l02 magnetismofsolids magnetic mineralogy ssrm mineralogy_ssrm.pdfmagnetic mineralogy magnetite and...

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Magnetism of SolidsE h F d M ti

Today’s Schedule

Exchange Forces and Magnetic Ordering in Minerals

AntiferromagnetismFerrimagnetism

Magnetic Minerals

y, iRocks.com

Magnetic Characterization of Magnetic Minerals (Part I)

Magnetite octahedra from Cerro Huanaquino, Bolivia.

Photo by

Rob

Lavinsky

Paramagnetism: Molecular Field Theory and Exchange Interactions

Curie Law of Paramagnetism

CT

Curie‐Weiss Law of C Paramagnetism: T

Produced by interactions between electron ‘neighbors’: Hm=M (=const.)Total field acting on an internal “molecular” moment: H=Ha+M

Assume a Molecular field acts on the magnetic ions (Hmis very large)

CApplied field + Molecular Field

,

( )

,

a a m

a a

CM H H H H HT

CH H MTC C C

T C T

FerromagnetismTheory of Ferromagnetism must explain

Magnetite

Shape of M‐H curve Shape of M‐T curve1.0

a. SDMagnetite50

Tauxe, 2009

‐1.0

‐0.5

0.0

0.5

‐0.5 ‐0.25 0 0.25 0.5

M/M

s

Field (T)

50 nm

Large magnetic moments in low magnetic fields (< 1 T) and high temperatures (300K)Spontaneous (saturation) Magnetization

(M0,B=0)

Fe3O4 480 kA/mFe 1700 kA/mParamagnet <1 A/m

Magnetic Ordering Temperature(Tc, Curie Temperature)

Fe3O4 580CFe 780CNi 358CCo 1121CT>Tc Curie‐Weiss law

Molecular Field Theory and Exchange Interactions

The interaction constant from Curie‐Weiss Law can be:

=0 paramagnetism unpaired spins independent (Curie‐Law)>0 ferromagnetism unpaired spins align parallel<0 antiferromagnetism unpaired spins align antiparallel

FerromagnetismCurie Temperature

Ferromagnetism T<Tc Paramagnetism T>Tc

Lowrie, 2007

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Weiss Molecular Field Theory of Ferromagnetism

Start with Langevin Equation for M(T,H)s

BM M LkT

Assume a molecular field dependent on Mw=molecular field constant mH wM

Total field H=Ha+HmMagnetization exists even when Ha=0 and T<Tc

w M0

0 1, ( 0)

s

s

wM LkT

wM M H MkT

MM

All spins are aligned

Weiss Molecular Field Theory of FerromagnetismTo solve for M when T<TcTwo simultaneous equations for M

0

(1) ( )

(2)

sM M L

kTMw

TT

T1T2

T3M/Ms

0.4

0.6

0.8

1.0

L(

)

T1T2

T3T4

Em>>kT Em<<kTEq (2)

Eq (1)

T4

Temperature0.0

0.2

0 2 4 6 8 10

T>T4, no solution, M=0, Ha=0, T4=Curie temperatureT=0 K, solution at =, L()=1, M=Ms

m


T>TC

Ferromagnetism

CT

Antiferromagnetism

O’Reilly, 1984


Curie TemperatureSlope of line at T=T4=slope of Langevin Function at origin

kT kTM

0 0

0

0

,

lim ( ) / 3, / 3

3

c c

s

sc

kT kTMMw w

ML M

M wTk

M it d f M l l Fi ld t 0 K3 ckTH wM w kT Magnitude of Molecular Field at 0 K

0

,m s cH wM w kT

Fe: Tc=1063 K, =2.2B: H~108 A/m, B~ 1000 T

Field is much larger than that produced by magnetic dipole‐dipole interaction between spins (a=lattice spacing, ~0.3 nm)

5

310 A/m (0.1 T)B

a

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FerromagnetismTheory of Ferromagnetism must explain

Large magnetic moments in low magnetic

Shape of M‐H curve1.0

a SDMagnetite Large magnetic moments in low magnetic fields (< 1 T) and high temperatures (300K)Spontaneous (saturation) Magnetization

(M0,B=0)

Fe3O4 480 kA/mFe 1700 kA/mParamagnet <1 A/m‐1.0

‐0.5

0.0

0.5

‐0.5 ‐0.25 0 0.25 0.5

M/M

s

Fi ld (T)

a. SDMagnetite50 nm

Field (T)

Weiss Domains: How to explain the demagnetized stateMagnetic Domains and Walls

Fe3O4

13x13 m (110) Fe3O4

Fe3O4

(100) Titanomagnetite

http://www.ifwdresden.de/institutes/imw/sections/24/members/schaefer/magnetic‐domains/5

(100)‐oriented silicon iron crystal

Some ways to image domainsBitter patterns

Magnetic Force MicroscopyMagnetooptic Kerr effect

Quantum Theory of Magnetically Ordered Materials

The source of the molecular field is exchange interactions between

1. Direct Exchange between neighboring electron spins

overlapping electron orbitals resulting in:

2. Superexchange between unpaired electrons spins couple through covalent interactions with intervening ligand (e.g. O2‐)

Tauxe [2008]

Exchange Forces

12 1 22

exe J s s Exchange energy: energy related to the exchange of

two electron spins (s) between atoms

J12 is the exchange integral (constant)f d>0 ferromagnetic ordering

<0 antiferromagnetic ordering

Depends on degree of orbital overlap between electrons

ra=interactomic distancer3d=mean radius of 3d orbital

ra/r3dSmall AF orderingLarger values FM ordering

ll l l d /Tc=631 K

Tc=1403KTc=1043K

Bethe‐Slater curve

Still larger values dia/paramagnetism

Cullity, 1972

Tc=96KCurie Temperature is related to strength of exchange integral

12 B cJ k T

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Magnetically Ordered Materials

When the applied field is zero, the internal field is still present and leads to Positivemagnetic ordering and spontaneous magnetization.

Positive interactions

Negative Interactions

Ferromagnetism Fe, Ni, Co, NiFe, Gd

Antiferromagnetism MnO, FeTiO3

Ferrimagnetism Fe3O4, MOFe2O3, where M=transition metal

Main Types of Magnetic Ordering

Egli

Antiferromagnetism

Two separate magnetic sublattices (A and B) with negative (antiparallel) magnetic superexchange coupling

A

p g

MB=‐MA at all temperaturesMs=MB‐MA =0

CT

B

MB

Néel Temperature (Tn)

M=0

MA

Molecular Field Theory of Antiferromagnetism (Néel Theory)

Molecular Field “seen” by A and B ions on A and B sublattices

molecular field constant (>0)

( / 2) AB

CTC

T>TN

00( ) B A

A A JA Ag JM T M B HkT

Brillouin Functions

A AB B

B AB A

H MH M

AB = molecular field constant (>0)

Applied Field to spin axis

1

AB

T<TN

T/TN

Cullity, 1972; Dunlop and Özdemir, 1997

0

00

( )

( )

( ) ( ) ( ) 0

A A JA A

B BB B JB B

S B A

kTg JM T M B HkT

M T M T M T

Applied Field // to spin axis

MAMB

MAMB

// ( 0) 0T

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1 00

1.20

1.40

Marcasite (FeS2)

axAntiferromagnetism

Tn

T>TN

Goethite (‐FeOOH)Özdemir and Dunlop (1996)

0.20

0.40

0.60

0.80

1.00

0 50 100 150 200 250 300

X/X

ma

Temperature (K)

TN=393 K

CT

1 2||3 3p

Temperature (K)

Mineral TN (K) Ms (Am2/kg)

Type

Ilmenite (FeTiO3) 40 0 AFM

Ulvospinel (Fe TiO 120 0 AFM

Antiferromagnetic Minerals

Ulvospinel (Fe2TiO4 120 0 AFM

Hematite (‐Fe2O3) 948 0.4 canted

Goethite (‐FeOOH) 393 ~0.5 defect

Lepidocrocite (‐FeOOH) 52 ~0.1 defect

Siderite (FeCO3) 37 0.38 canted

Rhodocrosite (MnCO3) 34 0.46 canted

Vivianite (Fe3[PO4]28H2O ~12 0.06 (?) defect?( 3[ 4]2 2 ( )

Ferrihydrite (Fe5HO84H2O) ~500 6‐12 non‐compensated

Data from various sources

FerrimagnetismTwo separate magnetic sublattices (A and B) with negative (antiparallel) magnetic superexchange coupling

Different numbers or kinds of cations or valence states on h bl tti diff t it di tieach sublattice + different site coordination

MAMB at all temperaturesMs=MB ‐MA > 0

T>TC FerrimagnetismT<TcC

T

Ferromagnetism

O’Reilly, 1984

Similar magnetic properties to ferromagnetic materials (Ms, hysteresis, remanence) C

T

Ferrimagnetism

( ) ( )~ 0 3 0 5s cM T T T

0.0

0.5

1.0

/Ms

a. SDMagnetite50 nm

Magnetite

0.3 0.5

‐1.0

‐0.5

‐0.5 ‐0.25 0 0.25 0.5M

Field (T)

Tauxe, 2008

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Molecular Field Theory of Ferrimagnetism

Cullity, 1972

Molecular fields acting between ions (AA, BB, and AB)

Molecular Field Theory of Ferrimagnetism (T<TN)

H M M

00( ) B A

A A JA Ag JM T M B HkTg J

Molecular Field “seen” by A and B ions on A and B sublattices

AA , BB , and AB : molecular field constants for interactions

Brillouin Functions JA, JB= quantum spin numbers

A AA A AB B

B BB B AB A

H M MH M M

00( )

( ) ( ) ( )

B BB B JB B

S B A

g JM T M B HkT

M T M T M T

Q‐TYPEAA=BB

P‐TYPEAA > BB

N‐TYPEAA < BB

Different combinations of AB, AA, BB interactions give rise different types of Ms‐T curves

Temperature Dependence of Magnetization

Q

QP

Néel Model

Q

N

Q

Harrison

, 2000

Nickel‐Iron Vanadates (NiFe2‐xVxO4 )Variation as a function of composition (O’Handley, 2000)

Titanomaghemite (Fe2.3Ti0.7O4)Variation as a function of oxidation (Readman and O’Reilly, 1972)

P

Mineral TN (K) Ms (Am2/kg)at 300 K

Magnetite (Fe3O4) 853 92

M h it ( F O ) 863 948 73

Ferrimagnetic Minerals

Maghemite (‐Fe2O3) 863‐948 73

Greigite (Fe3S4) Unknown, >593 59

Pyrrhotite (Fe7S8) 593 20

Jacobsite (MnFe2O4) 673 77

Trevorite (NiFe2O4) 713 51

Daubreelite (FeCr2S4) ~170 ~30 (at 70 K)

Fe O ~510 ~15‐Fe2O3 510 15

Feroxyhyte (‐FeOOH) 440‐460 ~12


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SummaryMagnetic Ordering in Solids

– Diamagnetism: No unpaired e‐

– Paramagnetism: Unpaired e‐, disordered and fluctuating– Ferromagnetism: All unpaired e‐ spins aligned parallel– Antiferromagnetism: Unpaired e‐ aligned antiparallel– Antiferromagnetism: Unpaired e aligned antiparallel– Ferrimagnetism: Unpaired e‐ aligned antiparallel but don’t fully cancel out

Magnetic Mineralogy

Magnetite and Titanomagnetites (Fe3‐xTixO4)Hematite and Titanohematites (Fe2‐yTiyO3)Maghemite and TitanomaghemitesMaghemite and TitanomaghemitesChemical Change

ExsolutionHigh temperature oxidation , T>600 C (oxy‐exsolution)Low temperature oxidation (titanomaghemites)

Magnetic Oxyhydroxides, Sulfides, and Fe‐Ni

Exsolved titanomagnetite grain (width of image =320m)

Magnetite (Sicily Strait, Dinare‐Turell et al., 2003)

Magnetic Mineralogy

Sources of magnetic minerals Igneous and metamorphic processesSoil formation and diagenesis

Magnetic minerals follow the rock cycle

gCosmic dustBiomineralizationIndustrial pollutionarcheological materials

Transformation of magnetic phaseschemical weatheringlow/high temperature oxidation

/dissolution/precipitationbiogenic formation/alteration

Magnetic Minerals can undergo significant chemical/physical changes after formationPhysical/Chemical changes can destroy primary remanence or produce new remanence

Ternary diagram for iron‐oxides

Most Important Magnetic Phases

Fe‐Ti oxidesFe‐sulfidesFe‐oxyhydroxides

Tauxe, 2008

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Titanomagnetites

Solid Solution Series (T>600 C)Cubic mineralsSpinel crystal structure

3 4

3 4 2 4(1 )

x xFe Ti O

x Fe O xFe TiO

Spinel crystal structure

Tauxe, 2008

TM0= magnetiteTM60= Fe2.4Ti0.6O4

Spinel Crystal Structure

S i l i th M Al bSpinel is the Mg-Al member of the larger spinel group of minerals.

MgAl2O4

A‐sites: tetrahedral coordination

http://en.wikipedia.org/wiki/File:Spinel.GIF

A sites: tetrahedral coordinationB‐sites: octahedral coordination

Superexchange Interactions in Spinels

Superexchange between unpaired electrons spins couple through covalent interactions with intervening ligand (e.g. O2‐)

Dunlop & Özdemir, 1997; Tauxe [2008]

Bond angles in Magnetite

Angles near 90 are unfavorable for superexchange interactions

AB, BB, and AA interactions

Crystal Structure of Magnetite

Magnetite octohedra from Cerro Huanaquino Bolivia

Unit cell contains 32 O2‐ anions arranged in a face‐center cubic network

Cerro Huanaquino, Bolivia.

Two types of cation sitesA‐sites: tetrahedral coordination (8 sites per unit cell)B‐sites: octahedral coordination (16 sites per unit cell)

Butler, 1992; Walz, 2002

2 4( )[ ]A B OGeneral Structural Formula:

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Magnetite

Fe3O4(Fe3+,Fe3+,Fe2+)O 4

2‐

In magnetite, there are 16 Fe3+ and eight Fe2+ cations per unit cell

Fe2+ : S=2, = 4BFe3+ : S=5/2, = 5BWhat’s the magnetic structure of Magnetite?

Ferromagnetism (all spins are aligned)Ferrimagnetism (A and B sites are AF coupled)

Coupling Spin Moment

Ferromagnetic All spins aligned 4+5+5 =14B

Ferrimagnetic (5+5)‐4=6B

Ferrimagnetic (5+4)‐5=4B

2 2

4( ) [ , ]

A BFe O 3+ 3+Fe Fe

2 2

4( ) [ , ]

A BFe O 3+ 3+Fe Fe Observed value

~4.1B

Temperature dependence of the sublattice magnetizations

Magnetite

Stacey and Banerjee 1974

Curie Temperature: 853 K (580 C)Saturation Magnetization at 23 C

92 Am2/kg480 kA/m

Stacey and Banerjee , 1974

Tauxe, 2008

0.70

0.80

dM/dT

Verwey Transition in MagnetiteChange in lattice symmetry at Tv=121 K Monoclinic Magnetite

1990

0.20

0.30

0.40

0.50

0.60

SIRM

(AM

2 /kg)

T>Tv CubicT<Tv

Monoclinic

3 mm Single Crystal

dM/dT

Relationship between the low‐T monoclinic axes (a, b and c), the h b h d ll di t t d ll ( lid

Kakol,

0.00

0.10

0 50 100 150 200 250 300

Temperature

Saturation IRM (SRM) given at 10 KMeasurement on warming to 300 K

rhombohedrally distorted cell (solid line), and the high‐T cubic unit cell (dashed line)

c‐axis is tilted ~0.2° from cubic <100>

Maghemite (‐Fe2O3 )

Isochemical with hematite (‐Fe2O3 ) but maintains the inverse spinel structure of magnetite.

C ll F 2+F 3+ d i i h b l

Partially oxidized magnetitesz=0 magnetite; z=1 maghemite

op and

Özdem

ir, 1997

Convert all Fe2+Fe3+ and maintain charge balance

3 2 3 2 3 3 2

4 5/3 1/3 4( )[ , ] ( )[ ]Fe Fe Fe O Fe Fe O

3 2 31 1 2 /3 /3 4[Fe ] [Fe Fe ] OA z z z B

Dunlo

Ms (300K)= 74.3 Am2/kgT ~ 863 948 K (590 675 ° C)

0.8

1.0

0)

maghemite

TC 863‐948 K (590‐675 C)No Verwey like transition

0.0

0.2

0.4

0.6

0 50 100 150 200 250 300

Sirm

/Sir

m(2

0

Temperature (K)

magnetite

Magnetite is oxidized to maghemite by changing the valence state of two thirds of the original Fe2+ to Fe3+ while simultaneously removing one third of the original Fe2+ from the B sublattice.

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‐Fe2O3 is metastable and typically inverts to ‐Fe2O3 (hematite) when heated to T> 300° C

Maghemite (‐Fe2O3 )

Synthetic maghemite

mir and Du

nlop

, 1988

Inversion to ‐Fe2O3

Synthetic maghemiteD~0.025 m

Özdem

ir, 1990

Synthetic maghemiteD~0.47 m

Axial ratio 9:1

Tc=645°C

Özdem

Differential thermal analysis (DTA) heated in 02.

Inversion T>Tc

Distribution of cation between A and B sites and the exchange coupling between/within sublattices control intrinsic magnetic properties ( e.g., Msand Tc) of titanomagnetites

Cation Distribution and Magnetization in Titanomagnetites

Normal spinel: similar cations/ occupy the same sublatticeInverse spinel: different cations occupy the same sublattice

3 2 3 2 4 21 (2 2 ) 4( ) [ ]b b x b b x xFe Fe Fe Fe Ti O A B

Generalized Cation Distribution Formula for Titanomagnetites

b= distribution parameter (no. of A‐site Fe3+ )x= compositional parameter

Generalized Cation Distribution Formula for Titanomagnetites

Cation Distribution and Magnetization in Titanomagnetites

Single‐crystal data3 2 3 2 4 2

1 (2 2 ) 4( ) [ ]b b x b b x xFe Fe Fe Fe Ti O A B

b= distribution parameter (no. of A‐site Fe3+ )x= compositional parameter

2 2

4( ) [ , ]

A BFe O 3+ 3+Fe Fe

For example (x=0, Magnetite)

b=0: normal spinel

Kakol et a

l., 1991

2 2

4( ) [ , ]

A BFe O 3+ 3+Fe Feb=1: inverse spinel

Variation in saturation magnetization (Bohr magnetons pfu at 0K)

6(1 ) 2x b

Variation of saturation magnetization with composition x for various models of cation distributions.

Crystal Chemistry in Spinels: Cation Site‐Preference for transition metal ions in A and B Sites

8Certain cations have a preference for

Navrotsky and

Klepp

a, 1968

B‐sites

3 3 2 24

2 2 4 24

( ) [ ]

( ) [ ]

Fe Fe Fe O

Fe Fe Ti O

A B

A B

Certain cations have a preference for occupying a particular coordination site

A‐sites

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Magnetic Properties of Titanomagnetites

In the titanomagnetite series (Fe3–xTixO4)

Ti4+ substitutes for Fe3+ as Ti content increases. The ionic substitution is 2Fe3+ Fe2+ + Ti4+

30

40

50

60

70

80

90

100

Ms(Am

2 /kg)

Saturation Magnetization at 300 K

Lattard et al., 2006Natural titanomagnetites can also contain Al3+,Mg2+, Mn2+

0

10

20

30

0 0.2 0.4 0.6 0.8 1x‐parameter


Different combinations of AB, AA, BB interactions give rise different types of Ms‐T curves

Néel Classification of Ms‐T curves

Magnetic Properties of Titanomagnetites

Temperature dependences of the sublattice magnetizations are not always the same, but all go to 0 as TTc

1.02

1.04

1.06

nt

TM60

P‐type behavior in TM60

0.90

0.92

0.94

0.96

0.98

1.00

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Normalized

Mom

en

T/Tc

Dunlop and Özdemir, 1997

Hematite: Fe2O3

Canted AntiferromagnetismSEM image of hematite. Image is ~800 m across.

Wikipedia Com

mon

sFe3+

Weak ferromagnetism

T > 263 K T < 263 K

Dunlop and Özdemir, 1997; Butler, 1992

pure cantedAF AF

Morin Transition

Curie Temperature: 948 K (675 C)Saturation Magnetization at 300 K

0.5 Am2/kg2 kA/m

T > 263 K T < 263 K

Morin TransitionSpin‐flop transition

Hematite: Fe2O3

Özdemir et al, 2008

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Titanohematites: Fe2‐yTiyO3TN= 953 K TN= 63 K

Phase diagram for titanohematites

Dunlop and Özdemir, 1997; Lagroix et al., 2004

Magnetic Structure

Both end‐members have rhombohedral crystal structures and are antiferromagnetic below TNOrdered Phase: Fe2+ uniquely distributed within the A layer and Ti4+ within the B layer

mir, 2007

Titanohematites: Fe2‐yTiyO3

y=0.7

Tc=418 K 04

Dunlop

and

Özdem

Canted‐AF

temperature

d2M

/dT2

Lagroix et al., 200

Compositions around y=0.5 have the property of self‐reversed thermoremanent magnetization

Applied a field in the positive direction and cool through Curie temperature an negative remanence is produced!

Primary Fe‐Ti oxidesOriginally crystallized from igneous melts

~1‐5% by volumecrystallize T~1300C > > Tc

Cooling rate has major impact on grain size and microstructureextrusive volcanic rocks: TM grain sizes < 50 m many <1mextrusive volcanic rocks: TM grain sizes < 50 m, many <1mintrusive igneous rocks : TM grain sizes > 100m

Composition of the melt

Mafic magmas (enriched in Fe, low Si)Basalts and Gabbros TM’s 0<x~0.8 ,Ferrimagnetic, Tc >0CTH’s 0.8<y<0.95, Paramagnetic, T> 0C

Felsic magmas (higher in SiO2)rhyolites, granitesTM’s x~0TH’s 0 <y<0.8

Tauxe, 2008

Magnetic Inclusions in Paramagnetic Silicate Minerals

Ti‐poor TM (< 1m) can be exsolved from silicate minerals

Dunlop and Özdemir, 1997; 2007

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Chemical Change

O’Reilly, 1984

Chemical Change and Exsolution (T>500C)Quench by rapid cooling (volcanic rocks)

single phase composition

Exsolution by slow cooling (t>106 yr) Igneous intrusionsmultiphase compositionsmultiphase compositions

Hemoilmenite(two‐phase composition

Important ChangesMagnetic properties due to change in compositionDecrease in effective magnetic grain size

Tauxe, 2008

Ilmenite‐Hematite Exsolution in granulites from S. Sweden

McEnroe et al., Elements, 2009

Lamellar MagnetismExsolution in Titanohematites

Lamellar magnetism is associated with boundary

Monte Carlo simulation of cation ordering and resulting magnetic signal of hematite‐rich and ilmenite‐rich phases in exsolved titanohematite

Lamellar magnetism in the hematite‐ilmenite series as an explanation for strong remanent magnetization (up to 55 kA/m)

layers between the exsolved phases.

Robinson et al. (2002)

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Slow Cooling and Oxy‐Exsolution

Solid solution between magnetite

Oxidation that occurs during original cooling of an igneous rock is deuteric oxidation

Solid solution between magnetite and ulvospinel exists in principle, intergrowths of these two minerals are rare

Natural titanomagnetites interact with oxygen in the melt to form intergrowths of low Ti magnetite with Ilmenite (deuteric oxidation)

Ti‐rich ilmenite

Fe‐rich TM

Tauxe, 2008

Titanomagnetite grain displaying typical high temperature oxidation texture (i.e. produced during cooling).

Titanomagnetite (brown) is sub‐divided b il it ( l b ) S l llby ilmenite (pale brown). Some lamellae and patches of hematite are also present (width of image =320m).

http://geography.lancs.ac.uk/cemp/atlas/atlas_frm1.htm

Optical photomicrograph of ilmenite lamellae within titanomagnetite grain; note the symmetrywithin titanomagnetite grain; note the symmetry of the ilmenite planes that are parallel to (111) planes of the host titanomagnetite.

Butler, 1992

Low‐Temperature (T<300C)Titanomaghemites

Weathering at ambient surface conditionsHydrothermal alteration Ocean floor basalts

MaghemitizationNo change in crystal structure (spinel)Convert all Fe2+Fe3+ & maintain charge balance

Ph ll d cation deficient spinels (non stoichiometric TM)

titanomaghemites

Phases are called cation‐deficient spinels (non‐stoichiometric TM)

3 2 3 2 4 21 (2 2 ) 2 (3 ) ( ) (9 ) (1 ) 3 4( ) [ ]

0 (1 ) / (9 )b b x x b b x x xFe Fe Fe Fe Ti O

x x

A B

Tauxe, 2008

Oxidation parameter: 0 ≤ z≤ 1, z=9For x=0: z=0 magnetite

z=1maghemite

Consequences of Maghemitization

MSTca

Titanomagnetite ( x = 0.6) is the dominant primary FeTi oxide in oceanic pillow basalts (upper 0.5 km of oceanic crust).

A magnetite crystal (∼ 30 μm) undergoing maghemitization. Because of the change in volume, the crystal begins to crack. [From Gapeyev and Tsel’movich, 1988.]

Tauxe, 2008

During seafloor weathering, titanomagnetites oxidize totitanomaghemite.

Titanomaghemite is one of the most abundant FeTi oxides in the earth’s crust.

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Cell‐parameter

Curie‐Temperature and Cell‐parameter contours for titanomagnetites‐titanomaghemites

Curie‐Temperature

Readman and O’Reilly, 1972

Ocean‐Floor Basalts

Temperature dependence of magnetizationInversion of titanomaghemite

i t th f F i h TM d Ti i h il it

If titanomaghemites are heated, by burial beneath later flows on the seafloor for example, they become unstable and ‘invert’.

intergrowth of Fe‐rich TM and Ti‐rich ilmenite

TMht

TM0

Inversion


Inversion temperature

Other Common Magnetic Phases

TN=393 KGoethite (‐FeOOH)

Common weathering product and precursor to hematite in sediments and soils.

Saturation Magnetization ~0.1 Am2/kg1‐2 kA/m

Özdemir and Dunlop (1996)

Goethite (‐FeOOH)Néel and Curie Temperature

Thermomagnetic heating

TC=120° C

Thermomagnetic heating curve (strong field) Step‐wise thermal demagnetization of

thermoremanent magnetization (TRM)

Özdemir and Dunlop, GRL, 23, 921‐924, 1996

Weak ferromagnetism has a TC=TN

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Iron‐Sulfides: Pyrrhotite

Monoclinic pyrrhotite (Fe7S8): FerrimagneticTc=593 K (320C)Ms=~20 Am2/kg ( 80 kA/m)

○ vacancies

Hexagonal pyrrhotite (Fe10S11 ,Fe9S10)structural transition from an (imperfect) antiferromagnet to ferrimagnet at about 200C.

Fe cations are FM coupled within c planes and AF coupled

Fe2+

Tauxe, 2008; Dunlop and Özdemir, 1997

within c‐planes and AF coupled between layers via S2‐ ions

0.8

1.0

(20)

“Besnus” Transition (T=34 K) in Monoclinic Pyrrhotite

Thermal demagnetization of 20 K SIRM

0.0

0.2

0.4

0.6

0 50 100 150 200 250 300

Sirm

/Sirm

(

Temperature (K)

pyrrhotite

Transition ~34K is diagnostic of the

Room‐Temperature SIRM

Transition 34K is diagnostic of the presence of monoclinic pyrrhotite

Rochette et al, 1990

Physical Origin of transition is likely related to crystallographic transformation

Pierre Rochette, Gérard Fillion, and Mark J. Dekkers, IRM Quarterly, 21:1, Spring 2011

Crystal Structure: Cubic, Inverse spinel Magnetic Structure: Ferrimagnetic

Iron‐Sulfides: Greigite (Fe3S4)Ro

berts e

t al., 2010

Ms=125 kA/m, 59 Am2/kg

Tetrahedral (A) site Octahedral (B) site

Chang et al., 2008

Low‐temperature measurements of Ms

Synthetic greigite

Iron‐Sulfides: Greigite (Fe3S4)Tc unknown but must be T> 603 K (330C)Chemically unstable at high T and decomposes at T< Tc

High‐field M‐T curves (heated in air)

High‐temperature hysteresis data (measured in air)

Robe

rts e

t al., 2010

Chang et al., 2008

Natural Greigite Samples Synthetic Greigite

Formation of high Hc phase

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1.0

Iron‐Sulfides: Greigite (Fe3S4)

No low‐Temperature transition

Coarse grained

0.2

0.4

0.6

0.8

Sirm

/Sir

m(2

0)

greigite

magnetite

t al., 2010

Coarse grained SD/PSD/MD samples

0.00 50 100 150 200 250 300

Temperature (K)

Low‐temperature SIRM

Robe

rts e

t

NanophaseUnblocking behavior

Biogenic greigite produced by bacteria Sedimentary greigite and pyrite

Iron‐Sulfides

Greigite and Pyrrhotite occur in reducing environments and both tend to oxidize to various iron oxides leaving paramagnetic pyrite as the sulfide component.

Bazylinski et al., 1994; Moskowitz et al,2008 Roberts et al., 2010

Iron and Iron–NickelFerromagnetism

Iron and iron–nickel are the principal NRM carriers in lunar rocks and most meteorites.

AFM

FM FM

Phase diagram of Fe–Ni

Dunlop

and

Özdem

ir, 2007

Body‐centered cubic kamacite (‐Fe) does not exist for >30 mol.% Ni. Face centered cubic taenite (‐Fe) is the stable phase at high T and at all T for >30 mol.% Ni. The ordered phase tetrataenite can exist for 50–55 mol.% Ni.

Phase Formula Ms (Am2/kg) Tc (K)

Gibeon IVA fine octahedrite

Phase Formula Ms (Am /kg) Tc (K)

Kamacite Fe (bcc cubic)Low‐T phase

218 1038

Taenite Fe (fcc cubic)High‐T phase

AFM

tetrataenite Fe0.5Ni0.5 ~823

Awarunite Ni2Fe to Ni3Fe 120 893

http://www.meteorlab.com/METEORLAB2001dev/widpatrn.htm

Widmanstätten pattern of a polished and etched slice showing kamacite (light bands) taenite (dark areas)

Magnetic Mineral Identification: Part 1

Different Approaches:

ImagingDiffraction ck

s.com

DiffractionMagnetic Characterization

Looking for changes in magnetic behavior at characteristic temperatures (2‐1000 K)

High‐Temperature (>300 K) methodsMagnetite octohedra from Cerro Huanaquino, Bolivia.

Photo by

Rob

Lavinsky, iRoc

Most magnetic minerals that carry stable remanence (SD/PSD) are small (< 10 μm) and in low concentrations ( < 1%)

The challenge is to associate a particular component of NRM (identified from partial demagnetization) with a particular ferrimagnetic mineral.

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Imaging Magnetic Minerals

Exsolved magnetic & ilmenite in an igneous rock

Detrital titano‐magnetites from Chinese loess.

Hematite rosettes on a smectite grain.

Industrial pollutant (“fly‐ash”)

Interstellar Dust Particle (IDP)

Just looking at your samples in a microscope can help explainJust looking at your samples in a microscope can help explain A LOT about the magnetic behavior of your samples.

The shape, grain‐size, texture, and associated non‐magnetic minerals all give clues to the magnetic mineralogy in a sample and their origins.







Frequently used modes of

Tauxe, 2008

Frequently used modes of imaging:

Reflected light microscopy.

Easy, cheap, fast! Reflected light image of ilmenite hematite intergrowths.http://www.ngu.no/prosjekter/Geode/Tellnes/Tellnes%20ore%20photographs.htm







Tauxe, 2008


Scanning Electron Microscopy (SEM)Electron MicroProbe Analysis (EMPA)

Better spatial resolution (~0.1 µm), observation of 3D morphology of grains, and ability to measure composition (EDS) and crystal structure (via EBSD)

EDS: Energy‐dispersive X‐ray spectroscopy, EBSD: Electron backscatter diffraction







Tauxe, 2008


Transmission Electron Microscopy (TEM)Synchrotron Radiation

Best spatial resolution (0.1‐5 nm), ability to measure composition (EDS or EELS), and crystal structure (SAED).

EELS: electron energy loss spectroscopy, SAED: selected area electron diffraction

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Magnetic Characterization of Minerals

Characteristic Temperature Changes

All FM minerals have a Curie or Néel point 08

Derivative curves

Typical Strong‐field (~1 T) thermomagnetic experiment.

or Néel point

One of the ways we characterize samples is by heating them up and determining these key temperature transitions.

Tauxe 20

0

Measurement of Ms(T) in whole rock samples.

Experiment measures induced magnetization in a very strong field.

Peak shows temperature of maximum curvature, interpreted as the Curie temperature

Vibrating sample magnetometer


lop & Özdem

ir, 2007

Identification of Magnetic Minerals by thermomagnetic analysis is complicated by two main factors

1. Each magnetic mineral has its own unique Curie (or Neel) temperature, but different minerals can

Dun

Normalized Ms(T ) dependences for five common magnetic minerals

have the same Tc or Tn (solid solution series titanomagnetites and titanohematites

2. Chemical alteration of samples can occur during thermomagnetic analysis (heating samples up to 500‐700 °C

Mineral Tc or TnMagnetite (Fe O ) 580° C

TitanomagnetitesMagnetite (Fe3O4) 580 C

TM60 120

Pyrrhotite (Fe7S8) 320

Maghemite (‐Fe2O3) 600?

Greigite (F3S4) 350(?)

Hematite (‐Fe2O3) 675

Goethite (‐FeOOh) 120

Tauxe, 2008

Magnetic Characterization of MineralsExamples of thermomagnetic curves

Standard Experimental practice is to measure thermomagnetic curves during a heating and cooling cycle

Butle

r 199

2 The Curie temperature is the same on both heating and cooling (~575˚C). This is termed reversible behavior.

Heating in air (not typically recommended)Heating in vacuum, or inert gas (N2, Ar) to reduce effects of oxidation

No (likely) chemical alteration occurred during heating and cooling cycle


Examples of thermomagnetic curves

The Curie temperature of ~200˚C is observed with reversible behavior.

Could be either titanomagnetite (x~0.55) or titanohematite (y~0.5)

(Optical microscopy shows that titanohematite is the dominant magnetic phase in the sample.)

Butler 1992

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Non‐reversible behavior

Usually indicates a change in the magnetic mineralogy has occurred during heating (oxidation, reduction, crystallographic change).

This sample contains intergrowths of Fe7S8 and Fe9S10

The change 225‐320˚C is a transition from antiferromagnetism to ferrimagnetism in the Fe9S10

Butler 1992

Irreversible Behavior: Titanomaghemites

TMAGH Fe‐rich TM + Ti‐rich ilmenite


Titanomaghemite (ocean floor basalts from DSDP site 417D (Moskowitz, 1980)

Characteristic irreversible thermomagnetic curve of a partially oxidized TM60 (Özdemir and O’Reilly, 1982)

Irreversible Behavior: ‐FeOOH ‐Fe2O3‐Fe2O3


Özdemir and Dunlop, 1993

“Irreversible” Behavior: Sluggish (reversible) phase transformations producing thermal hysteresis

Fe Fe‐Ni


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Some samples have more than one magnetic phase

Curie temps at 580˚C and 680˚C are due to magnetite and hematite respectively.

This is a nice example where you can see a mineral with a large Ms (magnetite) and a low Ms (hematite).

Sometimes, the stronger Ms mineral

Butler 1992

, gswamps the signal, making it hard to detect the presence of other magnetic phases.

Often, you need to combine information from different kinds of experiments.

Magnetic Characterization of MineralsExamples of thermomagnetic curves

Identification of the ferromagnetic minerals in a pelagic limestone by determination of their Curie temperatures in concentratedextracts Lo

wrie

, 2007

Magnetic Characterization of Minerals ‐ Susceptibility

Tauxe 2008Hopkinson Peak

Tauxe, 2008

Paramagnetism has a 1/T dependence.

So the χ100 should be twice as large as χ200

χ100

χ200

T

Susceptibility experiments measure induced magnetization in a very weak field.

Diamagnetism is independent of T

1.00

1.20

Susceptibility Measurements

0 20

0.40

0.60

0.80

X/X

300

Titanomagnetite (TM28)Fe

2.72Ti

0.28O

4

0.00

0.20

300 400 500 600 700 800 900

Temperature (K)

increases with rising T because wall pinning decreases and resistance to wall motion decreases

2013 SSRM 5/29/2013

22

coolingheating

FG_1951_02_04_BOT_05

/kg]

1.3e0

1.2e0

1.1e0

1.0e0

9.0e-1

8.0e-1

FG_1951_01_01_BOT_01

m3/

kg]

1.20e-5

1.10e-5

1.00e-5

9.00e-6

8.00e-6

B=1500 mTB=0.375 mT (ac)

Problems with Paramagnetism and high‐field measurements

Susceptibility Measurements

Paramagnetic (~1/T)

coolingheating

FG_1951_02_04_BOT_041.1e01.0e0

9.5e-19 0e 1

T [K]900800700600500400300

M [A

m2/

7.0e-1

6.0e-1

5.0e-1

4.0e-1

3.0e-1

2.0e-1

1.0e-1

T [C]700600500400300200100

susc

eptib

ility

[m 7.00e-6

6.00e-6

5.00e-6

4.00e-6

3.00e-6

2.00e-6

1.00e-6

B=500 mT

Paramagnetic ( 1/T) “contamination”

T [K]900800700600500400300

M [A

m2/

kg]

9.0e-18.5e-18.0e-17.5e-17.0e-16.5e-16.0e-15.5e-15.0e-14.5e-14.0e-13.5e-13.0e-12.5e-12.0e-11.5e-11.0e-15.0e-2

B 500 mTLooks more like a Tc

High‐field thermomagnetic (Ms) and NRM unblocking Temperature curves

Young Oceanic CrustEast‐Pacific Rise


NRM unblocking Temperature curves

Hi h Fi ld R lt T ~150 C idi d TM60

Kent and Gee, Science, 1994

High‐Field Results: Tc~150 C, unoxidized TM60NRM unblocking: TB >300 C, oxidized TM60

Why discrepancy between High‐field results and NRM results?

Isothermal Remanent Magnetization (IRM)The magnetization acquired during

exposure to a short‐lived magnetizing field.

Usually at room T and in large fields. -0.1

0.0

0.1

0.2

M

(Am

2 /kg)

05E1 05E305E4

2005

Room temperature

Low‐coercivity minerals

Saturation <300 mTferrimagnetsFe3O4, ‐Fe2O3

High coerci it minerals007

-1000 -500 0 500 1000-0.2

Applied Field (mT)

05E4

Dust on Snow (Red Mountain Pass, San Juans CO)

High‐coercivity minerals

Saturation >1000 mTOpen loop to high fields‘imperfect’ Antiferromagnets‐Fe2O3, ‐FeOOH

Natural goethite

Berquo

et a

l., 2006, 20

2013 SSRM 5/29/2013

23

Imparting IRMs in the Lab

Isothermal Remanent Magnetization

ASC Impulse MagnetizerASC Impulse MagnetizerThe magnetic field is produced by discharge of energy from a capacitor

ElectromagnetsBmax ~2 T

Superconducting MagnetsBmax ~ 5‐20 Tg gy p

bank through a coil surrounding the sample cavity. The capacitor bank is first charged to the desired voltage (corresponding to the desired field). It is then discharged through the coil very quickly (Bmax~ 1‐2 T)

Bmax 2 T Bmax 5 20 T

Highest magnetic field for a continuous field magnet (Guinness World Record) 45 T

Highest field for a resistive magnet 36.2 T

Highest field for a long‐pulse magnet 60 T

Highest field for a non‐destructive magnet 90 T

Highest field for superconducting magnet 33.8 T

National High Magnetic Field Laboratory www.magnet.fsu.edu

IRM Acquisition: Samples can be exposed to progressively higher fields

IRMs can be very useful in the lab.


SIRM and the resulting magnetization can be measured.

The magnitude of the IRM is sensitive to the magnetic mineralogy, concentration, and grain size of the assemblage.

M i IRM i k SIRM

Irm Acquisition

Tauxe, 2008

Maximum IRM is known as SIRM (saturation IRM) or Mr

IRM Demagnetization After saturation has been reached, the sample can be turned around and subjected to increasingly large back‐fields


Back‐field CurveIRM demagnetization

xe, 2008

fields.

A some point the back‐field strength will be strong enough to flip half of the moments in a sample, resulting in a net moment of zero. This is the Coercivity of Remanence (Hcr).

Sometimes people use a term called ’’’ hi h i h fi ld i d

Taux

H’’’cr, which is the field required to impart half the SIRM.

The acquisition of IRM is one of the many tools we use to characterize the magnetic minerals in a sample

Coercivity of Remanence (Hcr).

Verosub and Ro

berts, 1995

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IRM Acquisition and Demagnetization Curves

Slope ~ 2~0.5

Henkel plot : graphs IRM (demag) as a function of IRM (acq) at equivalent field levels

Cisowski plot: simultaneously graphs the acquisition curve together with the AF or DC demagnetization curve as a function of field

Wohlfarth (1958) Relationships for Non Interacting SD grains

Slope =~‐2

Wohlfarth (1958) Relationships for Non‐Interacting SD grains

MIRB= DC back field demagnetization MIRA= DC acquisition MIR(HAF)= AF demagnetization

IRB IRA

IR IRA

( ) 2 ( )

( ) ( )AF

M H SIRM M H

M H SIRM M H

Slope of Henkel plot =‐2Crossover point for Cisowski plot =0.5

Jackson, 2007

IRM Acquisition and Demagnetization CurvesCisowski Test for magnetic interactions

A crossover ratio R < 0.5 (equivalent to a trajectory on the Henkel

SD MDR

R

A crossover ratio R < 0.5 (equivalent to a trajectory on the Henkel plot that “sags” below the line of slope 2) indicates that MAF > MDF

Ensemble is harder to magnetize than to demagnetize

Such behavior is a hallmark of either a negatively interacting SD population or MD carriers (for which self‐demagnetization produces the equivalent effect)

Cisowski, S., 1981. Interacting vs. non‐interacting single‐domain behavior in natural and synthetic samples. Physics of the Earth and Planetary Interiors, 26: 77–83.

Coercivity Analysis (IRM acquisition curves)

Coercivity in AFM phases like hematite and goethite is much larger than that observed in ferrimagnetic phases like (titano)magnetite i li tphases like (titano)magnetite.

During IRM acquisition, it is more difficult to saturate AFM phases than magnetite.

marine limestone containing magnetite

Butle

r, 1992

Note the change in scale on the x‐axis (titano)magnetite: Hcmax ~ 300 mT Hematite/ goethite: Hcmax >1 T.

Jurassic limestone containing goethite

So the point at which an IRM curve reaches a plateau tells you something about the mineralogy in a sample.

IRM acquisition curves up to 57 TRoom‐Temperature

Non‐saturation of the defect moment of goethite and fine‐grained hematite up to 57 T (Rochette et al, 2005)

Goethite powders

Rock samples

Anomalous decrease in between 34=39 T is due to field reversal between the two coils used.

Rock samples RR,RD: goethiteTO: fine‐grained hematite

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Magnetic Characterization ‐ IRM and Coercivity Analysis

In (a) we see an IRM experiment that is rapidly magnetized up to ~200‐250 mT

Butle

r 199

2

rapidly magnetized up to 200 250 mT.

At this point, the acquisition of IRM slows down considerably, but does not plateau (even by 800 mT).

This is evidence for the presence of hematite or goethite.

It does not exclude the presence of magnetite.

A Curie temperature of 580°C is evident, but there is no indication of hematite (680°C ) or goethite (120°C)


(b) thermal demagnetization experiment for the IRM acquired in (a).

Butle

r 199

2

This experiment measures the remaining magnetization after the sample is heated in zero field to progressively higher temperatures.

Most of the remanence is gone by the Curie temperature of magnetite (580˚C).

However, a portion of the magnetization is still present at temps >580˚C.

This is the remanence held by hematite.


(c) strong‐field thermoremanent experiment for the same sample.

We see a Curie temperature around 580˚C (and also near 425˚C), but no sign of a Curie temperature associated with hematite or goethite.

This highlights the need for complementary experiments!

Butler 1992

Combining thermal and isothermal Magnetizations

3D IRM Test:

Acquisition of 3‐component IRM (‘Triaxial’ IRM)


1. Apply 2 T field along z‐axis2. Apply 0.4 T along y‐axis3. Apply 0.12 T along x‐axis

Thermal demagnetization of a 3‐axis IRM. Each component is plotted separately. (Ta

uxe 2008)

2 T field: AFM phases (hematite/goethite)0.4 T0.4 T field: (titano)magnetite0.12 T field: MD magnetite

Curve is dominated by a phase with a TBmax = 550◦ ‐600◦C and coercivity < 0.4 T, but > 0.12 T (PSD Magnetite)

Small fraction of a high coercivity (>0.4T) mineral with a maximum unblocking temperature > 650C (Hematite)

2013 SSRM 5/29/2013

26

Hematite is present in both (a) and (b), because SIRM requires fields > 1T and

Examples of the identification of magnetic minerals by acquisition and subsequent thermal demagnetization of IRM

because SIRM requires fields 1T and thermal demagnetization of the hard fraction persists to T= 675°C

In (a) the soft fraction that demagnetizes at T= 575°C is magnetite

In (b) no magnetite is indicated but pyrrhotite is present in all three

Lowrie

, 2007

fractions, shown by thermal unblocking at T = 300‐330°C

l02 magnetismofsolids magnetic mineralogy ssrm mineralogy_ssrm.pdfmagnetic mineralogy magnetite and...

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