Magneticminerals

Element Approximate % by weight Oxygen46.6 Silicon27.7 Aluminum8.1 Iron5.0 Calcium3.6 Sodium2.8 Potassium2.6 Magnesium2.1 All others1.5 Abundances of the Elements in the Earth's Crust is present in all natural magnetic minerals Also important: Titanium (Ti)

Magnetite (Fe 3 O 4 ) Unit cell: A-sites (8 Fe 3+ ) B-sites (8 Fe 3+ and 8 Fe 2+ ) Ferrimagnetism Normal Spinel (ZnFe 2 O 4 )Inverse Spinel (Fe 3 O 4 ) AB Zn 2+ Fe 3+ AB Fe 2+ => 5µB5µB 5µB5µB 4µB4µB

Magnetite (Fe 3 O 4 ) M s ≈ K The Verwey transition (at ≈ 120 K): from cubic to monoclinic symmetry T > T V T C = 580 °C cubicmonoclinic

Occurences of magnetite in nature Can be present in all types of rocks (igneous, sedimentary, metamorphic) Product of initial magma cooling Biogenic magnetite Product of rock alteration

Titanomagnetite series (Fe 3-x Ti x O 4 ) x Fe 2 TiO 4 · (1-x) Fe 3 O 4 (0 ≤ x ≤ 1) Titanium ions gradually replace Fe 3+ iron ions in B-sites (2 Fe 3+ → Fe 2+ and Ti 4+ ) Fe 3 O 4 - ferrimagnetic AB Fe 3+ Fe 2+ 6µB6µB 6µB6µB 5µB5µB Fe 2 TiO 4 - antiferromagnetic AB Fe 2+ Ti 4+ 5µB5µB 5µB5µB ulvöspinelmagnetite

Titanomagnetite series (Fe 3-x Ti x O 4 ) x Fe 2 TiO 4 · (1-x) Fe 3 O 4, 0 ≤ x ≤ 1 Curie temp, T C unit-cell dimension Mole fraction of Fe 2 TiO 4 (x) Curie temperature, T C (°C)Unit-cell dimension (Å)

Maghemite (γ-Fe 2 O 3 ) Fe 3 O 4 = Fe 3+ [Fe 3+ Fe 2+ ] O 4 Fully oxidized analog of magnetite (i.e. all Fe 2+ → Fe 3+ + e - ) γ-Fe 2 O 3 = Fe 3+ [Fe 3+ Fe 3+ 2/3 □ 1/3 ] O 3 AB Basic properties: M s ≈ K T C = °C (estimate) Converts to hematite when heated (at 250 to ≥ 750 °C) 2+ Fe 3 O 4 AB

Maghemite (γ-Fe 2 O 3 ) Fe 3 O 4 = Fe 3+ [Fe 3+ Fe 2+ ] O 4 γ-Fe 2 O 3 = Fe 3+ [Fe 3+ Fe 3+ 2/3 □ 1/3 ] O 3 AB Fe γ-Fe 2 O 3 Basic properties: M s ≈ K T C = °C (estimate) Converts to hematite when heated (at 250 to ≥ 750 °C) Fully oxidized analog of magnetite (i.e. all Fe 2+ → Fe 3+ + e - ) AB

Magnetite-maghemite oxidation series (aka cation-deficient magnetite) Oxidation parameter (z) Unit-cell dimension (Å) Saturation magnetization, Ms (µ B ) z = 0 z ≠ 0 Fe z/3 Fe 2+ 1-z □ z/3 O 4 z – oxidation parameter MsMs

Hematite (α-Fe 2 O 3 ) - Fe 3+ Side view The same composition as maghemite, but a different structure (hexagonal) Spin-canted (non-perfect) antiferromagnetism

Hematite (α-Fe 2 O 3 ) The Morin transition (at ≈ -15 °C): α-F e 2O 3 becomes a perfect antiferromagnet Figures from Dunlop (1971) and Özdemir and Dunlop (2005) M s ≈ K Temperature (°C) Normalized magnetization, M s /M s0 T C = 675 °C T < T M T > T M

Occurences of hematite in nature Red beds Can be present in all types of rocks (igneous, sedimentary, metamorphic) Product of initial magma cooling Product of rock alteration

Titanohematite series (Fe 2-y Ti y O 3 ) α-Fe 2 O 3 FeTiO 3 Imperfect antiferromagnetism for 0 ≤ y ≤ 0.5 Ferrimagnetism for ) TCTC y ≈ 0.5

Major magnetic minerals: Summary ChemicalM s T c Magnetic formula(kA/m) (°C) structure IronFe ferromagnet MagnetiteFe 3 O ferrimagnet Maghemiteγ-Fe 2 O ferrimagnet Titanomagnetite (x = 0.6)Fe 2.4 Ti 0.6 O ferrimagnet Hematiteα-Fe 2 O 3 ≈ imperfect antiferromagnet Titanoilmenite (y ≈ 0.5) Fe 1.5 Ti 0.5 O ferrimagnet Goethiteα-FeOOH≈ 2120imperfect antiferromagnet PyrrhotiteFe 7 S 8 ≈ 80320ferrimagnet GreigiteFe 3 S 4 ≈ 125 ≈ 330ferrimagnet

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 dolomite limestone sandstone shale Sedimentary rocks Volcanic rocks granite gabbro basalt Magnetic susceptibility, κ (SI units) Median values and ranges of the magnetic susceptibility in common rock types 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.) 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 O Ti Fe a.b.

Non-magnetic analytical techniques (X-ray diffractometry, Mößbauer analysis, etc.) 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) Hematite Magnetite

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.)

Magnetic measurements at high temperatures (magnetic behavior on heating/cooling, Curie temperature ) T C ≈ 580 °C Temperature (°C) Susceptibility (SI) 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) Temperature (K) 20K (memu) Thermal demagnetization of M rs imparted at 20 K from an Archean doleritic dike sample. Measured at the Institute for Rock Magnetism in November Verwey transition Hematite Magnetite Quartz

2kT Grain size dependence of ferrimagnetic properties M r (t) = M r0 exp - t τ VM s B c τ = A exp T = 0 E m = VM s B c /2 MsMs T ≠ 0 E m = VM s B c /2 E T = kT (k = 1.381∙10-23 J K-1) t MrMr M r0 τ - relaxation time

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 τ < t experiment (eg., 100 s) Grain size dependence of ferrimagnetic properties 2kT VM s B c τ = A exp

Single-domain state M r (t) = M r0 exp - t τ t MrMr M r0, τ >> t experiment (eg., billions of years) At a constant T, τ depends on V 2kT VM s B c τ = A exp

Magnetic domains Bloch domain wall DW width E total = E m = (1/2) NM s 2 V Single-domain Two and more domains E total = E m + E DW Domain wall energy Minimization of the total energy E d (domain size) E DW EmEm E total d0d0

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

REMANENTMAGNETIZATIONSINROCKS

Natural remanent magnetism (NRM) M = M induced + M remanent 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) 3.As T decreases, B c increase. At some temperature (blocking temperature) τ becomes very large. TCTC 20°C BcBc Grain volume, V τ = 100 s τ = 10 b.y. T ≈ 1000 °C. Magnetic minerals are formed, but are in paramagnetic state 1 2.T <≈ T Curie. Spontaneous magnetization appears. VM s B c 1 υ0υ0 τ = exp 2kT For SD grains: 2 3 At high T (i.e., VM s B c < kT), grains behave superparamagnetically (τ < t observation ) V = const T BL H H TRM

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) VM s B c 1 υ0υ0 τ = exp 2kT BcBc Grain volume, V τ = 100 s τ = 10 b.y. CRM is a remanent magnetization acquired when a ferromagnetic grain grows in the presence of a magnetic field T, M s, B c are constant, V increases CRM

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 → αFe 2 O 3 + H 2 O PRIMARY CRM

Depositional remanent magnetization (DRM) Depositing magnetic particles are oriented by the geomagnetic field Inclination error field direction DRM

Post-Depositional remanent magnetization (pDRM) Compaction / de-watering Lock-in depth ≈10 cm H H Water-sediment interface 100 years for lacustrine sediments years for pelagic marine sediments 10 cm

Viscous remanent magnetization (VRM) BcBc Grain volume, V τ = 100 s τ = 10 b.y. Particles with intermediate τ Log time VRM VRM = S log(t) Secondary magnetization in some rocks. Less stable than TRM/CRM/DRM.

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