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There are many different microscopy techniques in use across the various scientific industries. Some microscopes are used in many different scientific fields, whereas others have found most of their use in specific fields. In this article, we’re going to look at thin-section microscopy, which has found a lot of use within the geological, mining, and mineralogical fields.
Thin-Section Microscopy and its Applications
Thin-section microscopy is a type of polarized optical microscopy that measures a thin section (or an ultra-thin section) of rock, ore, mineral, or other geological samples. It is one of the widest used techniques for analyzing rock and mineral samples as a stand-alone technique. If more information is required on a sample, thin-section microscopy can also be used in conjunction with other complementary microscopy techniques, such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
Thin-section microscopy is a non-destructive technique that can be used to analyze both crystalline and amorphous samples, and is primarily used with solid samples. It is a technique that is known to have a high spatial resolution—hence it is complimentary with other high-resolution microscopy techniques—and it can be used to study the phases in the texture of the sample, such as the structure, fabric, phase assemblage, phase relationships and reaction textures of the material’s surface, by identifying specific optical and morphological properties on the surface of the material. These distinct phases in the surface of the material can also be used to identify the sample if it is unknown. Thin-section microscopy can also be used to provide an estimate on the chemical composition of the material as well as insights into the history of the material.
In most cases, the rocks and/or mineral samples being analyzed are very hard in nature (there are some which hare soft, but most are relatively hard). To analyze them under a polarized microscope, the samples need to be cut into very thin sheets. Due to their inherent hardness, very hard cutting tools—such as a saw with diamonds at the end of the blades—is needed. The sample is then made optically flat, which is achieved by making it into a smooth surface. To change the rough (and often coarse) surface into a smooth surface ready for analysis, an abrasive grit is used. The abrasive grit can also be used to reduce the thickness of the sample (if it is too thick), as most samples need to have a maximum thickness of around 30 microns. The thickness is often compared to a reference sample (made of an abundant mineral such as quartz) to ensure that it is accurate.
In some cases, ultra-thin sample sections need to be produced, generally with a thickness anywhere between 2 and 12 microns. It is common to produce ultra-thin sections when the material being analyzed has a high birefringence (as well as for finer-grained rocks and minerals). To prepare the ultra-thin slides, both sides of the sample are worked on and smoothed out. Rather than using an abrasive grit, a fine diamond paste is used to reduce the thickness from both sides, while smoothing them.
When either the thin or ultra-thin sample section has been smoothed out into the desired thickness, it is then placed in a glass slide ready for analysis. The sample is illuminated after the light has passed through a set of polarized optics which are set at right angles to each other. The user can then look through the eyepiece—like any microscope—to visibly see how the surface optical effects arise from the interactions between the polarized light and the surface. These include the change in the color and intensity of the light. As most minerals and rocks have distinct optical features under this light (which arises from how the crystalline lattice or amorphous structure refracts, absorbs and reflects light), it enables them to be easily identified without complex software. As well as seeing it personally, like many modern-day microscopy techniques, the microscopes can be coupled with imaging software to take photos of the sample during the analysis.
Thin-section microscopy, like any microscopy technique (or any analytical technique for that matter), does have some disadvantages. Thin-section microscopy is limited in the information it can obtain when the chemical composition of the material being analyzed is a complex solid solution, or it has grains which are very fine in nature—as this prevents the different and distinct surface phases from being identified.
When these scenarios arise, thin-section microscopy is used alongside other microscopy techniques. Even with these disadvantages, thin-section microscopy has established itself as a go-to imaging technique in applications that involve rocks, minerals, and geological samples.
Sources and Further Reading
- “GUIDE TO THIN SECTION MICROSCOPY, Second Edition”- Raith, M. M., Raase P., and Reinhardt J., 2012, ISBN 978-3-00-037671-9