week6_Lecture_16 - Microanalytical Techniques III

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Purdue University *

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243

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Oct 30, 2023

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EAPS 243 (Micro)Analytical Techniques I D+G+T Chapter 9 – Relevant Techniques We will discuss the different types of microanalytical techniques we use in the laboratory to understand the mineralogy and chemistry of rock samples.
zircon Stibnite Dioptase Topaz Vivianite Corundum Mercury Ammineite Calcite Perovskite Arsenuranospathite Cacoxenite Kyanite Rhodochrosite Zircon Stibnite Dioptase Corundum Perovskite Rhodochrosite Kyanite Calcite Calcite Zircon Stibnite Perovskite Zircon Perovskite Zircon
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Midterm Review • Mixture of question types • Short answer, matching, fill in the blank, multiple choice, identifying things on an image, basic calculations (bring a calculator)
Labs this week – Time of lab classes will be the same. Meet in front of Wetherill Hall Room 101 Wear closed-toed shoes. Email me if you get lost/can’t find the group.
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+ VNIR Reflectance Spectroscopy
How does it work? What’s the external stimulation? What happened to the electron in the inner shell? What’s the difference between K a and K b ? What’s the difference between different atoms?
EDS vs. WDS • Energy dispersive spectrometers (EDS) sort the X- rays based on their energy • More efficient for an unknown specimen • Measures across a broad range of energies in the spectrum • Wavelength dispersive spectrometers (WDS) sort the X- rays based on their wavelengths • Superior peak resolution of elements • Superior sensitivity of trace elements
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How does it work? A finely focused beam of electrons (the energy source) displaces inner- shell electrons in the atoms of the material being studied. When outer- shell electrons fill these inner-shell vacancies, X-rays are produced with wavelengths that are characteristic of the elements present. Electron Microprobe Analyzer (EMPA) Same technique as SEM and TEM What is this detector?
Electron Microprobe Analyzer (EMPA) Wavelength Dispersive Spectrometer (WDS) WDS sort X-rays based on their wavelength using an analyzing crystal and a detector. X-rays hit the crystal, diffract (Bragg’s Law!) and enter the detector. Only X-rays of a given wavelength will enter the detector at any one time. To measure X-rays of another wavelength, the crystal and detector are moved to a new position.
A finely focused beam of electrons (the energy source) displaces inner-shell electrons in the atoms of the material being studied. When outer- shell electrons fill these inner- shell vacancies, X-rays are produced with wavelengths that are characteristic of the elements present. Et voila! Now you know what ’ s in the minerals and rocks you ± re investigating! X-ray wavelengths measured using X-ray spectrometer or an energy-dispersive detector (EDS) Electron Microprobe Analyzer/ Electron Probe Microanalyzer (EMPA) Commonly used tool to determine the chemical compositions of minerals (EMPA)
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Strengths of EMPA and WDS • Non-destructive quantitative analyses of spots as small as a few µm, enabling quantitative chemical analyses of e.g., mineral zoning • Detection levels as low as a few 10s of ppm • When the electron beam is rastered, the WD spectrometers can allow X-ray image maps of individual elements to be constructed
Limitations of EMPA and WDS • Because WDS cannot determine elements below atomic number 5 (boron), several geologically important elements cannot be measured with WDS (e.g., H, Li, and Be). • Despite the improved spectral resolution of elemental peaks, some peaks exhibit significant overlaps that result in analytical challenges • WDS analyses are not able to distinguish among the valence states of elements (e.g., Fe 2+ vs. Fe 3+ ) • The multiple masses of an element (i.e., isotopes) cannot be determined by WDS • Only for solid materials
Reflectance Spectroscopy What happens when we shine light (photons) onto a surface? 1) Some photons pass through the grain 2) Some photons are reflected from grain surfaces 3) Some photons are absorbed, through several processes
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Reflectance Spectroscopy • The variety of absorption processes and their wavelength dependence allows us to derive information about the chemistry of a mineral from its reflected light The fraction of light that is reflected back What’s happening here?
Reflectance Spectroscopy For this class, we will mostly worry about the visible – near/shortwave infrared wavelengths (0.3-3.0 μm) This spectral region is commonly used in earth/planetary science because it is extremely sensitive to both primary and secondary minerals: 1. Water, on the surface and in hydrated minerals 2. Other absorptions in alteration minerals 3. Iron in many igneous minerals On Earth, this range is also sensitive to water, chlorophyll, and other absorptions in vegetation and soils.
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Absorption Features/Bands • Absorption features/bands can behave like fingerprints for a type of rock/mineral/surface • Similar to X-ray energy and wavelengths being indicative of specific minerals, so too are absorption bands
What causes those absorption features? 1) Electronic transitions (near-IR) • When an atom absorbs a photon of a given wavelength, its electrons move from a relatively low electron state to a higher one 2) Vibrational transitions (mid-IR) • Vibrations are quantized (only certain frequencies are allowed) so only photons of specific energies can excite a vibration 3) Rotational transitions (mid-IR) • The bonds in a crystal lattice or molecule vibrate like springs.
Photons can excite crystal field absorptions in transition metals near 1 and 2 μm Olivine and pyroxene exhibit strong crystal field absorptions. Fantastic for investigating mafic planetary surfaces!
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Other Important Absorption Features: Water 1. Liquid water on the surface 2. Atmospheric water vapor 3. Adsorbed water on the surface 4. Water within mineral structures Liquid water on the surface is almost totally absorbing beyond 1 micron
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Adsorbed water can darken a surface and cause large water absorptions at 1.4/1.9 microns 1.4 μ m 1.9 μ m Wet soil Dry soil Interlayer adsorbed water in clays Phyllosilicate layer (~1nm) Interlayer H 2 O (~0-4nm) Types of adsorbed water
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SO 4 -OH Fe Many minerals that form due to interactions with water contain water within their crystal structure Water is part of the crystal structure of hydrated minerals like Gypsum CaSO 4 •2(H 2 O) S-O 1.4 1.9 H2O
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Which spectrum is montmorillonite, and which is kaolinite? OH and H 2 O have different absorption bands – only H 2 O exhibits a strong 1.9 um band
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Strengths of Reflectance Spectroscopy • Spectroscopy can be used up close (e.g., in the laboratory) to far away (e.g., to look down on the Earth, or up at other planets) • Sensitive to both crystalline and amorphous materials • The variations in material composition often causes shifts in the position and shape of absorption bands in the spectrum enabling detailed understanding of mineralogy • Minimal sample preparation required
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Limitations of Reflectance Spectroscopy • The variations in material composition often causes shifts in the position and shape of absorption bands in the spectrum which can make interpretation very complicated • The vast variety of chemistry typically encountered in the real world, spectral signatures can be quite complex and sometimes unintelligible • Requires a spectral library to identify minerals
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Microscope, SEM, TEM 28 ~200 kV, ~0.2 nm resolution ~20 kV, ~2-5 nm resolution 400 nm h: Planck’s constant, m 0 : rest mass of an electron, E: kinetic energy of the accelerated electron The maximum resolution d : Bottom up Bottom up up Bottom transmission transmission reflection
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29 STEM In a STEM the electron beam is focused into a narrow spot which is scanned over the sample in a rastering mode . EDX SEM EDX SEM SEM-BSE SEM-CL HAADF (Z-contrast) EELS
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e - EDS Spatial Resolution = 0.14 nm (atomic); World Record = 0.05 nm EELS Transmission Electron Microscopy (TEM) Similar to an SEM, a focused beam of high energy electrons are accelerated towards a sample. However, our sample has been thinned to electron transparency (<100 nm).
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JEOL JEM2100F TEM ThemisZ
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1) Ultramicrotomy: Diamond Knife Sample <100 nm Sample Preparation for TEM
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