Analytical Techniques for Minerals

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Analytical Techniques for Minerals

• Electron microscopy look at techniques which
  utilize how electrons (shot through a sample of
  mineral) interact with minerals imaging
  possible to very small sizes
• Scanned-proximity probe microscopy techniques
  look at forces between probe tip and sample to
  measure a property (height, optical absorption,
  magnetism, etc)
• Spectroscopy different methods of studying how
  different parts of the electromagnetic spectrum
  (of which visible light is a small part) are
  affected by minerals

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Analytical Techniques for Minerals

• XRD (X-ray diffraction) is one of the most
  powerful tools for mineral identification,
  structural/chemical refinement, and size
  determination we will study it in detail (both
  lecture and lab).
• Microscopy Optical techniques are another very
  powerful tool for mineral identification,
  identification of physical/ chemical history of
  minerals/rocks, and mineral association which we
  will also study in detail (both lecture and lab)

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More analytical techniques

• Sychrotron Different techniques (many similar
  to spectroscopic techniques) that utilize
  particles accelerated to very high speeds and
  energies and how they interact with minerals
• Magnetic different techniques that utilize the
  magnetic properties of minerals
• Size techniques to determine the sizes of
  different minerals
• Chemistry/isotopes techniques to probe chemical
  and isotopic signatures in minerals

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Plancks law Ehn hc/l Where n is frequency, l
is wavelength, h is Plancks constant, and c is
the speed of light
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Electron Microscopy

• What we can see using visible light is limited at
  the small end of spatial scales by the wavelength
  of light (hundreds of nanometers)
• To image things smaller than this, need to use
  energy of smaller wavelengths
• Because energy is inversely proportional to
  wavelength (Ehc/l), higher energy particles have
  smaller wavelengths and can image smaller things
  (e- are easy to generate and accelerate ? faster
  particle has more energy)

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Scanning electron microscope (SEM)

• The electron gun, produces a stream of electrons
  accelerated by applying a pulsed potential across
  the gun (e- is pulled towards a positive
  potential, pushed from a negative potential)
• A series of condensers and apertures (i.e. lenses
  and holes) restricts the electron flow to a nice,
  tight beam
• A set of coils then "scan" or "sweep" the beam in
  a grid fashion (like a television), dwelling on
  points for a period of time determined by the
  scan speed (usually in the microsecond range)
• The final lens, the Objective, focuses the
  scanning beam onto the part of the specimen
  desired.
• Before the beam moves to its next point
  instruments count the number of interactions and
  display a pixel on a CRT whose intensity is
  determined by this number (the more reactions the
  brighter the pixel).
• This process is repeated until the grid scan is
  finished and then repeated, the entire pattern
  can be scanned 30 times per second.

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Electron Microscopy/ Spectroscopy

• Interaction of electrons with a sample

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e- scattering off bulk sample

• Backscattered Electrons Caused by an incident
  electron colliding with an atom in the specimen
  which is nearly normal to the incident's path.
  The incident electron is then scattered
  "backward" 180 degrees.
• Utilization -The production of backscattered
  electrons varies directly with the specimen's
  atomic number. This differing production rates
  causes higher atomic number elements to appear
  brighter than lower atomic number elements.
• Secondary Electrons Caused by an incident
  electron passing "near" an atom in the specimen,
  near enough to impart some of its energy to a
  lower energy electron. This causes a slight
  energy loss and path change in the incident
  electron and the ionization of the electron in
  the specimen atom. This ionized electron then
  leaves the atom with a very small kinetic energy
  (5eV) and is then termed a "secondary electron".
  Each incident electron can produce several
  secondary electrons.
• Utilization - Production of secondary electrons
  is very topography related. Due to their low
  energy, 5eV, only secondary electrons generated
  very near the surface (lt10nm) can exit the sample
  and be examined. Any changes in topography that
  are larger than this sampling depth will change
  the yield of secondary electrons yielding an
  image of the surface of the sample.
• Auger Electrons and X-rays these emmissions are
  caused by the formation of secondary electrons
  and rearrangements of electrons as a result of
  emission.
• Utilization Auger electrons or X-rays emitted
  from the atom will have a characteristic energy
  which is unique to the element from which it
  originated.

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e- penetration into a sample

• Details dependent on mineral composition and
  accelerating voltage of e- beam, but for SEM
  applications

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SEM what do we get?

• Topography (surface picture) commonly enhanced
  by sputtering (coating) the sample with gold or
  carbon

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But wait thats not all!

• SEM also detects electrons and x-rays which
  identify electron density distributions
  (Back-Scattered Electrons) and chemical identity
  (Auger and X-rays) using specific detectors (BSE,
  EDX, Auger)

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Electron Microscopy/ Spectroscopy

• Interaction of electrons with a sample can make
  the sample thinner and shoot e- with higher
  energy, more of them go through the sample.

SEM
TEM
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Transmission Electron Microscopy (TEM)

• Electron beam transmitted just as in the SEM
• This transmitted portion is focused by the
  objective lens into a sample
• Optional Objective and Selected Area metal
  apertures can restrict the beam
• the Objective aperture enhancing contrast by
  blocking out high-angle diffracted electrons,
• the Selected Area aperture enabling the user to
  examine the periodic diffraction of electrons by
  ordered arrangements of atoms in the sample
• The image is passed down the column through the
  intermediate and projector lenses, being enlarged
  all the way
• The image strikes the phosphor image screen and
  light is generated, allowing the user to see the
  image. The darker areas of the image represent
  those areas of the sample that fewer electrons
  were transmitted through (they are thicker or
  denser). The lighter areas of the image represent
  those areas of the sample that more electrons
  were transmitted through (they are thinner or
  less dense)

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e- that go through a sample

• Unscattered Electrons - Incident electrons which
  are transmitted through the thin specimen without
  any interaction occurring inside the specimen.
• Utilization - The transmission of unscattered
  electrons is inversely proportional to the
  specimen thickness another topographical image!
• Elasticity Scattered electrons - Incident
  electrons that are scattered (deflected from
  their original path) by atoms in the specimen in
  an elastic fashion (no loss of energy). These
  scattered electrons are then transmitted through
  the remaining portions of the specimen.
• Utilization electrons passing through at a
  similar angle are scattered, these electrons can
  then be collated using magnetic lenses to form a
  pattern of spots each spot corresponds to a
  specific atomic spacing (a plane). This pattern
  can then yield information about the orientation,
  atomic arrangements and phases present in the
  area being examined.
• Very small particles will yield rings instead of
  spots (nanometer-sized xstals)
• Inelastically Scattered Electrons - Incident
  electrons that interact with specimen atoms in a
  inelastic fashion, loosing energy during the
  interaction. These electrons are then transmitted
  trough the rest of the specimen
• Utilization Electron Energy Loss Spectroscopy
  The inelastic loss of energy by the incident
  electrons is characteristic of the elements that
  were interacted with. These energies are unique
  to each bonding state of each element and thus
  can be used to extract both compositional and
  bonding (i.e. oxidation state) information on the
  specimen region being examined.

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TEM ( HRSTEM) What do we get?

• See smallest features with this sub-nm!
• Morphology size, shape, arrangement of
  particles on scale of atomic diameters
• Crystallographic information from diffracted
  electrons, get arrangement and order of atoms as
  well as detection of atomic-scale defects

• Compositional information Chemical identity,
  including redox speciation (distinguish Fe2 and
  Fe3 for instance)

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Electron Microprobe

• Very similar to SEM and TEM in many respects, but
  utilizes thick sections and a set of detectors
  which measure the emitted X-Rays from e-
  bombardment and excitation more accurately than
  the detectors used in SEM or TEM analyses
• These detectors are wavelength dispersive
  spectrometry (WDS) detectors, there are usually
  an array of 3-5 which record over some range of
  wavelength much more accurately than the EDX
  detector available with SEM and TEM instruments