X-ray crystallography - CH8
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What are different analytical techniques that geologists use to identify minerals?
- Petrographic microscopes
- electron microscope
- scanning electron microscope
- mass spectrometer
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
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
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
What is d-spacing?
distance between planes of atoms in a crystle structure
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
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
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
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