Magnetic
minerals
Abundances of the Elements in the Earth's Crust
Element
Approximate
% by weight
Oxygen
46.6
Silicon
27.7
Aluminum
8.1
Iron
5.0
Calcium
3.6
Sodium
2.8
is present in all
natural magnetic
minerals
Also important:
Potassium
2.6
Magnesium
2.1
All others
1.5
Titanium (Ti)
Magnetite (Fe3O4)
Unit cell:
A-sites (8 Fe3+)
B-sites (8 Fe3+ and 8 Fe2+)
Normal Spinel (ZnFe2O4)
A
Zn2+
B
Fe3+
Inverse Spinel (Fe3O4)
A
Fe3+
B
Fe3+
Fe3+
5µB 5µB
Fe2+
4µB
=> Ferrimagnetism
Magnetite (Fe3O4)
Ms ≈ 480 kA/m @ 300 K
The Verwey transition (at ≈ 120 K):
from cubic to monoclinic symmetry
T > TV
monoclinic
TC = 580 °C
cubic
Occurences of magnetite in nature
Can be present in all types of rocks (igneous, sedimentary,
metamorphic)
Product of initial magma cooling
Product of rock alteration
Biogenic
magnetite
Titanomagnetite series (Fe3-xTixO4 )
Titanium ions gradually replace Fe3+ iron ions in B-sites (2 Fe3+ → Fe2+ and Ti4+)
Fe3O4 - ferrimagnetic
A
B
Fe3+
Fe3+ Fe2+
6µB 6µB
5µB
Fe2TiO4 - antiferromagnetic
A
B
Fe2+
Fe2+ Ti4+
5µB 5µB
x Fe2TiO4 · (1-x) Fe3O4
ulvöspinel
magnetite
(0 ≤ x ≤ 1)
Titanomagnetite series (Fe3-xTixO4 )
600
8.54
400
8.50
200
8.46
0
8.42
-200
0
0.4
0.6
0.8
Mole fraction of Fe2TiO4 (x)
1
8.38
Unit-cell dimension (Å)
Curie temperature, TC (°C)
x Fe2TiO4 · (1-x) Fe3O4 , 0 ≤ x ≤ 1
Maghemite (γ-Fe2O3)
Fully oxidized analog of magnetite (i.e. all Fe2+ → Fe3+ + e-)
A
B
Fe3O4 = Fe3+ [Fe3+ Fe2+] O4
Fe3O4
2+
A
2+
B
γ-Fe2O3 = Fe3+ [Fe3+ Fe3+2/3 □1/3] O3
2+
2+
Basic properties:
Ms ≈ 380 kA/m @ 300 K
TC = 645-675 °C (estimate)
Converts to hematite when heated (at 250 to ≥ 750 °C)
Maghemite (γ-Fe2O3)
Fully oxidized analog of magnetite (i.e. all Fe2+ → Fe3+ + e-)
A
B
Fe3O4 = Fe3+ [Fe3+ Fe2+] O4
γ-Fe2O3
Fe2+
A
3+
B
γ-Fe2O3 = Fe3+ [Fe3+ Fe3+2/3 □1/3] O3
3+
3+
Basic properties:
Ms ≈ 380 kA/m @ 300 K
TC = 645-675 °C (estimate)
Converts to hematite when heated (at 250 to ≥ 750 °C)
Magnetite-maghemite oxidation series
(aka cation-deficient magnetite)
Fe3+2+2z/3 Fe2+1-z □z/3 O4
z≠
=0
Unit-cell dimension (Å)
8.40
4.0
8.39
3.8
8.38
3.6
8.37
Ms
8.36
8.35
3.4
3.2
0
3.0
0.2 0.4 0.6 0.8 1.0
Oxidation parameter (z)
Saturation magnetization, Ms (µB)
z – oxidation parameter
z=0
Hematite (α-Fe2O3)
The same composition as maghemite, but a
different structure (hexagonal)
- Fe3+
Side view
Spin-canted (non-perfect) antiferromagnetism
Hematite (α-Fe2O3)
Normalized magnetization, Ms/Ms0
Ms ≈ 2.5 kA/m @ 300 K
The Morin transition (at ≈ -15 °C):
α-Fe2O3 becomes a perfect
antiferromagnet
TC = 675 °C
T < TM
Temperature (°C)
Figures from Dunlop (1971) and Özdemir and Dunlop (2005)
T > TM
Occurences of hematite in nature
Can be present in all types of rocks (igneous, sedimentary,
metamorphic)
Product of initial magma cooling
Product of rock alteration
Red beds
Titanohematite series (Fe2-yTiyO3)
α-Fe2O3
FeTiO3
y ≈ 0.5
TC
Imperfect antiferromagnetism for 0 ≤ y ≤ 0.5
Ferrimagnetism for 0.5 < y ≤ 1 (Note: TC is below
room temperature for y > 0.7)
Major magnetic minerals: Summary
Chemical
Ms
Tc
Magnetic
formula
(kA/m)
(°C)
structure
Iron
Fe
1715
765
ferromagnet
Magnetite
Fe3O4
480
585
ferrimagnet
Maghemite
γ-Fe2O3
380
590-675
ferrimagnet
Titanomagnetite (x = 0.6)
Fe2.4Ti0.6O4
125
150
ferrimagnet
Hematite
α-Fe2O3
≈ 2.5
675
imperfect antiferromagnet
Titanoilmenite (y ≈ 0.5)
Fe1.5Ti0.5O3
100
20
ferrimagnet
Goethite
α-FeOOH
≈2
120
imperfect antiferromagnet
Pyrrhotite
Fe7S8
≈ 80
320
ferrimagnet
Greigite
Fe3S4
≈ 125
≈ 330
ferrimagnet
Identification of magnetic minerals in rocks
Rocks are assemblages of diamagnetic,
paramagnetic, and ferrimagnetic minerals.
Concentration of ferrimagnetic minerals is
usually very small (< 1 %)
Microphotograph of a thin section of norite
Magnetic susceptibility, κ (SI units)
Median values and ranges of the magnetic susceptibility in common rock types
1
Sedimentary rocks
10-1
sandstone
10-2
10-3
granite
gabbro
basalt
shale
limestone
dolomite
Volcanic rocks
10-4
10-5
Modified from Lowrie, 1997
Identification of magnetic minerals in rocks
• Direct observation
• Non-magnetic diagnostic techniques
• Magnetic measurements
- at room temperature
- at high temperatures
- at low temperatures
Direct observation
(scanning and transmission electron microscopy, optical petrography, etc.)
a.
b.
O
Ti
Fe
Scanning electron microscope image (a) and energy dispersive spectrum (b) of
a titanomagnetite (TM60) grain
1.35 µm
Transmission electron microscope image of magnetosomes
from the Ocean Drilling Program Site 1006D
Non-magnetic analytical techniques
(X-ray diffractometry, Mößbauer analysis, etc.)
Hematite
Magnetite
X-ray diffraction spectra from
Lake Chiemsee sediments (Pan
et al., 2005); M – magnetite,
mh – maghemite, Q - quartz
Moessbauer spectrum of Alaskan loess
samples (Solheid, 1998)
Magnetic measurements at room temperature
(IRM acquisition, magnetic hysteresis, etc.)
Acquisition of isothermal remanent magnetization
(IRM) among the samples units (Evans et al., 2002)
Magnetic measurements at room temperature
(IRM acquisition, magnetic hysteresis, etc.)
Isothermal remanent magnetization (A/m)
600
400
200
0
0
500
1000
1500
Magnetic field (mT)
2000
2500
Magnetic measurements at high temperatures
(magnetic behavior on heating/cooling, Curie temperature )
TC ≈ 580 °C
Susceptibility (SI)
1600
1200
800
400
0
0
200
400
600
Temperature (°C)
Temperature dependence of magnetic
susceptibility measured from a doleritic
dike (Smirnov and Tarduno, 2004)
Magnetic measurements at low temperatures
(identification of low-temperature magnetic transitions)
Verwey
transition
Mrs @ 20K (memu)
6
Hematite
4
Magnetite
2
Quartz
0
100
200
Temperature (K)
300
Thermal demagnetization of Mrs imparted at 20 K from an Archean doleritic dike
sample. Measured at the Institute for Rock Magnetism in November 2005.
Grain size dependence of ferrimagnetic properties
Ms
T=0
Em = VMsBc/2
Mr0
Mr
t
T≠0
Em = VMsBc/2
ET = kT
(k = 1.381∙10-23 J K-1)
t
Mr(t) = Mr0 exp τ
VMsBc
τ = A exp
2kT
τ - relaxation time
Grain size dependence of ferrimagnetic properties
VMsBc
τ = A exp
2kT
τ < texperiment
(eg., 100 s)
Superparamagnetism – randomization of the spontaneous magnetization vectors in very
small particles (but the atomic moments are aligned within a grain)
Paramagnetism – randomization of atomic moments
Single-domain state
VMsBc
t ,
Mr(t) = Mr0 exp τ = A exp
τ
2kT
At a constant T, τ depends on V
τ >> texperiment
(eg., billions
of years)
Mr0
Mr
t
Magnetic domains
Minimization of the total energy
Single-domain
Etotal = Em = (1/2) NMs2V
Two and more domains
Domain wall energy
Etotal = Em + EDW
E
Etotal
Em
DW width
EDW
d0
d (domain size)
Bloch
domain wall
Domain observations: Bitter patterns
Upper and lower size limits for single-domain state
in equidimensional grains at 20°C
from Dunlop and Özdemir, 1997
Domain state as a function of grain size and shape
from Lowrie, 1997
REMANENT
MAGNETIZATIONS
IN
ROCKS
Natural remanent magnetism (NRM)
M = Minduced + Mremanent
NRM = primary NRM + secondary NRM
Information about past
geomagnetic field (useful
for paleomagnetism)
Parasitic magnetizations
acquired during geological
life of a rock
Thermal remanent magnetization (TRM)
1
T ≈ 1000 °C. Magnetic minerals are formed, but are in paramagnetic state
22.
T <≈ TCurie. Spontaneous magnetization appears.
For SD grains:
H
1 exp VMsBc
τ=υ
2kT
0
At high T (i.e., VMsBc < kT), grains behave superparamagnetically (τ < tobservation)
As T decreases, Bc increase. At some
temperature (blocking temperature)
τ becomes very large.
TC
TBL
H
20°C
V = const
Grain volume, V
33.
TRM
τ = 10 b.y.
τ = 100 s
Bc
Thermal remanent magnetization (TRM)
TRM is a remanent magnetization acquired when a ferromagnetic material is
cooled from above its Curie point in the presence of a magnetic field
Primary magnetization in extrusive and
intrusive igneous rocks (basalt, dolerite)
Chemical remanent magnetization (CRM)
(aka crystallization remanent magnetization)
CRM is a remanent magnetization acquired when a ferromagnetic grain grows
1 exp VMsBc
τ=υ
2kT
0
T, Ms, Bc are constant,
Grain volume, V
in the presence of a magnetic field
CRM
τ = 10 b.y.
V increases
τ = 100 s
Bc
Chemical remanent magnetization (CRM)
Mostly secondary magnetization in igneous, metamorphic, and
sedimentary rocks (eg., red beds)
E.g., formation of hematite
within goethite grains
FeOOH → αFe2O3 + H2O
CRM
CRM
CRM
CRM
Depositional remanent magnetization (DRM)
Depositing magnetic particles are oriented by the geomagnetic field
Inclination error
DRM
field
direction
Post-Depositional remanent magnetization (pDRM)
Water-sediment
interface
H
Lock-in depth
≈10 cm
Compaction / de-watering
10 cm
100 years for lacustrine sediments
10000 years for pelagic marine sediments
Viscous remanent magnetization (VRM)
Particles with intermediate τ
VRM
Grain volume, V
VRM = S log(t)
τ = 10 b.y.
τ = 100 s
Bc
Secondary magnetization in some rocks.
Less stable than TRM/CRM/DRM.
Log time
Magnetizations in rocks: Summary
TRM: Material (a rock) is cooled from above its Curie
temperature in the presence of a magnetic field
CRM: Magnetic grains are growing at a constant
temperature in the presence of a magnetic field
DRM: Magnetic grains are depositing in water in the
presence of a magnetic field
VRM: Material (a rock) is exposed to a magnetic field
for a long time (at a constant temperature)
If for a short time, then an isothermal remanent
magnetization is acquired