2.0 X-ray diffraction

2.0 X-ray diffraction - XRay Crystallography The most...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: XRay Crystallography The most important technique for mineralogy Direct measurement of atomic arrangement Direct measurement of what was originally deduced from crystal faces XRays Electromagnetic radiation similar to visible light = 0.02 to 100 = 0.002 to 10 nm Visible light = 400 to 750 nm XRay generation Heat filament, which discharges electrons Electron accelerated with 20 to 100 kV toward "target" Target is Cu or Mo (also Co, Fe and Cr) Very hot requires continuous circulation of cooling water Fig. 82 XRay generated K the most intense (highest energy) Continuous spectrum of Xray energy (wavelengths) are produced X-rays K K Electrons Characteristic spectrum when incoming electrons dislodge electrons from K shell Electrons drop from either M shell (K) or L shell (K) Target material Use of Xrays requires single wavelength Must filter out the continuous spectrum, and leave only one of the characteristic spectrum Typically K peak most intense Similar to one color of visible light Absolute length is important for measurement Referred to as Cu K radiation Filtering Use a monochrometer Typically a thin piece of nickel foil Foil allows most CuK radiation to pass Blocks all wavelengths shorter than K X-rays pass through filter Absorption edge, Ni filters out these wavelengths X-rays blocked by filter Variety of detectors Detection Detectors are arranged so that Xrays reflected off of mineral surfaces can be recorded Detector Sample Scintillation counters (light flashes) Gas proportional counters XRay diffractometer XRay diffraction Wavelength of X rays 1 to 2 About the same length as spacing of atoms in minerals Cu K = 1.5418 Typically 1 to 2 Called d spacing XRay diffraction Waves in phase, only if angle is such at that the additional distance pqr traveled by wave 2 is an integer number of wavelengths (here 1 wavelength) pqr = n pq = d sin Bragg Equation pqr = n pq = d sin pqr = 2pq = 2d sin = n Bragg Equation Planes of atoms Example Halite {111} planes have d spacing of 3.255 Cu K radiation, = 1.5418 Solving Bragg equation shows = 13.70 for n = 1 = 28.27 for n = 2 = 45.27 for n = 3 = 71.30 for n = 4 When X-rays hit mineral at these angles, they will reflect off with constructive interference Multiple possible atomic planes {110}, {100}, {001} etc. Orienting a single grain unlikely to reflect Xrays off of any of these planes Better to use multiple grains with random orientations Powder Diffraction method Powder Diffraction Sample crushed to small size, typically < 0.05 mm Placed on glass slide or hollow holder Sample placed in XRay diffractometer Blasted with Xrays as sample and detector rotate from around 2 to 70 Strip recorder records intensity of signal from detector Intensity of reflection Degrees 2 Data reduction Powder diffraction files 4 major peaks, d spacing All peaks, d spacings relative intensity, reflecting plane Cards with the intensity and d spacing for all minerals Take rock, sediment, mineral sample Grind sample Mount Measure all d spacings Compare with the powder diffraction files ...
View Full Document

This note was uploaded on 07/06/2011 for the course GLY 5245 taught by Professor Staff during the Spring '11 term at University of Florida.

Ask a homework question - tutors are online