X-ray crystallography - CH8

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X-ray crystallography - CH8
2012-10-08 12:20:22
ray crystallography

X-ray crystallography
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  1. What are different analytical techniques that geologists use to identify minerals?
    • Petrographic microscopes
    • electron microscope
    • scanning electron microscope
    • mass spectrometer
  2. Explain the basic physics of XRD
    • - Free electons produced by heating a (cathode) filmaent, accelerated and strike a target (typically copper)
    • -When the electon hits the target, X-rays produced.
    • -These X-rays, after filtered, monochromatic X-rays are then directed towards the sampleof interest
    • the x-rays are then diffracted from parallel planes of atoms
  3. Why are X-rays diffracted by minerals?
    • -Caused by X-rays hitting parallel planes of atoms
    • -the rays 1 and 2 impinge on layers of atoms that are distance d apart
    • -
  4. What is Braggs law? What does the diagram look like?
    • this law demonstates that interference effets are observable only when radiation interacts with physical dimensions that are apporximately the saem size as the wavelentgth of the radiation
  5. What is d-spacing?
    distance between planes of atoms in a crystle structure
  6. why do peaks of different minerals have diferent position and intensities
    Intensity of peaks depend on number and kind of atoms present along a plane

    position of peaks depends on size and symmetry of a unit cell
  7. how is powder X-ray diffraction used to identify a mineral?

    What do peak intensities show?
    finely grounded powder packed into a sample holder, grain size <0.0704mm

    powderis used so that the specimen contains a veyr large number of tiny randomly oriented crystals. (not good for micas)

    • -use copper x-ray radiation
    • -run deterctor through a 2¬†angle between 5 dgrees and 70 degrees
    • - peaks in intensitiy indicate that the mineral possesses atomic planes who d-spacing is appropriate to "reflect" X-rays for that particular angle \theta1
  8. Why is powder X-ray diffraction used?
    The crystal must be correctly orientated, so powder methd is good because grains are all in different orientations