Coherent Petrographic Material Preparation
Petrology is the study of rocks and how they form. Petrography is under petrology but concentrates on the textural relationships within rocks and its mineral content. The most essential tool used in petrology is, of course, the petrographic microscope.
Preliminary Petrographic Material Examination
Samples need to be obtained from rocks since the latter could be too large to fit a microscope’s stage. The process of extricating samples is, however, mainly destructive so one should first take preliminary notes and photographic evidence of anything that could affect the process of extricating and processing rock samples. These include but are not limited to the following:
Location – Where will you obtain the rock sample from? To whom does it belong? What kind of place is it? Are there any field, environmental, and weather conditions that could affect your sample?
Sample Condition – What is the appearance and condition of the sample? Are there any unusual features you’ve noticed? Evaluate its surface. What about embedded materials? Are there any, and if so, where is it embedded, how large is it, and what is its condition? Look for any signs of sealants, joints, cracks, and voids as well as physical and chemical deterioration.
Techniques in Petrographic Material Preparation
When preparing petrographic materials, you should be careful to follow the steps systematically and completely. Missing or overdoing a step could both damage your sample. There is a variety of techniques you can use for preparing samples in petrography. You can generally use two or more techniques right after each other when preparing a petrographic sample.
Cleaning – Samples must be cleaned before and during preparations to wash away the debris found when the sample was taken from the site and those produced naturally in each preparation step like sectioning and polishing. You must only use appropriate cleaning materials and ingredients as something especially potent could also damage the quality of the sample.
Sectioning – Taking a thin section as sample is required when the rock you wish to observe is too big. In sectioning, there are usually advised shapes for certain types of rocks to reveal as much detail as possible.
Lapping and Grinding – This technique can remove any saw marks made during sectioning as well as any other surface irregularities and deformations. Lapping and grinding will ensure that the surface of your sample is even, smooth, and with a matt-like finish. Grinding a rock sample is commonly achieved with a grinding or lapping wheel and finer abrasives such as water or oil-based carrier.
Polishing – When you polish a sample, you can achieve the same effects of lapping and grinding and at the same time cause the surface to appear bright and shiny enough to produce reflection. Polishing is often a follow-up step to grinding, removing irregularities and deformations that the prior step wasn’t able to eliminate.
Tips on Transporting Petrographic Material Preparation
Preparation isn’t done until the sample arrives safely in the laboratory or wherever your petrographic microscope is and placed on a glass slide. While you might have successfully avoided damaging your sample from extrication to polishing, transporting your sample from one place to another has its own share of risks, too.
Use the appropriate wrap for your rock sample. If there’s no bubble wrap available, use one of similar material, something that could protect it from scratches and cracks but soft enough not to cause damage to the sample itself.
Certain rock samples, like shale for instance, do not react well to dehydration. In such cases, using aluminum foil and coating it with wax will ensure that your sample will stay sufficiently moist throughout the trip. Place the packed sample in a sealed back for added protection.
If you have an unconsolidated sample, consider having it shipped express or overnight so that it will arrive at your laboratory in its still frozen state.
Using a Petrographic Microscope
A petrographic microscope is generally operated the same way other microscopes are used but it is dissimilar with other models due to its use of polarized light microscopy. Polarized illumination is made possible due to the inclusion of a polarizer and analyzer in the microscope’s design. The polarizer can be found underneath the microscope’s condenser and on top of the illumination system. The analyzer on the other hand is located above the objective lens.
Because of polarized light microscopy, the petrographic microscope Is capable of supplying great detail regarding absorption color and help the user differentiate anisotropic materials from isotropic ones. Further information about petrographic materials, such as structure and composition, may also be obtained.
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