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.

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Presentation transcript:

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 X-Ray Source Powdered sample Detector

Sample XRD Pattern

background radiation strong intensity = prominent crystal plane weak intensity = subordinate crystal plane

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)

background radiation strong intensity = prominent crystal plane nλ = 2dsinθ (1)(1.54) = 2dsin(15.5 degrees) 1.54 = 2d(0.267) d = 2.88 angstroms

d-spacingIntensity

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 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