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Electrical
and Magnetic Properties
Electricity and magnetism are two
expressions of a single force, called "electromagnetism". Two
electrical properties that have important industrial and electronic applications
are piezoelectricity
and pyroelectricity, although these properties
are not typically used in mineral identification. These properties are
shown by certain classes of minerals or synthetic crystalline materials that
lack a center of symmetry. Magnetism is an
important diagnostic tool for a few minerals. Radioactivity,
or the spontaneous decay or disintegration of an unstable atomic nucleus
accompanied by the emission of electromagnetic energy, can be detected in some
minerals with a Geiger counter and for these minerals, it is a diagnostic
property.
 Piezoelectricity
is the ability of a mineral or crystal to acquire opposing electrical charges on
opposing surfaces when mechanical stress (such as bending, stretching, or
compression) is applied to the crystal. The piezoelectric effect is caused by
the displacement of ionic charges within a crystal structure, and the magnitude
of the charge generally is proportional to the amount of stress applied.
Removal of the stress reverses the effect. This electrical charge can be
converted into a voltage using a charge amplifier.
Of the minerals in the twenty crystal
classes that lack a center of symmetry, only a few are piezoelectric to any
significant degree. Piezoelectricity was first discovered in quartz
crystals. This effect allows them to be used in certain sorts of radio
tuners (first done in 1921, but now largely replaced by other types), in the
timing mechanisms of quartz watches, and in the electronics industry.
Tourmaline, another strongly piezoelectric mineral, is used in gauges to measure
transient blast pressures.
 Pyroelectricity
is the ability of a mineral or crystal to acquire opposing electrical charges on
opposing surfaces as a result of heating. Tourmaline was recognized as a
distinct mineral with a highly variable composition because of
pyroelectricity. The pyroelectric property of the tourmaline shown on the
right made photographing it very difficult, because the heat of the lights that
were used to illuminate it produced an electrical charge that attracted a heavy
coating of dust. The dust had to be removed before the photograph could be
taken. (quickly before more dust was attracted!)
 Magnetism
and electricity are closely related, and are regarded by physicists as two
expressions of a single force, called "electromagnetism".
Although only minerals that are strongly attracted to a magnetic field are
considered to be magnetic, all matter has some magnetic properties, even though
it may be too small to be detectable. This is because all matter is made
up of moving charged particles (electrons and protons), whose motion sets up
magnetic fields.
The magnetic properties of atoms and
molecules are primarily dependent on the spin of electrons in a substance.
Electrons can spin in one of two opposite directions and any two electrons in
the same orbit are constrained to spin in opposing directions. A spinning
electron behaves as a small magnet and will produce a magnetic field while
orbiting around the nucleus of an atom. The opposing spins of two
electrons in an orbital results in a net zero magnetic moment. Diamagnetic
substances have zero net magnetic moment because they have the same number of
electrons of opposing spin. Diamagnetic substances are actually slightly
repelled by a magnetic field due to the negative charge of the electron
clouds. Quartz and calcite are diamagnetic.
A paramagnetic
substance has a random arrangement of magnetic dipoles of the atoms in its
structure until the material is placed within a magnetic field. At that
time the dipoles will align themselves with the external magnetic field.
Olivine and pyroxene are paramagnetic.
Ferromagnetic
substances show strong magnetic attraction when subjected to a magnetic
field and will remain magnetic after the removal of the magnetic field unless
heated above their Curie temperatures. Magnetism occurs because the
magnetic dipoles of domains that make up a ferromagnetic substance align when in
the presence of a magnetic field. Metallic iron, nickel, cobalt, and
numerous alloys of these transition
metals are ferromagnetic. Heating above the Curie temperature of a
ferromagnetic substance causes the magnetic dipoles in the domains to randomly
realign and the material will behave as a paramagnetic substance.
Ferrimagnetic
substances are characterized by strong permanent magnetic susceptibility
caused by ionic spin moment that are antiparallel. Magnetite and
pyrrhotite are ferrimagnetic.
 Radioactivity
is the spontaneous decay or disintegration of an unstable atomic nucleus of an
atom of one element to produce one or more new nuclides and the emission of
radiation (energy). The radioactive isotopes of potassium,
strontium, thorium, uranium and samarium (40K, 87Sr,
232Th, 238U, 235U, 247Sm) are
geologically important for absolute age dating and as a diagnostic tool.
The radioactivity of some uranium- and thorium-rich minerals is great enough
that it can be detected by a Geiger counter and is therefore a diagnostic
property. The radioactivity of these and other radioactive minerals may
also pose a health risk.
There are three mechanisms of
radioactive decay: alpha decay, beta decay and electron
capture. Alpha decay results in the emission of an alpha
particle. The alpha particle (a) is identical to the nucleus of 4He
(helium) atom and consists of two protons and two neutrons. The decay of
uranium-238 is an example of alpha decay. Alpha decay reduces the mass
number of the nucleus by four and the atomic number by two due to the loss of
two protons and two neutrons from the parent isotope.
238U
=> 234Th + a + g + Energy
Beta decay results in the emission of a
beta particle (b). A beta particle is equivalent to a negatively charged
electron. The beta particle is formed through the decay of a neutral
neutron particle into a positively charged proton.
87Rb => 87Sr
+ b- + Energy
Electron capture occurs when an orbital
electron is captured by the nucleus. The decay of potassium-40 to argon-4o
occurs through electron capture. The nuclear charge decreases by one
without any significant change in mass.
40K + orbital
e- => 40A + g +
Energy
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