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Leslie Kanat – October 2006 The following instructions are to aid in the production of thin sections using the equipment in the Johnson State College Rock Laboratory. The equipment was purchased with funds from a National Science Foundation grant and money for construction of the room was supplied by the College. The Rock Laboratory is to be used by students, staff, and faculty to make thin sections for teaching and research, and to make polished rock slabs for display. A set of instructions for the preparation of thin sections follows. In order to develop a thin section, one must start with a rock chip of a size that will fit onto a glass slide. If the rock is large, then cut a 1-3 cm thick slab on the slab saw. Make sure the rock is solidly seated in the vise. All rocks cut on the slab saw must be washed immediately because the oil-based cutting lubricant. Take the slab, or a small specimen, and use the trim saw to cut the rock chip to the proper size. The rock chip should have parallel sides and no rough edges. Grind the rock chip on the lap using the 45µ plate. Continue to grind the chip until all rock saw cuts and undulations in the surface are removed. Use #600 grit on the #600 glass plate and polish the rock chip. One should use either a circular or a figure eight motion when polishing the chip. Make sure to utilize the entire glass plate so it wears evenly. It is imperative that all scratches are removed from the rock chip and that it is perfectly planar. The chip is finished when the reflection of the polished surface is shiny and continuous (view when dry and at a low angle to light source). The polished chip must be washed thoroughly with warm, clean water, and gently scrubbed with a toothbrush. All grit must be removed from the chip. Place the chip on a hot plate, at low heat, in order to dry it. The glass slide, to which the chip will be bonded, must be frosted with #600 grit in a similar fashion as described previously. Thoroughly wash the glass slide, using a toothbrush, and let it dry. Once the chip and the slide are dry, they should be cleaned with isopropyl alcohol. Wipe the surface of the chip and slide with a dust free cloth and place in a dust free environment. The epoxy should be mixed thoroughly, although slowly and gently, in a plastic mixing boat. Do not induce air bubbles into the epoxy by the mixing process. Continue mixing for one full minute. Use a metal spatula and evenly spread the epoxy on the polished and cleaned rock chip. Place the chip on the table with the epoxy side face-up. Place the slide onto the chip at an angle, starting at one side of the chip and gently lowering it until the entire chip is in contact with the slide. There should be about 1 cm of glass at the side of the chip where contact was initiated; the amount of glass on the other side will depend on the length of the chip. Work out all air bubbles by slightly moving the slide on the chip. The eraser end of a pencil may be of assistance at this point. Once all air bubbles have been removed turn the chip over and place it on the bonding jig. Make sure the chip is aligned with the edges of the glass slide. Let it rest undisturbed for at least 18 hours. Remove the bonded chip from the jig and pencil in the identification number on the frosted side glass slide. Use a razor blade and remove any glue that may be found on the bottom of the glass slide. Use a diamond awl to write the number on the glass on the reverse side. Make sure the arm of the cut-off saw is clean and wet. Wet the back of the slide and place it onto the arm of the cut-off saw in such a way that the portion of the slide which has the most glass is farthest away from the blade. Hold this part of the slide securely, and slowly proceed to cut the chip off of the slide. Place the slide, with the chip cut off, on the grinder. Advance the grinder at 2µ per pass. One pass is defined as a complete cycle (back and forth) over the diamond grinding wheel. Advance by 0.5µ as the section becomes thinner. Take the rock chip down to a thickness of 33µ. Rapid grinding will fracture the minerals within the thin section – go slowly. Use #600 grit to take the thin section down to exactly 30µ. Quartz and feldspar should look slightly straw-yellow in polarized light when the thickness is 30µ. Thoroughly wash the thin section, using a toothbrush (softly) and let it air dry. Do not use heat at this point. Once dry, clean it with isopropyl alcohol in order to prepare it to be covered. Clean the cover slips with isopropyl alcohol and dry with a dust free cloth. Place a drop of Canada Balsam in the center of the chip on the glass slide and gently lower the cover slip into place. Gently work out any air bubbles which may exist. Once the glue has dried, use a single edged razor blade in order to trim excess glue from the thin section. The thin section is now finished. |
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