Transcript Introduction to Environmental Geochemistry

Magnetism
GLY 4200
Fall, 2012
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Early Observations of Magnetism
• Ancient Greeks, especially those near the
city of Magnesia, and Chinese, observed
natural stones that attracted iron
• The naturally magnetic stones are called
lodestone
• The Chinese discovered a steel needle
stroked by a lodestone became magnetic,
and, if suspended, pointed N-S
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What is Magnetism?
• Although discovered relatively early in
man’s history, and exploited, the causes of
magnetism were not understood
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Orbiting Electrons
• Moving electrical currents generate
magnetic forces
• This includes electrons orbiting and
spinning around a nucleus
• Each orbiting electron possesses a magnetic
moment equal to 1 Bohr magnetron (μB), or
0.927 x 10-23 Am2 (Amps meter2)
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Isolated Ions
• Net magnetic moment is equal to the sum
of:
 Orbital contributions
 Spin contributions
• Filled orbitals give a net contribution of
zero to the magnetic moment since the two
electrons orbit and spin oppositely
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Net Magnetic Moments
• Generated only in atoms or ions with incomplete
electronic shells
• Most important subshells likely to be incompletely
filled are the 3d (first transition row) and the 4f
(rare earth elements)
• The second and third transition rows (4d and 5d
electrons) also produce magnetic moments but the
elements, and hence the minerals, are rare
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Three d Electrons
• Three d electrons have large spin and relatively
low orbital contributions to magnetic moments
• In compounds the orbital contribution is affected
by, and largely negated by, bonding to other ions
• Since the 4s electrons are outside the 3d, the 3d
electrons are partially shielded and the orbital
contribution will not be entirely negated
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Spin Contribution
• Spin contribution is largely responsible for
the 3d electrons contribution to the
magnetic moment and is proportional to the
number of unpaired d electrons
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Four f Electrons
• In 4f electron containing elements, the
electrons are well-shielded by outer
electrons
• The 4f electrons are not involved in bonding
and both orbital and spin effects contribute
to the total magnetic moment
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Magnetic Susceptibility
• An aggregate of ions or atoms may behave
much differently than an individual ion
• Magnetic Susceptibility is the ratio of
induced magnetization to the strength of the
external magnetic field causing the induced
magnetization
• Magnetic susceptibility may be grouped
into different classes of behavior
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Diamagnetism
• Minerals possessing ions with totally paired
electron spins
• No transition elements are present, and the
net magnetic moment is zero
• In a strong magnetic field diamagnetic
materials exhibit a small negative magnetic
susceptibility, which means they are weakly
repelled from the magnet
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Paramagnetism
• Transition metals ions are present but the magnetic
moments are randomly distributed
• Net field is zero, although an external field will
produce some alignment of dipoles, which
disappears when the external field is removed
• Alignment of the magnetic dipoles produces a small
positive magnetic susceptibility and these minerals
are attracted to a magnet in a strong magnetic field
• Example: olivine (Mg, Fe)2SiO4
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Ferromagnetism
• Adjacent moments are aligned
• After an external field is applied the dipoles
interact and the field remains locked in
• The magnetism is due to unbalanced electron spin
in the inner orbits of the elements concerned
• The ionic spacing in ferromagnetic crystals is such
that very large forces, called exchange forces,
cause the alignment of all atoms to give highly
magnetic domains
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Making a Magnet
• In unmagnetized metal these domains are
randomly oriented
• After a strong magnetic field is applied the
domains align and the material remains a strong
magnet after the external field is removed
• Examples of ferromagnetic materials are the
metals cobalt and nickel, and alloys such as alnicol
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Curie Temperature
• Upon heating the domains may become
randomly aligned once again
• This transition to a paramagnetic state is
called the Curie temperature, after Pierre
Curie, who was instrumental in elucidating
the behavior of paramagnetic materials
• In metallic iron the Curie temperature is
770 ̊C
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Antiferromagnetism
• Alternate atoms have oppositely directed
moments
• Magnetic susceptibility is low but increases
with increasing T up to the Néel
temperature
• Above this temperature the susceptibility
falls and the material is paramagnetic
• Examples include Cr metal, and compounds
like MnO, MnS, and FeO
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Louis E.F. Néel
• The Néel temperature is
named after L.E.F. Néel, who
discovered the phenomenon
of the transition from
antiferromagnetism to
paramagnetism in 1930
• Born 1904, died 2000
• Nobel prize in physics, 1970
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Ferrimagnetism
• Adjacent atoms have antiparallel alignment, but
the magnitude of the magnetic moments of
different ions is different
• Cancellation is incomplete and strong magnetism
may exist
• Alternatively, the number of magnetic moments
aligned in one direction may be different than in
another direction
• Ferrimagnetic materials may have magnetism
similar to that of ferromagnetic materials
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Incorrect Identification
• Some minerals have been incorrectly described in
the literature as being ferromagnetic when in fact
they are ferrimagnetic
• Examples include ilmenite FeTiO3, magnetite
(Fe3O4 or Fe2+Fe23+O4) and pyrrhotite (Fe1-xS, x =
0.0 → 0.2)
• Curie temperature for magnetite is 85 ̊ C, much
lower than for metallic iron
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Magnetic Separation
• Magnetic separation, based on the differing magnetic
susceptibilities of different minerals, is used in processing
minerals since many minerals, especially those containing iron,
are attracted to or repelled from a magnet in a strong magnetic
field
• Magnetic separation is used in both laboratory and commercial
scales for mineral separation
• Picture: Frantz laboratory magnetic mineral separator
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Aerial Remote Sensing of
Magnetism
• An airplane flies over an area towing a
magnetometer, which measures local
perturbations of the earth's magnetic field
• These aircraft fly low (100 to 300 meters)
and use highly sensitive magnetometers
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Diagram of Magnetometer Tow
• Catalina aircraft fitted with a
magnetometer
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Sulfide Ore Bodies
• Many sulfide ore bodies are associated with
magnetite and, although the magnetite itself may
have no economic value, the sulfides often are
valuable
• This method is rapid and relatively cheap,
especially in areas of rough terrain
• Any positive magnetic anomalies must be verified
by subsequent geophysical and geochemical
exploration
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Paleomagnetism
• Ferrimagnetic minerals are permanently
magnetized
• Study of natural remnant magnetism of
rocks yields a record of the earth’s magnetic
field through time
• This reveals polarity reversals, and can aid
in the study of plate motions
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Polarity
Reversal
Record
• Paleomagnetic
record of 0-4
MYBP
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