With PetroGlyph you can also study each thin section with an electron microscope. Electron microscopy can be used to determine the shapes and sizes of small grains or even their chemical composition. A electron microscopes are capable of imaging very small areas within a sample. These microscopes (or microprobes) contain a filament of wire, usually tungsten, which serves as an electron source. The filament is heated and the electrons are accelerated toward the sample by an oppositely charged anode. The electron beam passes through several magnetic condensing "lenses" and spray apertures which narrow the beam and focus it on the sample. A vacuum is maintained inside the instrument to avoid filament oxidation and interference with the electron beam. Some materials also give off visible light when irradiated by an electron beam.
The electron microscope techniques used in PetroGlyph include cathode luminescence, backscattered electron imaging (BSE), X-ray derived element mapping, and energy dispersive X-ray analysis.
Cathode luminescence occurs because many minerals emit visible light when they are bombarded by an electron beam. The electrons in the target atoms jump to higher energy levels when bombarded. As the excited electrons drop back down to lower energy levels, excess energy is emitted as light. Defects in the crystal structure and trace element substitutions can control the intensity and color of the emitted light. Thus, a given mineral will not always have the same color or even the same brightness in all rock samples. In the image below, plagioclase glows a bright blue, but amphibole and titanite do not luminesce.
To view the cathode luminescence image of a thin section:
 | Select the Cathode Luminescence icon |
A backscattered electron (BSE) image records the intensity of electrons that bounce off a sample at high angles. Electron backscattering can be the result of a single, high-angle deflection or several lower angle deflections. High-angle deflection is typical of target elements with high atomic numbers. Lower angle deflection is typical of elements with lower atomic numbers and generally results in a greater loss of energy per electron. Thus, BSE images are generally used to reveal compositional differences between grains and to examine chemical zonation within grains. The brightest grains are made of elements with high atomic numbers and those with dark colors are made of light elements. The BSE maps in PetroGlyph are false-color images in order to emphasize compositional variations. In the example shown below, the brightest grains are titanite, the mineral with the highest average atomic number in this sample. The yellow-orange grains are hornblende. The dark grains are feldspar, which has the lowest average atomic weight.
To view the backscatter electron image of a thin section:
| Select the Backscattered Electron Image icon |
Several maps showing the concentration of various elements in each thin section are available on PetroGlyph. Element maps are obtained by examining the intensity of the characteristic X-ray emissions of an element. X-rays are produced while the sample is bombarded with electrons. To form the image, the electron beam is moved across the sample. Emitted X-rays are collected and the data are organized into a map. The brightest areas indicate the highest concentrations of the element, while the darkest areas indicate the lowest concentrations. This technique is useful for determining the minerals present as well as their compositions. The element maps in PetroGlyph are in false-color.
To view an element map:
| Select the Element Map icon |
| Select the desired element map from the menu displayed |
Al element map of amphibolite.
Fe element map of amphibolite.
Energy
Dispersive X-ray Spectra
Energy dispersive X-ray spectra for all mineral phases can also be displayed. Energy dispersive analysis utilizes a type of X-ray spectrometer. Bombardment by a narrow beam of electrons causes each mineral to emit X-rays with energies characteristic of its elements and intensities proportional to their concentrations. The detector produces pulses that correspond to the energy of the X-rays. While the electron beam is focused on a specific mineral, the X-ray energy spectrum is collected over several minutes. Peaks of certain characteristic energies correspond to specific elements.
To view the energy dispersive spectrum for a mineral
| Select the Energy Dispersive icon |
| Click on the grain |
PetroGlyph will display the spectrum for the mineral; the peaks are labeled with the element they represent. You must be in one of the electron microscope modes to use this function.
Energy dispersive X-ray spectrum for hornblende.
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