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 precipi- tation 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 Little is 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 mag- netite 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 mag- netite 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 magnet- ite 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 ef- fectsof 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 increasingtemper- ature 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,