X Rays & Crystals Characterizing Mineral Chemistry & - - PowerPoint PPT Presentation
X Rays & Crystals Characterizing Mineral Chemistry & Structure J.D. Price Light - electromagnetic spectrum Wave behavior vs. particle behavior If atoms are on the 10 -10 m scale, we need to use sufficiently small wavelengths to
J.D. Price
Wave behavior vs. particle behavior If atoms are on the 10-10 m scale, we need to use sufficiently small wavelengths to explore this realm if we want to learn something about atoms and lattices.
E.B. Watson
E.B. Watson
wave property
E.B. Watson
Crystalline structure diffracts x-rays (XRD)
Crystal with unknown d spacing X-ray source with known
Diffraction lines are generated by any plane within the crystal
the root planes to the unit cell, but it also includes all of the possible diagonals. Miller indices are used to label to the lines resulting from the planes (you know all about indexing). In a powdered sample, grains typically orient in a myriad of directions*, such that many diffraction lines are simultaneously generated
*exception – sheet silicates
(100) 4.1341! (011) =3.259! (110) 2.3868!
The rather ordinary rock slab on the left is composed of small (1- 5mm) grains of quartz and feldspar. The feldspar below is large (15 mm) but is concentrically zoned.
Feldspars are solid- solutions and exhibit a range of compositions. How might we determine the composition of the minerals in our rocks? What is unique about each element?
M M T T T T
Electron structure is element specific. In other words, Ephoton is the result of a specific jump in a specific element. Fluorescence: electromagnetic radiation results from moving electrons closer to the nucleus
Photo by Elizabeth Frank
Visible light is produced by energies in U.V. light.
Examples of transition levels in Barium K 37.44 keV LI 5.99 keV LII 5.63 keV LIII 5.25 keV So LII to K (K 1) is… 31.81 keV
Heavier atoms have many energy levels
So LIIto K is 31.81 keV or 31,810 eV The wavelength of the photon produced by this jump is = h c / E h = 6.626 10-34 m2 kg/s c = 3 108 m/s E = 31,810 eV 1.602 10-19 J/ eV = 5.096 10-15 J So = 3.900 10-11 m
Electrons are charged particles that can be focused and redirected using a magnets Lower energy example: the CRT Raymond Castaing formulated the technique for microanalysis and built the first working unit by 1951.
Recall that crystalline structure diffracts x-rays (XRD)
Crystal with unknown d spacing X-ray source with known
Castaing’s machine: focused electron beam that produces x-rays in an unknown, that may be counted at known diffraction angles. Wavelength dispersive spectrometry (WDS)
The intensity of x-rays is much smaller relative to those generated from a tube (as in XRD) The EMP wavelength spectrometer uses crystals with curved lattices and ground curvature to reduce lost x-rays
Locate the following: Cathode and anode Beam Magnets Sample Crystal Detector
The Cameca SX100
The new RPI facility Cameca SX 100 EMP Rontec EDS detection Gatan mono CL
Electron forces jump
Glancing background phn Produced photon adsorbed – may produce Auger e- Electron bounces off atom (high E): backscattered Electron knocks out another e- (low E): secondary
EMPA does not analyze surfaces (thin film), but penetrates a small volume of the sample. The collectable products of electron collision origin originate from specific volumes under the surface.
Secondary electrons emitted from the first 50 nm Images surface topography Backscattered electron intensity are a function of atomic density Images relative composition
The x-ray volume changes as a function of a number variables. A sample with higher average atomic density will have a shallower but wider volume than one with a lower density. A beam with higher energy (keV) will produce a larger volume than one with a lower E0.
From the excitation volume behavior, it is clear atomic density (Z) makes a difference in the emitted intensities. Some of the x-rays are absorbed into atoms within and adjacent to the excitation volume. Some of the x-rays promote electron jumps in atoms within and adjacent to the excitation volume.
The number of x-rays counted at the appropriate diffraction angle is proportional to the concentration of the fluorescing
Quantification requires comparison to a well-characterized standard. Standard analyzed by
Your sample with unknown composition
Castaing’s micro WDS machine was a breakthrough. By 1960, advances in semiconduction permitted the construction of a new detector that could collect all of the emitted x-ray energies (pulses and background) within a few seconds.
EDS spectrum for a 15kV beam on a gemmy crystal from the Adirondacks (M. Lupulescu, NYSM). Al K & Si K & K K K K
EMPA traverses of spinel using WDS Formula for the spinel