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Lecture 21 (12/11/2006) X-Ray Diffraction (XRD) Theory and Analytical Technique PowerPoint PPT Presentation


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Lecture 21 (12/11/2006) X-Ray Diffraction (XRD) Theory and Analytical Technique. X-Ray Analysis. X-rays discovered in 1895 Fundamental to understanding of crystal structure and symmetry

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Lecture 21 (12/11/2006) X-Ray Diffraction (XRD) Theory and Analytical Technique

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Lecture 21 (12/11/2006)X-Ray Diffraction (XRD)Theory and Analytical Technique


X-Ray Analysis

  • X-rays discovered in 1895

  • Fundamental to understanding of crystal structure and symmetry

  • Powder diffraction analyses are a simple and inexpensive method for identifying minerals, especially fine-grained minerals


X-ray Generation

X-rays – High energy*, highly penetrative electromagnetic radiation

*E = hc/λλ(X-rays) = 0.02-100Å (~1)

λ(visible light) = 4000-7200Å

X-ray Vacuum Tube

Cathode (W)– electron generator

Anode (Mo, Cu, Fe, Co, Cr) – electron target,

X-ray generator


X-ray Spectra

  • Continuous spectra (white radiation)– range of X-ray wavelengths generated by the absorption (stopping) of electrons by the target

  • Characteristic X-rays – particular wavelengths created by dislodgement of inner shell electrons of the target metal; x-rays generated when outer shell electrons collapse into vacant inner shells

  • K peaks created by collapse from L to K shell;

    K peaks created by collapse from M to K shell

K

K

X


X-ray Diffraction (Bragg Law)

nλ = 2d sinθ

Defines the spacing (d) of atomic planes and incident angle (θ) at which X-rays of a particular wavelength will reflect in phase (i.e., diffract)

GE+EH = nλ

θ’

≠ nλ


Powder Diffraction Method

  • Requires random orientation of very fine crystals

  • Incident beam of a certain X-ray wavelength will diffract from atomic planes oriented at the appropriate θ angles for the characteristic d spacing

  • Random orientation of crystals will produce more intense diffraction peaks for particular angles that correspond to characteristic atomic planes


Powder Diffraction Plots

θ=arcsin (nλ / 2d)

λ(Cu) = 1.54Å

d - Qtz [101] = 3.342

θ = 13.32° ; 2θ = 26.64°

Quartz


X-Ray Powder Diffraction Records70,000 experimental patterns


Utility of XRD in Chemical ID


XRD Structural State Analyses

Silver Bay anorthosites

LPD

Duluth Complex

Anorthositic Series

Silver Bay anorthosites and Duluth Complex

Anorthositic Series analyzed by Miller (1986)

Leveaux Porphyritic Diorite (LPD) analysed by Albers (2006)

(diagram after Smilth, 1975)


Last Lecture 12/13/06

Final Review:

Optical Mineralogy Trivia


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