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Home , Magnetic mineralogy

MagneticProperties of Rocksand

ChristopherP. Hunt, BruceM. Moskowitz,Subir K. Banerjee

1. INTRODUCTION composition-dependentmagnetic parameters of a varietyof -bearingminerals. This is an updatedcollation of magneticparameters of rocksand mineralsfor geologists,geochemists, and geo- 2. physicists.Since the publication of theprevious edition of Handbookof PhysicalConstants [74], two othercollations Magnetic susceptibilityis a measureof the magnetic have appeared[16, 18]. In addition,selected magnetic responseof a materialto an externalmagnetic field. The parametershave alsobeen assembled[19, 22, 38, 41, 88]. volumesusceptibility k, measuredin dimensionlessunits, is Ratherthan produce a fully comprehensivecollection, we definedas the ratio of thematerial magnetization J (perunit have aimed for high-precisiondata obtainedfrom well- volume) to the weak externalmagnetic field H: characterizedsamples. Bothtables and figures have been used for presentingthe J=kH. (1) data,and best-fit equations have been provided for someof thedisplayed data so that interpolations can be made easily. Alternatively,the specificor masssusceptibility Z, mea- In an attemptto discouragethe useof the outdatedcgs suredin units of rn3kg4, isdefined as the ratio of the material system,all valuesare in theSI system(see Moskowitz, this magnetizationJ (per unit mass)to the weak externalmag- volume). Referenceshave been cited for the sourcesused netic field H: here. However,a morecomprehensive bibliography has alsobeen provided from which information can be extracted (2) for sampleswhich havenot beenincluded. The single-crystalconstants and their variation with tem- All materialshave magneticsusceptibility, which can be peratureand compositionare for useby rock magnetists. eitherpositive (paramagnetic) or negative(diamagnetic). In Paleomagnetistsand magnetic anomaly modelers have been materialswhich displayhysteresis, the initial slopeof the providedwith the magneticproperties of rocksand poly- hysteresisloop is takento be theinitial or low-fieldsuscep- crystallinemineral samples. Lastly, we havemade an effort tibility Zo. Magnetic susceptibilityvalues are useful in to addressthe needsof environmentalmagnetism, a new geophysicalexploration, and in models of both crustal groupof researcherswho requirethe valuesof size- and magnetizationand magneticanomalies. Table 1 liststhe (initial) susceptibilityfor commonrocks and minerals. C. P. Hunt, B. M. Moskowitz,and S. K. Banerjee,University In ferro-, ferri-, or cantedantiferromagnetic materials, of Minnesota,Institute for Rock Magnetismand Department hysteresisand the presence of magneticdomains cause the of Geologyand Geophysics, 310 PillsburyDrive SE,Minneap- initial susceptibilityto becomegrain-size dependent. This olis, MN 55455 dependencefor magnetiteis plottedin Figure 1. Rock Physicsand PhaseRelations Initial magneticsusceptibility is temperaturedependent. A Handbookof PhysicalConstants The susceptibilityof paramagneticmaterials is inversely AGU Reference Shelf 3 proportionalto absolutetemperature, but the susceptibility

Copyright1995 by the AmericanGeophysical Union. 189 190 MAGNETIC PROPERTIES

TABLE 1. MagneticSusceptibilities of SelectedRocks and Minerals Rock/ Chemical Formula Density Volume k MassZ References (103 kg m-3) (104 SI) (104 m3kg-•) IgneousRocks andesite 2.61 170,000 6,500 114 basalt 2.99 250-180,000 8.4-6,100 95, 107, 114, 115 diabase 2.91 1,000-160,000 35-5,600 114 diorite 2.85 630-130,000 22-4,400 114, 115 gabbro 3.03 1,000-90,000 26-3,000 95, 107, 114, 115 granite 2.64 0-50,000 0-1,900 95, 107, 114, 115 peridotite 3.15 96,000-200,000 3,000-6,200 114 porphyry 2.74 250-210,000 9.2-7,700 114 pyroxenite 3.17 130,000 4,200 114 rhyolite 2.52 250-38,000 10-1,500 114 igneousrocks 2.69 2,700-270,000 100-10,000 22 averageacidic igneous rocks 2.61 38-82,000 1.4-3,100 114 averagebasic igneous rocks 2.79 550-120,000 20-4,400 114

SedimentaryRocks clay 1.70 170-250 10-15 114 coal 1.35 25 1.9 114 dolomite 2.30 -10-940 -1-41 95, 114 limestone 2.11 2-25,000 0.1-1,200 22, 107, 114, 115 red sediments 2.24 10-100 0.5-5 22 sandstone 2.24 0-20,900 0-931 107, 114, 115 shale 2.10 63-18,600 3-886 114 averagesedimentary rocks 2.19 0-50,000 0-2,000 114

Metamorphic Rocks amphibolite 2.96 750 25 114, 115 gneiss 2.80 0-25,000 0-900 107, 114, 115 granulite 2.63 3,000-30,000 100-1,000 126 phyllite 2.74 1,600 60 114 quar•ite 2.60 4,400 170 114 schist 2.64 26-3,000 1-110 114, 115 serpentine 2.78 3,100-18,000 110-630 114 slate 2.79 0-38,000 0-1,400 107, 114, 115 averagemetamorphic rocks 2.76 0-73,000 0-2,600 114

Non-Iron-BearingMinerals C 2.16 -80--200 -3.7--9.3 16, 95, 107, 114 calcite CaCOa 2.83 -7.5--39 -0.3--1.4 16, 18, 22, 114 CaSO4 2.98 -14--60 -0.5--2.0 16, 18, 95 gypsum CaSO•.2H•O 2.34 -13--29 -0.5--1.3 16, 107, 114 ice H•O 0.92 -9 -1 107 KA1Si30s 2.57 -13--17 -0.49--0.67 16 MgCO3 3.21 - 15 -0.48 22 Mg2SiO4 3.20 -12 -0.39 16 serpentinite Mg3Si2Os(OH),• 2.55 3,100-75,000 120-2,900 107 halite NaC1 2.17 - 10-- 16 -0.48--0.75 16, 18, 107, 114 PbS 7.50 -33 -0.44 16 quarlz SiO• 2.65 -13--17 -0.5--0.6 16, 18, 22, 73, 95 cassiterite Sn02 6.99 1,100 16 114 celestite SrSO• 3.96 -16--18 -0.40--0.450 16 ZnS 4.00 -31-750 -0.77-19 16, 114 HUNT ET AL. 191

TABLE 1. (continued)

Iron-BearingMinerals A3B2(SiOn)3 3.90 2,700 69 16 illite clay w/1.4% FeO, 4.7% Fe203 2.75 410 15 18, 22 montmorillonite clay w/2.8% FeO, 3.0% F%O3 2.50 330-350 13-14 18, 22 K(Mg,Fe)3(A1Si3Oa0)(O•2 3.00 1,500-2,900 52-98 16, 18, 22 FeCO3 3.96 1,300-11,000 32-270 16, 18, 47, 73, 114 FeCqOn 4.80 3,000-120,000 63-2,500 114 orthoferrosilite FeSiO3 4.00 3,700 92 18 orthopyroxenes(Fe,Mg)SiO3 3.59 1,500-1,800 43-50 16, 18, 22 FenSiOn 4.39 5,500 130 18 (Fe,Mg)•SiOn 4.32 1,600 36 16 MnFe2On 4.99 25,000 500 18 fmnklinites (Zn,Fe,Mn)(Fe,Mn)•On 5.21 450,000 8,700 114

Iron Sulfides chalcopyrite CuFeS2 4.20 23-400 0.55-10 16, 114 arsenopyrite FeAsS 6.05 3,000 50 114 troilite FeS 4.83 610-1,700 13-36 16, 47, 73 pyrrhotites Fe•_xS 4.62 460-1,400,000 10-30,000 18, 20, 22, 29, 95, 114, 115, 127 Fe•S•2 4.62 1,200 25 16 pyrrhotite Fe•0S• 4.62 1,700 38 16 pyrrhotite Fe9S•0 4.62 170,000 3,800 16 pyrrhotite Fe, Ss 4.62 3,200,000 69,000 16 pyrite FeS2 5.02 35-5,000 1-100 16, 47, 95, 114

Iron- Oxides o•-FenO3 5.26 500-40,000 10-760 16, 18, 22, 47, 70, 73, 114, 115 •/-FenO3 4.90 2,000,000-2,500,000 40,000-50,000 1, 115 FeTiO3 4.72 2,200-3,800,000 46-80,000 16, 18, 22, 47, 106, 114, 115 FeaOn 5.18 1,000,000-5,700,000 20,000-110,000 16, 18, 22, 55, 61, 62, 75, 114, 115 titanomagnetite Fe3_xTi•On,x=0.60 4.98 130,000-620,000 2,500-12,000 18, 44, 62 titanomaghemiteFeo_•> •Ti,•Elm_•>O n, 4.99 2,800,000 57,000 22 R=8/[8+z(l+x)] ulvOspinel FenTiOn 4.78 4,800 100 22

OtherIron-Bearing Minerals iron Fe 7.87 3,900,000 50,000 22 goethite o•-FeOOH 4.27 1,100-12,000 26-280 16, 32, 47, 115 lepidocrocite ht-FeOOH 4.18 1,700-2,900 40-70 16, 47, 115 limonite FeOOH.nH20 4.20 2,800-3,100 66-74 16, 115 Notes: All susceptibilitieswere measured in weakfields at roomtemperature and at oneatmosphere pressure. Literature valuesfor susceptibilitieswere convertedto $I unitswhen necessary,and from volumeto massnormalization using acceptedvalues for materialdensities [22, 73, 114]. Susceptibilityvalues have been rounded to the numberof significant figures given in the original. Most valuescome from other tabulations,to which the reader shouldrefer for more information. Valuesfor the more importantmagnetic minerals (magnetite, titanomagnetite, hematite, pyrrhotite, and goethite)were collated from recentoriginal sources. 192 MAGNETIC PROPERTIES

ß • ß OzdenurandBanerice (1982) 0 HartsIra(1982a) Zfd= Z470,z- Z47oou x100%, (3) ß ß Dunlop(1986) o Dayet al. (1977) •470Hz • •o '* ß Maher(1988) o Dankers(1978) where,•470Hz and ,•4700Hzare the susceptibilityof a sample measuredat 470 Hz andthat measured at 4700 Hi, respec- tively. In magneticmaterials, there is a small window of • 6 grainsizes (near 20 nmin magnetite)which will bemagneti- cally unstable(superparamagnetic) at 470 Hi, but stable (single-domain)at 4700 Hi. Overrelatively long"observa- tion times" at 470 Hi, sucha grain will appearto be magneticallyunstable, and will contributesignificantly to

0 i [ I i i thetotal susceptibility of thesample. But overshorter times 0.01 0.1 I 10 100 at 4700 Hi, thesame grain will appearto be stable,and will Grain diameter (/.tm) contributelittle to thetotal susceptibility. A samplecontain- Fig. 1. The grain-sizedependence of initial magneticsus- inga significantfraction of suchgrains will thushave a high ceptibility(Zo) in magnetite.Experimental data from crushed value(up to about12%) of Zfd-This parameter can be used grains(open symbols) and grown crystals (closed symbols onlyqualitatively to detect the presence of ultrafinegrains of andcross). Solid linesare power-law fits for grown(GR) magneticmaterial such as magnetite or maghemite,which and crushed(CR) samples. areoften found in soils[e.g., 115]. Thesusceptibility of a samplecan also vary with direction, dependingon the fabric of the constituentminerals. Anisot- ropy of magneticsusceptibility (AMS) can be used to of diamagneticmaterials has no temperature dependence. In determinesedimentary flow directions,or metamorphic magneticmaterials, there is often a"Hopkinson peak" [e.g., deformationparameters [e.g., 57]. 88] where susceptibilityincreases just below the Curie temperaturebefore dropping to relatively small values. 3. GRAIN-SIZE DEPENDENCE Examplesare shownin Figure 2. The peak occursat high temperaturesbecause both number and mobility of domain Variousmagnetic properties show a stronggrain-size walls in MD grainsincrease, and thermalactivation of SD dependence[e.g., 41, 111]. This dependenceoccurs not momentsincreases, all leadingto an increasein responseto becauseof anyintrinsic control of grainsize on magnetiza- an external field. tion, but becausethese parameters are influencedby the Hydrostaticpressure does not affect the magnitudeof magneticdomain state of the samples,which in turn is a magneticsusceptibility in experimentsof up to 2 kbar [e.g., 76]. However,uniaxial stress will changethe susceptibility bothin amountand direction,dependent on the orientation 3 ß ' ' i • , i i ß 00•ftm magne.te. Dunlop11974J of theapplied stress relative to themagnetic field. When the 0.1ftrn magne.te, Durdop119741 ß 02-0.3lun..... lar ma•net ....Dunlop(1974, / appliedstress is parallel to the magnetic field thesusceptibil- serpent.....dperido .....Btna andHenrv (1990) / ity decreases;when the stressand field are perpendicular, deepupper-crustal...... l •rneta-andestte...... Will .... Sruve al and(1985/86) Fountatn •1988 ) / thereis anincrease in magneticsusceptibility [e.g., 82]. The œrarud....Kelso(1993) amountof changeis reversible,and is dependentboth on compositionand on magneticgrain size. Changesin suscep- tibility can be +40% at 2 kbar of differential stress[76, 82]. However, uniaxial stressesgreater than 1-2 kbar are un- likely to be sustainedin matehals residing at elevated temperaturesin the lower crust. Initial susceptibilityis dependentupon the frequencyat 0 100 200 300 400 500 600 whichit is measured.This is becausesusceptibility depends Temperature (øC) on the magnetic domain state of a sample, which in turn dependson the length of time over which the sample is Fig.2. Temperaturedependence ofinitial magnetic suscep- measuredor observed. The parameterknown as the "fre- tibility(Zo) for crustalrocks and syntheticfine-grained quencydependence of susceptibility"Zfd is usuallydefined magnetite.Note the presence ofonly a veryweak Hopkinson by peak in the rock data. HUNT ET AL. 193

function of grain size. Domain states change from TABLE 2. Reference Guide for Grain-Size superparamagnetic(SPM), to single domain (SD), and fi- Dependenceof MagneticParameters nally to multidomain(MD) with increasinggrain size. Mineral References Domain-transitiongrain sizes depend on composition,tem- perature,and microstructure. Magnetic granulometry is any hematite 25, 30 magneticmethod for determiningeither the physical or the titanomagnetite 28, 87, 100 titanohematite 125 magneticgrain size of a magneticmaterial. Magnetic grain pyrrhotite 20, 29, 31, 35, 77, 127 size refersto the magneticdomain state and behaviorin a goethite 32, 33, 34 magneticparticle, regardless of thephysical dimensions of maghemite 25 theparticle. Here, we are interested in magneticgranulometry of naturalmagnetic grains, such as magnetite (Fe3On), hema- tite (c•-Fe203),maghemite (T-F%O•), and the varioustita- nium-substitutedcompositions of these three minerals (titanomagnetite,titanohematite, and titanomaghemite). The ture, or equivalently,as a function of thermal activation compositionof the various Fe-Ti oxides and their solid- energy/stability. Examplesinclude the frequencydepen- solutionseries are shownin Figure 3. Theseminerals are denceof susceptibilitydiscussed above, and low-tempera- found in soils,in oceanand lake sediments,and in sedimen- ture thermal demagnetizationof remanence. In the latter tary, igneous,and metamorphic rocks. case,remanence acquired at low temperatureis lost upon Bothhysteresis and remanence are strongly dependent on warming, becauseof the thermal unblockingof magnetic grain size. Magnetichysteresis results when a magnetic grains,which is dependenton grain size. mineralis cycledbetween large positive and negative mag- Magnetitehas been one of the mostextensively studied neticfield valuesat roomtemperature. Remanence proper- magneticminerals, and the important grain-size-dependent tiesare measured in a field-freespace after a magneticfield hysteresisparameters for this mineral are presentedhere hasbeen applied to a sample. [e.g.,40]. Referencesfor grain-sizedependent properties in A secondcategory of magneticgranulometry is basedon otherminerals systems are listedin Table2. The grain-size measuringmagnetic parameters as a functionof tempera- dependenceof coerciveforce (H•), remanencecoercivity (H,•), andreduced saturation remanence (J•/J•) for magne- titefrom various studies are plotted in Figures4-6. Thedata, TiO2 (i.e., Fe•*) compiledfrom the literature,are from magnetitesthat have ruffle, brookire beensynthesized by severaldifferent methods: (1) grown crystals(GR) producedeither by hydrothermalrecrystalli- zationat high temperatures,or by aqueousprecipitation at low temperatures[e.g., 3, 40, 55, 75]; (2) crushedgrains (CR) producedby crushingand sieving large crystals [e.g., « FeT 25, 28, 50]; and(3) glass-ceramicsamples (GL) producedby quenchingiron-rich glasses from hightemperature and then «FeTiO3/ • annealingat temperaturesbelow 1000øC[ 128]. As shown in Figures4-6, variationsin a particularmagnetic param- eter,such as H G,for thesame nominal grain size are sensitive «=tio /6.0 o.: 0.2-,,, ulv6spinel/•'",,• "• - • •-• x • •pseudobrookite to the methodof samplepreparation [e.g., 40], and hence showthe importanceof microstructureand crystaldefects onmagnetic behavior. The uniquestress-strain histories that x differentsamples have experienced resultin different crystal defectpopulations. For example,hydrothermally recrystal- lized samplesare thought to havelow residualstrains and a (i.e., Fe2*) magnetite (i.e., Fe3*) low defectdensity [55]. In contrast,crushed grains that have hematite, been milled have probably undergoneextensive brittle/ maghemite plasticdeformation at low temperatures,resulting both in a Fig.3. Ternarydiagram of theiron-titanium oxides and their rapidincrease in thenumber of defectsand in a highdefect solid-solutionseries; x is the compositionparameter (Ti density.The grain-sizedependence and magnitude of coer- content)in thetitanomagnetite series, and z is theoxidation civity,remanence, and susceptibility are important, and are parameterfor titanomaghemites.Figure redrawn from [41]. usedextensively by environmentalmagnetists [e.g., 115]. 194 MAGNETIC PROPERTIES

100 . Worm and Mar•t (1987) 0 Hartstra ( 1982 ) C] Day et al. (1977) Day.etal. (1977) _ 10 Hartsira(1982)_a - CR 0.01 ß Dunlop(1986) ß Argyleand Dunlop (19901 ß Dunlop(1986) ß Argyleand Dunlop(19901 ß Levi and Merrtll (1978) ß Arntn et al. (1987) x Schmidbauerand $chemberat1987) ß Levt and Merrtll (19781 .- Heider et al. (1987)

0.001 ' -Heideretal.,1987,...... i ...... i ...... i ..... •+.i i x. - 0.01 0.1 1 10 1 O0 O0 0.01 0.1 I 10 100 1000 Grain diameter ½wm) Grain diameter (wm) Fig. 4. Grain-sizedependence of coercivity(H•) in magne- Fig.6. Grain-sizedependence ofreduced saturation magne- tite.Experimental data from crushed grains (open symbols), tization(J•/J,) in magnetite. Experimental data from crushed growncrystals (closed symbols and cross), and glass ceram- grains(open symbols), grown crystals (closed symbols and ics (hatchedsymbol). Solid lines are power-lawfits for cross),and glass ceramics (hatched symbol). Solid lines are grown(GR), crushed(CR), andglass ceramic (GL) samples. power-lawfits for grown,(GR), crushed(CR), andglass ceramic(GL) samples.

4. INTRINSIC PARAMETERS The compositiondependence both of J, (measuredat room temperature)and of Tc for titanomagnetites,titanomaghe- 4.1. SaturationMagnetization and mites,and titanohematites is shownin Figures7-9; Table3 Curietemperatures (Tc) and saturation magnetization listsT c andJ, datafor otherminerals. The thermaldepen- areintrinsic properties which depend on chemical composi- denceof J, for magnetiteand for hematiteis givenin Table tion and crystalstructure. Saturationmagnetization is a 4 andin Figure 10, respectively. functionof temperatureand disappears atthe Curie tempera- ture. Rapid thermomagneticmeasurement of Tc aids in determiningthe compositionof magneticmineral phases.

400 60- ß Dunlop(1986) 5O[- ß Argyleand Dunlop(1990) ß Levt and Merrtll (1978j 4o 40•- ß Maherß (1988) rn ¸

,. 2o 3O[- 'ß•ß GR o -200 o.o 0.2 o.4 0.6 o.8 1.o Fe• O, x-parameter We2TiO, 0 Hartsira (1982) 10•ß E Day et al. (1977) o Danl•er$ (1978) Fig. 7. Variationof room-temperaturesaturation magneti- zation(J,) and Curie temperature (Tc) withcomposition (x- 0.01 •.1 1 10 100 parameter)in the titanomagnefite(Fe3_xTi•On) solid-solu- Grain diameter (,t•n) tionseries. End members are magnetite (x= 0) andulvOspinel Fig. 5. Grain-sizedependence ofremanence coercivity (x = 1). Curietemperature data denoted by open circles, and in magnetite.Experimental data from crushed grains (open J, databy solidsquares. Solid lines are (1) linearfit tothe J, symbols)and growncrystals (closed symbols and cross). data [2, 59, 85, 100, 124]; (2) bestfit second-orderpolyno- Solidlines are power-law fits for grown(GR) andcrushed mialto T c data [2, 85,94, 100,103,120]. Best-fit equations (CR) samples. are givenin the Figure. HUNT ET AL. 195

betweenthe c-axis and the (0001) plane, and a triaxial 600 constantthat determinesthe in-planeanisotropy perpen- dicular to the c-axis. Theanisotropy constants depend on mineral composition, crystalstructure, temperature, and pressure, but are indepen- dentof grainsize. Room-temperaturevalues of anisotropy • 300 constantsfor titanomagnetites,maghemite, hematite, and pyrrhotiteare listed in Table5. The valuesof theanisotropy constantslisted in Table 5 were determinedfor single •r • • :- x = 0.5 (Ntshuant and Ko•, 19• crystalseither by high-fieldtorque measurements [e.g., 45, 46,113], orby analysisof magnetizationcurves [e.g., 5, 58]. The temperaturedependence both of thebasal plane anisot- ropyconstant for hematite, as well asof K• formagnetite, are z-parameter illustratedin Figures10 and 11, respectively. Fig. 8. V•a•on of Cu•e tem•rat•e (Tc) with deg•e of oxi•on (z-p•ame•0 for •e timnomaghemi• se•es 4.3. Low-TemperatureMagnetic Transitions •e•r•Ti•a•O n,R = 8/[8+z(1+x)], where• deno•sa Certainmagnetic properties may changegreatly as a lance vacancy). Solid lines •e line• •st-•m for each functionof temperaturebelow 300 K. Suchlow-tempera- •fies. turetransitions may be diagnosticof mineralcomposition (seeTable 5). In magnetite,there is a crystallographic Verweyphase transition near 118 K [e.g.,11]. Alsoassoci- atedwith this transition is a magneticisotropic point Long-rangemagnetic ordering below the Curie tempera- thetemperature where K• becomeszero as it changessign, ture is achieved by the mechanism of exchange or andthe characteristic easy directions ofmagnetization change superexchangeinteractions, and is usuallydescribed by an theirorientation (see Figure 11). A remanencegiven either exchangeconstant, A [e.g., 24], which is an important parameterin micromagneticdomain theory. From an analy- sis of a synthesisof publisheddata on inelasticneutron 4O scatteringin magnetite[54], thebest-fit fourth-order poly- 600 nomialfor the temperaturedependence of A(T), usefulfor reproducingthe experimentalresults, is givenby • 400 •..• , :_ _ .• • • T = 680(1-1.3y) • ß A(T) = (-1.344+ 2.339x 10-2 T- 0.706x 10-n T 2 +8.578x10-ST3-3.868x10-XXT)x10-•xjm -x, (4) whereT is the absolutetemperature.

4.2. MagnetocrystallineAnisotropy ...... • I , , , I ' ' * 0 Magnetocrystallineanisotropy and magnetostriction arise 0% 0•z 0.• 0.6 0.s •.0 fromspin-orbit coupling of ionicmagnetic moments, result- FezO• )'-pa•amete• FeTiO• ing in crystallographicallycontrolled easy and hard direc- Fig. 9. V•a•on of r•m-tem•rat•e •t•a•on magne•- tionsof magnetization[e.g., 11,24]. The magnetocrystalline za•on (JO•d C•e tem•ra•e (T• wi• com•si•on •- anisotmpyenergy (E•) for a cubiccrystal is givenby p•eter) • •e •ohema• •e•T•O•) •lid-solu•on •es. •d mem• • hema• • = 0) •d ilmenite • = 1). C•e •m•ra•e • isdenot• by o•n sym•ls, andJ,• by solidsym•. Sold s•ight l•e is a line• fit whereK• andK 2 are empirical anisotropy constants, and a' s to •e Tc • [56,108, 120]. Best-fitequa•on is given in •e arethe direction cosines of magnetizationwith respect to the Fig•e. J, ••om [125,69]. •e complexv••on of J, principalcubic axes. wi• com•si•on is due• a changein magneticorde•ng For a hexagonalcrystal, anisotropy can be expressedin from c•ted •tife•omagne•sm to fe•imagne•sm at termsof a uniaxialconstant that determinesthe anisotropy y • 0.45. 196 MAGNETIC PROPERTIES

TABLE 3. MagneticProperties of SelectedMinerals Mineral Composition MagneticOrder Tc' (øC) j,b (Am2kg-•) Oxides cobaltferrite CoFe204 ferrimagnetic 520 80 copperferrite CuFe204 ferrimagnetic 455 25 hematite ct-Fe203 cantedantiferromagnetic 675 0.4 maghemite ht-Fe203 ferrimagnetic • 70-80 ilmenite FeTiO3 antiferromagnetic -233 magnetite FelOn ferrimagnetic 575-585 90-92 ulv6spinel Fe2TiOn antiferromagnetic - 153 magnesioferrite MgFeaOn ferrimagnetic 440 21 jacobsite MnFeaOn ferrimagnetic -300 77 trevorite NiFe2On ferdmagnetic 585 51 Sulfides troilite FeS antiferromagnetic 305 pyrrhotite FeqS8 ferrimagnetic 320 20 Fe3Sn ferrimagnetic -333 -25 Oxyhydroxides goethite a-FeOOH antiferromagnetic/weakferromagnetic -120 <1 femxyhyte •5-FeOOH ferdmagnetic -180 <10 lepidocrocite ht-FeOOH antiferromagnetic(?) -196 Metals and Alloys cobalt Co ferromagnetic 1131 161 wairauite CoFe ferromagnetic 986 235 iron Fe ferromagnetic 770 218 nickel Ni ferromagnetic 358 55 awaruite Ni•Fe ferromagnetic 620 120 'Tc = Curietemperature (ferromagnetic materials) or N6el temperature(ferdmagnetic and antiferromagnetic materials). bJ, = Saturationmagnetization at roomtemperature. References: [24, 78, 115].

aboveor belowthis transition will be reducedupon passing defined as the strain dependenceof magnetocrystalline throughT v. In hematite,the transitionis calledthe Morin anisotropy.The linearsaturation magnetostriction constant transitionand occursnear 263 K in bulk samples,but is •, is thefractional change in lengthZXg/g of a materialwhen suppressedin fine grainsless than 20 nm becauseof internal it is magnetizedfrom a demagnetizedstate to saturation.It dilatationalstrain [84]. A newly discoveredtransition in canbe positive (elongation) or negative(contraction), and it pyrrhotiteoccurs near 34 K [35, 101], but its microscopic is usuallyanisotropic in singlecrystals. The microscopic causeis unknown. All thesetransition temperatures are originof magnetostrictionis the samespin-orbit coupling known to be sensitiveto impurities,grain size, and non- thatproduces magnetocrystalline anisotropy [e.g., 24]. stoichiometry;in somecases, the transitioncan be totally In cubic crystals,the linear magnetostriction•, is de- suppressed[e.g., 6, 11, 111]. Thus,low-temperature rema- scribedby the two-constantexpression [e.g., 24], nence transitions for mineral identification should be used with caution. Dependingon the type of experiment,a distinctionis made in Table5 betweenisotropic points (Tv), whereK• becomeszero, and remanencetransitions (TR), (6) wherea changein remanenceor susceptibilityoccurs. where•, is the strainmeasured in thedirection defined by 4.4. Magnetostriction directioncosines ]•i, and ai arethe direction cosines of the Magnetostrictionis thechange in crystaldimensions that magnetization.Both ]3• and a• (i = 1, 2, 3) are measured accompaniesthe processof magnetization,and can be relativeto the principalcubic axes. The magnetostriction HUNT ET AL. 197

TABLE 4. TemperatureDependence of SaturationMagnetization in Magnetite K 20 Absolute Saturation 0.4 Temperature Magnetization T (r) J,' (Am2kg-•) 0.3 20.4 98.80 77.1 98.37 o.• •o 284.6 92.14 325.2 90.36 372.7 87.49 415.9 84.82 452.3 82.15 o 498.9 78.40 - 1oo o 1oo 200 3 oo 400 500 600 700 539.5 74.84 Temperature (øC) 586.1 69.99 631.0 64.85 Fig. 10. Variationof saturationmagnetization (J,) and basal 678.4 58.82 planeanisotropy constant (K) withtemperature for a natural 728.4 51.51 crystalof hematitefrom Ascension. Note the effect of the 761.5 45.18 Morin transitionbelow-10øC. Figuremodified after [ 11, 790.3 37.86 45]. 830.1 22.54 98.86Am2kg 4 at absolutezero. Reference: [96]. constantsalong the <100> and<111> crystaldirections are &0oand •,•, respectively.A relatedparameter is the polycrystallinemagnetostriction constant •,,, givenby of a samplein theabsence of an externalmagnetic field, and thusoccurs only in materialswhich exhibit hysteresis.A remanencecan be a volume magnetization(magnetic mo- mentper volume, measured in unitsof Am-•),or a mass magnetization(magnetic moment per mass,measured in Like themagnetocrystalline anisotropy constants, magne- unitsof Am2kg-•). tostrictionconstants vary as a function of composition, Thermal remanentmagnetization (TRM) is the rema- crystal structure,temperature, and pressure. Room-tem- nenceacquired by a samplewhen it coolsto room tempera- peraturevalues of single-crystaland polycrystal magnetos- turestarting at or aboveits Curie/N6elpoint in thepresence trictionconstants for titanomagnetites,maghemite, hema- of an externalmagnetic field (usually50-100 }•T). TRM is tite, andpyrrhotite are listedin Table 5. The temperature often used as a laboratorymodel for the acquisitionof dependenceof themagnetostriction constants for magnetite is shownin Figure 12. -16 ' I ß I ' I '

ß Fletcherand O'Reilly (1974) -12 4.5. PressureDependence ß Syono(1965) Only a weak hydrostaticpressure dependence of mag- ß KakolMagnetite and Honig (1989) netocrystallineanisotropy, magnetostriction, and Curie tem- -8 peraturehas been detected in magnetite.The results of one study[83] werethat K• andK 2 decrease with pressure at the rate of -0.05%/MPa (-5%/kbar), whereas&0o and &n -4 increaseat the rateof +0.15 %/MPa (15 %/kbar). The results of another[ 105] werethat the Curie temperaturefor magne- titeand for varioustitanomagnetites increases with pressure I • I , I • I • I • I , I • at approximately0.02 K/MPa (2 K•bar). 100 200 300 400 500 600 700 800

Temperature (K) 5. REMANENCES Fig. 11. Variationof first-ordermagnetocrystalline anisot- Remanentmagnetization is thepermanent magnetization ropyconstant (K•) of magnetitewith absolutetemperature. 198 MAGNETIC PROPERTIES

TABLE 5. Room-TemperatureValues of Magnetocrystallineand Magnetostriction Constants Mineral K• K2 • •00 •, Tv' TR" References (x104Jm -3) (x104Jm -3) (x104) (x10-•) (x104) (K) (K) Titanomagne tites Fe304 [TM0] -1.23b 0.44 78 -20 39 120-130 118 5, 6, 47, 60, 113 TM04 - 1.94 • -0.18 • 87 -6 50 112 113 TM05 -1.5 0.2 95* 60 TM10 -2.50 0.48 96 4 59.2 92 113 TM18 -1.92 109 47 84.2 113 TM19 -2.04 1.0 <50 60 TM28 -1.92 -0.3 <50 60 TM31 -1.81 104 67 89.2 113 TM36 -1.6 0.3 68* 60 TM40 148.2 146.5 147.5 68 TM40 (PC)a 122.7 80 TM41 -1.4 0.3 125 60 TM52 133 117 TM55 =0 1.4 60 TM56 -0.70 228 113 TM59 211 117 TM60 79.3 137.9 102.7 68 TM60 (PC)a 111.3 80 TM60 243 117 TM65 262 117 TM68 0.18 --300 113 TM70 9.3 65.4 31.7 68

Other Minerals T-Fe304 -0.46 (SCF)d -(5-10) (PC)d None None 12 •-Fe203 7-188x10-5c •-8 263 45, 79 Fe7Ss 11.8f 32.2f <10g 35 14, 31, 110 Notes: An asteriskindicates an extrapolatedvalue. 'Tvis temperaturewhere K• = 0, TRis temperaturewhere a changein remanenceor susceptibilityoccurs. hAveragevalue from listed references. •Datafrom [60], whichhave the followingerror limits: K• +5%, K2 +20%. dpc= polycrystallinesample, SCF = singlecrystal thin film. tin-planeanisotropy constant. fK3and K4 anisotropy constants. gBasedon domain observations[110].

remanenceby magneticminerals in newly-formedigneous (peakfield about 100 mT) with a superimposedsteady field rockswhich cool throughtheir Curie temperaturesin the (usually50-100 pT). ARM hasbeen used as an analogfor Earth's field. Experimentalstudies have been made on TRM TRM, but avoidsthe possibilityof alteringthe magn'ctic asa functionof grainsize for magnetite(see Figure 13), and mineralsat hightemperature. It is alsocommonly used in asa functionof compositionin titanomagnetites[e.g., 26, environmentalmagnetism for magneticgranulometry [e.g., 87, 93,100, 119],in titanomaghemites[e.g., 86, 89], andin 115]. Theresults of severalstudies of thegrain-size depen- titanohematites[e.g., 69, 120]. ReverseTRM has been denceof ARM in magnetiteare plottedin Figure 15. foundfor certain compositions of titanohematite(see Figure Isothermalremanent magnetization (IRM) is a laboratory 14). remanenceacquired by a sampleat after exposure to a steady Anhystereticremanent magnetization (ARM) is a labora- externalmagnetic field at a given temperature. If the tory remanenceacquired by a sampleat room temperature externalfield is strongenough to saturatethe magnetic duringtreatment in an decaying,alternating magnetic field mineralsin thesample (typically 1 T), thenthe remanence is HUNT ET AL. 199

I ' I ' I ' i ' i [ , i ' • ' 0.2 ©,e•• Magnetite _ • ,• ß •ß ß B•ckfordet al. (1955) • 0.0 ß,,•/ •'111 • ß Klapela• Shtve(1974) • '• a Klerket al. (19•) •• o Moskowitz•1993)

-0.4

ß•. -o .6

_ • . ß• -o.s'

ß 100 200 360 400 500 660 700 800 900 O.12 , 0•.4 O. 16 , 0.8 I 1.0 Temperature (K) Fe.,O 3 y-parameter FeTiO•

Fig. 12. Variationof magnetostrictionconstants (•, Fig. 14. Variationof weak-fieldTRM intensitywith com- and •,) for magnetitewith absolutetemperature. Single position(y-parameter) fortitanohematites (Fez_yTiyOz). Note crystaldata from [13,67, 68]; polycrystallinedata from [80]. thereverse TRM acquiredwhen titanium content is in the •, calculatedfrom single crystal data or measured directly by range0.55 to 0.75. TRM datahave been normalized to an [80]. inductionfield of to 0.1 mT (80 Am-•, or 10e). called a saturation isothermal remanent magnetization field at the time of formation or alteration. NRM is the most (SIRM). SIRM isessentially the same asJ, froma hysteresis variableof magneticparameters because it dependsnot only loop. Variationsin IRM and SIRM are relatedto the on mineralogyand grainsize,but also on the mode of coercivityspectrum of a sample,and can thusbe usedfor remanenceacquisition, and on thermaland magnetichis- magfieticmineralogy determinations inenvironmental mag- tory. Nevertheless,NRM is a criticalparameter in crustal netismstudies [e.g., 65]. magnetizationstudies that try to model the sources of marine Naturalremanent magnetization (NRM) is theremanence andcontinental magnetic anomalies. A summaryof several in a rockbefore any demagnetization treatment in thelabo- modelsof oceaniccrust magnetization is shownin Figure16 ratory.Itis usuallyacquired parallel to the Earth's magnetic [116].

20 10- ß ß Magnetite

ß Dunlop(1972) ß Maher (1988) ßAA • ß ß0 •)z•mir Hartstra andBaneryee(1982b) (1982) o Dankers (1978)

0.5 03

0.2

o. 0.05 0.1

0.02]- 0.01 0.1 I 10 100 0.01 • 0.01 0.1 1 1 100 Grain diameter (gin) Grain diameter (#m) Fig. 15. Grain-sizedependence of weak-field anhysteretic Fig. 13. Grain-sizedependence of weak-fieldthermal rema- remanentmagnetization (ARM) intensityfor magnetite. nentmagnetization (TRM) intensityfor magnetite. Data Experimentaldata from crushed grains (open symbols) and fromcrushed grains (> 1 pm), growncrystals (< 1 pm), and growncrystals (closed symbols). ARM datahave been magnetiteof otherorigins compiled by [41]. Seeoriginal normalizedto an inductionfield of 0.1 mT (80 Am-•, or paperfor references.Figure modified after [41]. 10e). 200 MAGNETIC: PROPERTIES

CXNOE& STERNET KENTET •J_ WDO(1977) DUNLOP& LUYENOYK 8WIFTANO HAYUNG& AJ:IKA/4- T--• KENT 0e?e) I:•OT & DAY(1882) JOHNeON HAI:I:I• HAMEO OOP 0ee2) 0eM) (tees) (tell)

2A

2B

3A

38 ß /

I

cnAlal NRM(An1•) 7500 13015 6063 6500 48OO • 5660 • ?T•O 875O

AVERAGECRUSTAL TOTAL NATURAL REMANENT MAGNETIZATION - 7060 A Fig.16. Models of natural remanent magnetization (NRM) in the oceanic crust. Seismic layers are identified atfight. NRM valueswithin layers are in Am-•. Valuesat the bottom of columnsare the crustal total NRM in amperes,neglecting values from below the Moho. See original paper for references. Figure from [ 116].

TABLE 6. KoenigsbergerRatios for SelectedRocks TABLE 7. Reference Guide for Other Remanences Rocks Koenigsberger References Remanence References Ratio,Q, Chemical(CRM) 17, 48, 64, 91, 92, 97, 112 SedimentaryRocks Depositional(DRM) 4,7,8,71 marinesediments 5 18 Viscous(VRM) 10, 39, 43, 49 red sediments 1.6-6 18 siltstone 0.02-2 18 silty shale 5 18 avg sedimentaryrocks 0.02-10 18, 107 Therelative importance of NRM comparedwith induced Igneous Rocks magnetizationis characterizedby theKoenigsberger ratio granite 0.1-28 18, 107 Q•, a dimensionlessquantity given by granodiorite 0.1-0.2 18, 107 dolerite 2-3.5 18, 107 Q, = NRM / kHc, (8) diabase 0.2-4 18, 107 gabbro 1-9.5 18, 107 whereNRM isthe magnitude of thenatural remanent mag- oceanicgabbro 0.1-58.4 66 netization(per unit volume), k isthe volume susceptibility, intrusions 0.1-20 18, 107 andHc is themagnitude of theEarth's magnetic field at the volcanics 30-50 18 siteunder consideration (H,= 24-48Am-•, B, =/h•, = 30- subaerialbasalt 1-116 18, 98 601aT).Values of Qafor several rock types are collected in oceanicbasalt 1-160 18, 107 Table 6. seamounts 8-57 107 Othertypes of remanence--includingchemical (CRM), avg igneousrocks 1-40 18 depositional(DRM), viscous(VRM)--have alsobeen the subjectof extensivestudy, but are not included here. The MetamorphicRocks interestedreader can refer to thepapers cited in Table7 for granulites 0.003-50 63, 116, 122 further discussion. Others Acknowledgements.Thanks are due to PaulKelso for helpful magnetiteore 1-94 18, 107 input. This is contributionnumber 9301 of the Institutefor Rock manganese ore 1-5 107 (IRM). The IRM is fundedby the W. M. Keck lunar rocks 0.001-1 18 Foundation,the National Science Foundation, and the University of Minnesota. HUNT ET AL. 201

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