentat all temperatures in runsof only0.5 hours duration; this clearly cannot be a consequenceof hydrogen diffusion from the pressuremedium. It is reasonableto concludethat hydrogendiffusion through capsulewalls is of sec- ondary influenceon experimentalphenomena. Matthews (1974) found that increasingtemperature and solution acidity acceleratedthe oxidation of iron metal.Consequently, the rate of generationof hydrogenby the oxidation reactionwill also be in- creasedby higher temperatureand solution acidity. iron metal grain. Ftc. l. Magnetitecrystals surrounding an (1960) found that the rate of hematite Product from reactionof iron with hematitein water after 0.5 McKewen hoursat 545oC. reductionin hydrogen-watervapor mixturesat I at- mosphereis proportionalto the partial pressureof hydrogen.Thus, the rate-enhancingeffects oftemper- acidity. This independence in the chemical behavior ature and solutionacidity on the iron-hematitereac- of the two solid reactants,together with the SEM tion could have a dual origin. The productionof observations,suggests that two reactionsare occur- magnetitefrom the oxidationof iron is increased,but ring: also,because of the correspondingincrease inl(Hr), Oxidationof iron metalwith water

3 Fe + 4 HrO= Fe'On* 4 Hz (3) Reductionof hematitewith hydrogen

3 FerO, * H, =i 2 FerOr + HrO (l) In oxidation-reductionterms, the two reactionsare proceedingin oppositedirections. (3) occursso asto increasefHz),and hencedecrease/(Oz); whereas (l) resultsin increasedoxygen fugacity. Effectively(l) and (3) aretwo opposedflOr)buffer reactions. The 'slow'step' of the overallreaction is the oxidation of iron. Since,according to theabove equations, the iron oxidationprovides the sourceof hydrogen for the hematitereduction, it would be expectedthat the kineticsof (l) are controlledby those of (3). However,if the join hematite-capsulewater is itself consideredto containan oxidation-reductioncouple, undergoingpartial reaction,then additionalhematite reductionoccurs over that taking place by stoichiometric reaction.An alternativeproposal is that the non-stoichiometric reaction progress is a con- FIc. 2. Euhedralmagnetite crystals formed by the reductionof sequenceof hydrogen diffusion from the pressure hematite. Product from reaction of iron with hematite in water medium through the capsulewalls into the reacting after 576hours at 455oC. 930 ALAN MATTHEWS the rate of hematitereduction is enhanced.The ob- Precipitationof magnetiter servedacceleratory effects of pressuremay also be a 6Fe(OHL=2FerO,+2H2 +4H,O (lb) consequdnceof iron oxidation being acceleratedby increasingpressure. Conclusions Matthews' study revealedthat magnetiteinitially Magnetiteformation by the reductionof hematite nucleates heterogeneouslyfrom solution onto the in the presenceof water and a Ni/NiO buffer has surfaces of iron metal grains.The reactioninterface been used in calibratinga magnetite-wateroxygen then developsinwards into the iron grains,with the isotopegeothermometer at 300o-960"C(Bertenrath magnetitecrystals replacing and armoringthe metal. et al., 1973).However, the evidenceof this study is SEM examinationof the iron reactantin the inter- that the reduction of hematitein the presenceof a mediate stages of the iron-hematite reaction in- reducinglOr) buffer has a very rapid initial rate, variablyreveals the metalgrains to be surroundedby with substantialmagnetite formation occurringdur- a tightly packedlayer of magnetirecrystals (Fig. I ). It ing heatingof bombs up to maximum temperature. hasalso been noted previously that the iron shavings, This initial magnetiteformation createsa problemin coatedwith a surfacelayer of magnetite,can be ex- the interpretationof isotopicequilibrium; for unless tracted from reactionproducts. These observations substantialrecrystallization of magnetitetakes place indicatethat iron oxidationin the iron-hematitere- at maximum temperature,the oxygenisotope analy- action occursby a replacementand armoringmecha- sis of the solid will contain a memory reflectingthe nism. range of temperaturesoccurring during heatingup. The well-formed, often idiomorphic, magnetite Early-formedmagnetite crystals, though small, are crystalsrevealed by SEM examination(Fig. 2) are frequentlywell-formed and sometimeseuhedral; such formed by the reductionof hematite.The magnetite crystals will tend to grow, but it is questionable crystalsare not packedtogether in a mannerwhich whether they will recrystallize.In the case of the suggestspreservation of the grossmorphology of the magnetite-water geothermometercalibration, the hematitegrains, and contrastwith the magnetitepro- presenceof memory effectsof the heating-upperiod duced by the oxidation of iron, in which product would tend to give low estimatesof the equilibrium crystalsare tightly packed,and the grossmorphol- fractionation factor a(O18,/O16magnetite/OL8/OLo ogiesof the reactantiron grainsare preserved.These water), since da/dT is positive in the range 300- SEM observationsindicate that the reductionreac- 960'C. Such a fractionation,when combinedwith tion occursthrough dissolution of hematite,followed another calibration (e.g. quartz-water) to give a by heterogeneousnucleation and growth of magnet- geothermometer,would tendto giveerroneously high ite from solution.The magnetitegrowth appearsto temperatureestimates when applied to the ther- occur directlyfrom the bulk solution,and thereis no mometryof naturalassemblages. evidenceto indicatethat magnetitereplaces and ar- Evidence for the existenceof low-temperature mors the hematitereactant. Nucleation of magnetite memory effectscan be found by comparingthe frac- appearsto be a highly favoredprocess, as is attested tionation factors for magnetitessynthesized under by the rapid initial rate at which the hematitereduc- hydrothermalconditions at I kbar (a) by the reduction takesplace. tion of hematitewith iron, accordingto equation(2), A further questionis at what stagein the reaction and (b) by the oxidation of iron metal with water.A doesreduction occur: during dissolutionof hematite, magnetitesynthesized in this study(I:565oC, t : or at somelater stage?Studies on the dissolutionof 534 hours) gives a fractionationfactor l0ln a : thin films of ferric oxide at room temperaturehave -7 .831'whereas a magnetiteproduced by the oxida- shownthat direct dissolutionof the oxideis a signifi- tion of iron metalpowder with water,after 264 hours cantly slower process than reductive dissolution reactionat 560oC,gives a factor lffln a : -7.20 (Evans, 1930;Pryor and Evans, 1949;Evans and (analyticaldetails can be found in the Appendix). Berwick, 1952).Holser and Schneer(1961) found Both reactionsinvolve magnetiteformation by solu- that ferrousspecies dominate the solutionin equilibrium with magnetiteunder hydrothermal conditions. ' The inclusionof ferroushydroxide as solution species is for the Theseobservations suggest that a suitablemechanism purposesof satisfyingthe stoichiometriesof equations(la) and for the hematitereduction is: (lb). The true nature of the ferroussolution speciesexisting in hydrothermalconditions is not known. However,it is possibleto Reductive dissolution of hematite' write equationssimilar to (la) and (lb) usingalternative represen- 3 FerO,+ 3 H, + 3 HrO =6 Fe(OH), (1a) tationsfor the ferroussolution species; Fe(HrO)?"+, etc. MAGNETITE FORMATION L]NDERHYDROTH ERMAL CONDITIONS 931 tion and precipitation,with the inherentpossibility of t 0.1 per mil or better.Oxygen was liberatedfrom establishingisotopic equilibrium between the product the magnetitesusing the BrFu techniqueof Clayton phases.However, whereasextensive magnetite for- and Mayeda(1963), and from waterby theguanidine mation occursduring the run-up period of the hema- hydrochlorideprocedure ofBoyer et al.(1961).Anal- tite-iron reaction,only minor magnetiteformation ysesof the magnetitesfrom the hematitereduction : (- 20Vo)occurs during the run-up of the iron oxida- and iron oxidation respectivelyztQ 6sv6q 25.45 : tion (Matthews, 1974).The differencebetween the parts per thousand and 6s;aey 26.10 parts per two fractionationfactors may arisefrom the presence thousand.Analyses of the water usedin both reac- : of low-temperaturememory effectsin the isotopic tionsgave 6suow 33.51parts per thousand. analysisof the magnetiteproduced by hematitereduction.It is not possibleto commentin detailon the Acknowledgments calibration of Bertenralh et al. (1973), since they This work is basedin part on experimentsperformed while the present no data in support of their fractionation author was in receiptof an NERC studentshipat Manchester Beckinsaleand J. curve.However, it canbe notedthat their curvegives University.The authorwould like to thank R' D. for oxygenisotope analyses, and ProfessorsW' S' Fyfe a = -7 .9 at 565oC,a valuewhich corresponds J. Durham 1CIln and W. S. Mackenzie,and Dr. A' C. Dunham,for their encour- quite closely to that obtained above from the agementand help. Dr. Y. Kolodny is thankedfor reviewingthe iron-hematitereaction. manuscnpt. Ramdohr (1969) notes that naturally-occurring magnetiteis frequentlyformed by the pseudomorphic References replacementof hematite.This mode of formation Bnro, R. rNo J. J. Huzpt,arnN (1969) Cinetique de r6duction de (001). contrastswith that observedin this study for hydro- I'h6matite FezOs en magn6tite FesO. selon une face Bull' 92' 394. in which magnetiteis precipi- Soc. Fr. Mineral. Cristallogr' thermal conditions, BEcKTNsALE.R. D., N. J. Fnnrunu, M' C. JecrsoN, R. E. Powrll tated from the bulk solution,after the reductivedis- lno W. A. P. Your.rc (1973) A 30 cm radius 90o sector double solution of hematite.This suggeststhat magnetite collecting mass spectrometer with a capacitive integrator de- formation by pseudomorphicreplacement occurs in tector for high precision isotopic analysisofcarbon dioxide. lnt' the absenceof sufficientsolution, or underP, T con- J. Mass. Spectrom. Ion' Phys.12,299-308. I BERTENRATH.R.. H. FnIsonIcHsEN, AND E. Hru-NEn (1973) Die which diffusiveprocesses become favored 180/160 ditions at Fraktionierung der Sauerstoffisotopen im System Eisen- over solution and precipitation.An exampleof such oxid-Wasser. Fortschr. Miner' 30' 32-33. conditionsis givenin the Introduction;i.e. high tem- Bovrn, P D., D. J. GRAvEs,C. J' Sunlrrr, lro M. E. DrupsnY peratureand low Ps16.In contrast,some metamor- (1961). Simple procedure for the conversion of oxygen of ortho- phic magnetitesare found to exhibit a well-defined phosphate or water to carbon dioxide for oxygen-18 determina- 1906-1909. habit. Octahedral magnetite associated tion. Anal. Chem.33' crystalline CllytoN, R. N. rNo T. K. M.c.vsna (1963). The use of bromine with pyrite hasbeen found in chloriteschists at Ches- pentafluoride in the extraction of oxygen from oxides and sili- ter County, Vermont, and both simpleand twinned cates for isotopic analysis. Geochim. Cosmochim. Acta, 21, octahedronsin the chlorite schistatZillettal, Tyrol. 43-52. (1964)'Reduction of Dodecahedralmagnetite crystals have been found in ENoopr.A.. K. Hroonr rro G. LrsueNr with hydrogen.Arch. Eisenhuttentues, 35, 5'17-584 Rumania,and at Nordmark,Swe- hematite the Banatregion, (Chem.Abst. 6l-11659). \ den. At Traversella,Piedmont, well-formed magne- Eucsrrn, H. P. nr.rnD. R. WorqEs(1962). Stability relations of the tite occursat a contactof limestoneand diorite.The ferruginousbiotite, annite. J. Petol ,3,82-125. analogiesbetween these crystal forms and thoseob- EvnNs, U. R. (1930),Isolation of the film responsiblefor the Nature, 126, I 30-I 31' servedin this study suggestthat sufficienthot solu- passivityof an iron anodein acidsolution. -AND I. D. C. Bsnwtcr (1952),Passivity of metals,Part XI. present the precursoriron tion was to both dissolve Passivityof anodic behavior of oxide films' ,I Chem. Soc' solid species (commonly iron-bearing carbonate 3432-3437. rock) and allow direct precipitationand growth of Henn, Y., M. Tsucuvr, ,lNo S. KoNro (1969).Reduction of iron magnetite. oxide pelletswith hydrogen at high temperatures.Tetsu To Hagane,55,1297-13ll, (Chem. Abst 72-57944). HntzunNu,J. J. lNo R. B,qno(1966) Relations topotaxiques entre Appendix descristaux naturels d'h6matite FerO, et la magn6titeFeeO. qui en est issuepar reductionchimique. C. R. Acad Sci Parrs,Ser' Oxygenisotope analysis D.263.953-954. HoLssn,W. T. eNoC. J. ScsNnrn(1961) Hydrothermal magnet- Samples,as COz gas,were analyzed on a 30 cm ite. Geol.Soc. Am. Bull. 12,24'l-259. double collecting mass spectrometerdescribed by Husnr.,ren,J. S. (1971),Buffering techniques for hydrostaticsys- Beckinsaleet al. (1973).Standard reproducibility was temsat elevatedpressures. In G. C. Ulmer, Ed', ResearchTech' 932 ALAN MATTHEWS

pressures. niques for High Temperatures and High Springer-Ver- -AND-(1971) Cin6tiquede reductionen magn6tite,de lag, New Yotk. 123-l'17. monocristauxd'h6matite Fe.Or, selon leurs facesnaturelles. Kewesrr, E., J. SnxscurNrE ANDT. J. WALsH (1962).Kinetics of Bull. Soc. Fr. M ineral. Cristallogr. 94, 444-445. reduction of iron oxide with carbon monoxide and hydrogen. Nnsr,G. ANDW. K. Lu (1968)Reduction kinetics of hematiteto Am. Inst. Chem. Engrs. J. 8, 48-52. magnetitein hydrogen-watervapour mixtures.Trans. Metall. McKEwEN, W. M. (1960)Kinetics of iron oxide reduction.Zrans. Soc.Am. Inst. Mech.Eng.242,2471-24'17. Am. Inst. Mech. Engrs. 218, 2-6. Pnyon,M. J. nNo U. R. EvrNs (1949)The passivity of metals,part MettHrws, A. (1974) Oxygen isotope geology: experimental cali- X. The mechanismof the direct dissolutionof ferric oxide."/. bration of geothermometers with related experimental in- Chem.Soc 3330-3336. uestigations.Unpub. Ph.D. Thesis, University of Manchester. Rrnooun, P. (1969) The Ore Minerals and their Inlergrowths. MotNrlu, H rxo R. Blno (1967), Relations topotaxiques entre PergamonPress, Oxford, 1174p. cristaux d'h6matite Fe2O. et la magn6tite FesOl qui en est issue par la paris,$er.D,264, dissociation sous vide. C. R. Acad. Sci. Manuscript receiued, October 23, 1975; 432433. acceptedfor publication, March I, 1976.