Transcript X-Ray Diffraction - Earth Science

X-Ray Diffraction
The XRD Technique
• Takes a sample of the material and places a
powdered sample which is then illuminated with
x-rays of a fixed wave-length.
• The intensity of the reflected radiation is
recorded using a goniometer.
• The data is analyzed for the reflection angle to
calculate the inter-atomic spacing.
• The intensity is measured to discriminate the
various D spacing and the results are compared
to known data to identify possible matches.
Powdering Samples
• The samples are powdered to give a random
sampling of ALL atomic planes (crystal faces)
• Statistically accurate given samples are
powdered finely AND randomly oriented on
sample holder
– Intensities are a reflection of d-spacing abundance
• Problems arise with minerals that may
preferentially orient on sample holder
– Micas and clays have special preparation techniques
X-Rays
Wavelengths used
for XRD
What is X-Ray Diffraction??
• Crystalline
substances (e.g.
minerals) consist of
parallel rows of atoms
separated by a
‘unique’ distance
• Simple Example:
– Halite (Na and Cl)
• Crystalline substances (e.g. minerals) consist of parallel
rows of atoms separated by a ‘unique’ distance
• Diffraction occurs when radiation enters a crystalline
substance and is scattered
• Direction and intensity of diffraction depends on
orientation of crystal lattice with radiation
Schematic X-Ray Diffractometer
Detector
X-Ray
Source
Powdered
sample
Sample XRD Pattern
strong intensity = prominent crystal plane
weak intensity = subordinate crystal plane
background radiation
Determine D-Spacing from XRD patterns
Bragg’s Law
nλ = 2dsinθ
• n = reflection order (1,2,3,4,etc…)
• λ = radiation wavelength (1.54 angstroms)
• d = spacing between planes of atoms
(angstroms)
• θ = angle of incidence (degrees)
strong intensity = prominent crystal plane
nλ = 2dsinθ
(1)(1.54) = 2dsin(15.5 degrees)
1.54 = 2d(0.267)
d = 2.88 angstroms
background radiation
d-spacing
2.88
2.18
1.81
1.94
2.10
1.75
2.33
2.01
1.66
1.71
Intensity
100
46
31
25
20
15
10
10
5
5
Factors that affect XRD data
• Sample not powdered fine enough
– May not give all d-spacing data (not random
enough)
• Analysis too fast (degrees/minute)
– May not give accurate peak data
• Mixture of minerals??
• Not crystalline – glass!!
Mixture of 2 Minerals
Applications of XRD
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Unknown mineral ID
Solid solution ID (e.g. feldspars, olivine)
Mixtures of minerals
Clay analyses
Zeolites
Crystallographic applications
Material Science
Created by Nicolas Barth
2007
Geology 114A
University of California, Santa Barbara
Source material by Grant Yip