Making Petrology Slides with Home Improvement Tools
Introduction
Examining the petrology slides under polarizing microscope is a important method to help geologist investigating the origin and evolution of the parent rock. In order to examine the slide under a microscope, the geologists need to make very thin rock sections (about 30 microns) and mount the thin section onto a slide. The process requires precision cutting, imgregnating, mounting, triming, polishing the rock specimens and finally examining the specimen under a crossed polarizing light microscope. In a research rock laboratory, rotary diamond lapidary saw, thin section saw, lap station, poloshing machines and polaring microscopes are common equipments. The cost of these machines can be out of reach for more hobbysts. In the exercise, I am using some regular household home improvement equipments to create thin section slides and a regular compound microscope using polarize film and camera lens for examination.
Equipments and Materials
Figure 1 Rock thin section equipments: Dremel 300 rotatory tool with diamon cut-off wheel and stand, #500 and #1000 dry/wet silicon carbide sand paper.
Figure 2 a regular compound light microscope was used to examine the slide. Noted that a piece of polarizing film was sit on the top of stage and a linear polarizer on the top of light source.
- Dremel 300 rotatory tool
- Dremel rotatory tool stand
- Diamond cut-off wheel (1-9/16" in diameter) for dremel rotatory tool
- Dry-wet silicon carbide sandpaper #500, #1000
- Rock Chisel
- Scalpel
- Blank slide
- General purpose epoxy
- Summerville lace agate
- Compound microscope
- Polarizing film and linear polarizer
- Microscope camera for taking picture
Method
Figure 3. Unprocessed surface of a summerville lace agate.
Summerville lace agate (Figure 3) containing mostly quartz was used in this exercise. Size reduction: The agates were first broken up to small pieces with rock chisel. I had many attempts trying to make cube but most pieces were in irregular shape. I choosed a piece about the size slightly larger than a pea (~6mm in all dimensions). Grinding: The piece was grinded to create flat surface at in each dimension with Dremel N-300 rotatory tool with 1-9/16" inch diamond cut-off wheel (Figure 4). I pay particular attension to one dimension and made certainly that the surface was flat because I would use this side to glue to the slide. Sanding: The flat side then was sanded with #500 grid wet/dry silicon carbide sandpaper. To sand the specimen, the specimen was pressed against the sandpaper gently and moved in circular motion and dipped into water to make sure the sanding surface was wet during sanding. I visually examined the surface until the visible scraches were gone then used #1000 sand paper for another ten minutes. The rock were rinsed and brushed with regular tooth brush and examined under stereo microscope to makesure that the rock is free of silicon carbide. Bounding: The rock then leave dry overnight before bound it to a microscope slide. A regualr epoxy were used for bounding - I carefully mixed the content from the two tubes to minimize bubble formed during the process. Epoxy was applied to the flat surface of the agate then pressed against the slide to make sure the flat surface were lay flat on the slide. I wait another twenty four hours to allow the epoxy to dry and formed strong bounding between the agate and the slide. Thin section: Most agate bound to the slide were cut-off with Dremel rotatory tool again with the cut-off wheel again to leave about 1.5mm (Figure 5). The cutting was very slow since most of the diamond was one the side rather than the edge of the cut-off wheel. More grinding: More agate was removed using Dremel rotatory tool. The thickness of the agate was checked occassionally under polarizing light microscope which consists of a regular light microscope, polarizing film and linear polarizer (Figure 2). The linear polarizer can be turned to crossed position which results in a very dark background. The image was compared with the Michel-Lévy interference colour chart to determine the thickness of the thin section. Hand-sanding again: Until the thickness reached about 100 to 200 microns, I switched to hand-sanding with #500 silicon-carbide sandpaper (Figure 6). Again switch to #1000 sandpaper when the thickness reached about 40 microns. Final Touch: The thin section was clean thoroughly to ensure free of silicon carbide and dried overnight. Snall amount of epoxy was prepared and applied to the surface of the agate and covered it with a cover slide.
Figure 4. A break up piece of summerville lace agate. (This is not the same piece that I worked on. I just wanted to show you the size of the specimen that I choose to process. The side of the agate was flat but not the bottom. The bottom needs to be completely grinded and sanded flat.)
Figure 5 The majority of the agate was cut off the diamond cut-off wheel
Figure 6 Sanding the lace agate with silicon carbide sandpaper.
Results
Figure 7. Picture of summerville lace agate under polarizing light microscope*. The picture was taken right after the grinding with Dremel tool and before sanding with silicon carbide sandpaper. The color appears to be in second or third order so I estimated that the thickness was about 100 to 200 microns.
Figure 8. Picture of summerville lace agate under polarizing light after first pass of sanding.
Figure 9. Picture of summerville lace agate under polarizing light after second pass of sanding
Figure 10A
Figure 10B
Figure 10C
Figure 10. The picture of summerville lace agate under polarizing light microscope. The pictures were taken after final sanding. The picture 10A showed completely yellowish color which corresponding to about 40 microns on Michel-L'evy interference color chart for quatz (birefrigence of 0.009). Picture 10B and 10C showed some pale-tray blocks. These crystals appears to be in about 30 microns range.
Conclusion
In this exercise, I demonstrated that it is possible to use home improve tools for petrology slide making. With some patience, it is possible to achieve the thickness required for polarizing light microscope examine, however, it lacks some consistencies. It is also less efficient. It took me three evenings to make this slides but this is my first attempt. I have to frequently check the thickness due to the lack of experiences. The specimen produced in this method is much smaller than the regular petrology slides. In this exercise, the dimension of the specimen was about 6mm for easier handling. The width of the 4mm diamond cutting wheel is 1.2 mm, it is possible to make the thin section with 1cm wide but that requires some practice. Although I have the Dremel rotatory tool stand, I did not fully utilities. The Dremel tool was mounted on the stand but I used my hand to hold the two ends of the slides for grinding and cutting. It is less stable than having a cuting and griding that I can push the slide along while cutting and griding. The process will be easier and more consistant.
* The pictures of summerville lace agate under polaring light microscope were taken with Tucsen 3.3 MP Cooled CCD microscope camera.
Improvement after my initial attempt:
Other thin section instruction I found on the web:
Thin section instruction by Johnson State Rock laboratory
How to make rock thin sections, without much equipment by Greg Mchone