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

Info icon This 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

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern