Optical Microscopy, Electron Microscopy, XRD and XRF
Numerous instrumental methods are used in the analysis of geological materials, trace elements, and molecular structures of specimens. The decision on the type of method or instrument to use depends on the specific type of material being analyzed. There has also been much advancement in the instrumental techniques which have resulted to more powerful and precise instruments.
Types of Microscopy
- Optical Microscopy
Optical microscopy employs the use of electromagnetic spectrum in the range of visible light which is refracted through a series of lenses to achieve a virtual or magnified real image.
Depending on the magnitude required, a single piece of the lens or an array of lenses appropriately arranged can be used to magnify a minute particle being observed. An optical microscope hence can be called a light microscope.
If a single lens is used to, then this is termed a simple light microscope while if there are more than one lens e.g. an objective lens, a condenser unit and an eyepiece lens then such a lens can be termed a compound lens.
Since the power of an optical microscope is relatively low, to observe the finer details of a rock sample, selective staining of the surface can be used by using an appropriate staining medium such as iodine.
Optical microscopes almost entirely use convex lenses due to their image formation properties and the ease with which they bring images into sharp focus. The main image forming property of a convex lens, in this case, is the fact that the image is always real hence can be passed on to the next lens as a real object making its magnification feasible.
Along the path of the lens array, when sufficient magnification has been attained, an eyepiece lens can then be used to view the image. In this case, the eye is used to view the image hence the image should be virtual. It is, however, possible to direct a real image onto a camera. The camera will then capture and process the image and display it on a screen. Once this image is digitized, it can then be analyzed to finer details using computer programs that have the ability to estimate the volume of a particular mineral depending on the approximate area it covers on the image. This digital technique is hence an important tool used to find the relative composition of rocks with simple, recognizable grain structures.
Since such an analysis would only be based on the grain size and shape, it is not possible to very accurately categorize a rock based on its exact chemical composition but based on such physical features only.
Above is the image of Dunite under an optical microscope using polarized light.Note that the colors are easily discernable, but it is not possible to state the exact mineral composition.
The image of Dunite under an optical microscope using polarized light.
1. Optical Microscopy
2. Electron Microscopy
Electron microscopy exploits the science behind the interaction of electrons with matter. Some of the scenarios expected when an electron is injected into a target atom are:
a) The electron may be completely absorbed it had just the right amount of energy for the energy level it occupies upon entry into the target atom. This is not a practical scenario
b) The electron may be absorbed with a similar release of a characteristic for of energy e.g. light
c) The electron may dislodge/knock off more electrons from the target atom hence some emitted electrons are detectable around the target atom.
It is important to note that the emission of electrons from a particular atom occurs at particular energy levels, and the corresponding and accompanying energy emission is a characteristic that can be used as a signature to a particular atom and hence element within a rock
SEM is the method used to read off these backscattered electrons from a surface and use this information to determine the topography of the specimen and hence create an image of the same since the number of electrons scattered in a particular direction has a significant relationship with the orientation of that surface.
Since the electron has a higher depth of penetration compared to the visible light used in the optical microscope, the SEM has a greater depth of view and can also produce a finer detailed and three-dimensional images.
It’s possible to characterize the mineral composition of a rock using SEM as each mineral has a signature electron emission capability under the same electron beam. The resolution of SEM can be improved by improving the focus of the electron beam.it is much higher than the resolution obtained when using a light microscope.
Mineral composition of a rock using SEM
The image is an SEM image of graphite.
3. EDS (Energy-Dispersive X-ray Spectroscopy)
This is a method that relies on the principle that each element has a unique atomic structure that will interfere differently with an X-ray beam allowing unique/signature of peaks on its X-ray spectrum. This signature can then be used to map out the elements contained in the rock sample.
As outlined in the previous section above, a beam of electron bombarding a target atom can cause the emission of a corresponding and characteristic energy form. This can be in the form of light, but more often than not, it takes the form of x-ray. It is hence possible to track the pattern of x-ray emitted after a beam of electrons has been injected on a sample and depending on the peaks, the mineral characterization of a rock sample is possible.
4. XRF (X-ray fluorescence)
Just like bombarding a target atom with electrons may lead to the emission of electrons from the atom accompanied by a characteristic X-ray spectrum, bombarding an atom with an x-ray beam may lead to a similar behavior. In this case, however, an electron will absorb the x-ray energy and achieve a higher energy stage that may even lead to the dislodging of the electron from its parent atom. This reorganization will lead to the production of yet another x-ray spectrum that is characteristic of the parent atom.
It is hence reasonable to say that the result of both the EDS and XRF methods are the same (X-ray spectrum with characteristic peaks for natural minerals).The only difference is hence the means of achieving this spectrum. The EDS uses an electron beam to excite the sample to emit X-rays while XRF uses either gamma radiation or X-ray beam to induce secondary radiation. However, since electrons are negatively charged and hence are bound to experience repulsive interaction with the electrons in the sample, both the gamma rays and X-rays are not charge bearing hence XRF is likely to have a higher depth of investigation.
The resulting spectrum can hence be used to map the element component of a rock sample
5. XRD (X-ray Powder Diffraction)
This method is used to identify compounds based on their diffraction pattern. XRD is a logical method that is useful in the phase detection of crystalline materials. The action of XRD is based on its basic principles which are as follows: the monochromatic X-rays’ constructive obstruction and crystalline specimens. There is a cathode ray tube that produces the x-rays that are filtered to generate monochromatic radiation then determined through collimation before being aimed at the specimen. Productive obstruction is created through contact of incident rays. The diffracted x-rays are detected then counted.
Microscopes are fundamental and pivotal in determining the structure and composition of a rock sample. The technique employed depends on the type of analysis and properties targeted such as resolution and depth of investigation.