Lakehead University Geology - Journal Yearbook (Thunder Bay, Ontario Canada)

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Mix a small batch of Araldite Resin (such as 20 ml. of resin to 4 ml. of hardener), stir it well and leave it standing until the mass of air bubbles is out. Remove the Teflon plate and the slab from the oven and quickly apply a surface coat of the resin to the slab, and then return it to the oven and re-evacuate the air. When the resin has stopped frothing, close the vacuum valve and slowly open the air vent. The inrush of air will force the resin into the slab. Leave the unit in the oven to cure for two hours and then allow it to cool to room temoerature. Remove the slab from the Teflon plate and cut it to the size required for thin section. The grinding for mounting is the same as for a hard rock section. The mounting media should be Lakeside 70 and the sectioning is the same as before with the exception that the sandstone can be machined thinner, saving hand grinding. The covering is the same as before. THE IMPREGNATION OF RECENT SEDIMENTS The following report is on the technical procedure of making a thin section followed by a micro-polished surface of a recent fine grained sediment. The sediments which are wet when collected in core form are cut along their length to expose the variation in deposition. The selected parts are removed and dried out slowly to prevent excessive shrinkage and to minimize the number of cracks. Stage I ; The first stage is to grind a flat surface of the selected sample. This is done on dry abrasive paper of 400 grit, and then on 600 grit until the surface is flat, blowing off any excess with a compres sed air jet. Stage 1 1 : The impregnation media is Araldite Epoxy (Resin 502 Hardener 956) (a mixture of 10 parts resin to two parts hardener). Teflon should be used for the mould, as it is a self- lubricating plastic which does not require any releasing agents. It is convenient to have circular rings of varying depths and diameters - these rings are placed on a Teflon plate (one quarter of an inch thick) to prevent bending. The ring is held in place on the plate with a thin layer of silicon grease, which prevents leakage from the mould. Place the sample in the mould, prepare the resin and let is stand to allow any excess air to come to the surface. Wet the surface of the specimen with Methyl Ethyl Ketone, (a resin thinner which invades finer layers, allowing the resin to better impregnate the sample). Pour the resin into the mo uld so that it just covers the sample, and then put the unit in the vacuum oven and start to evacuate the chamber. At 15 pounds pressure, the resin will start to froth. Do not allow it to overflow the mould, and to prevent this close the vacuum control valve and slowly open the vent. The inrush of air will start the impregnation of the sample, a repetition of this process should be done until there is no bubbling under the vacuum. Close the vacuum control, switch off the pump and open the vent slowly. When the vacuum is released, set the oven heater at 60 degrees Centigrade and allow it to cure for two or more hours. Remove the unit from the oven and allow it to cool. Separate the ring from the Teflon plate and press out the sample. The method of preparation for thin section is similar to that of a sedimentary rock, with several exceptions. The surface to be mounted on the slide must be on an even plane with the surrounding resin so that there is no relief between the two. The only way to obtain a flat even surface, if the sediment does not fine grind flat, is by repeated surface impregnation, and careful fine grinding. Use a fresh mix of resin as a mounting cement. Apply a little to the slide and to the fine surface of the sample and press them together. Leave it to set for at least 24 hours. The now thin section is ground to its proper thickness, after machining as before, on a wet glass plate using 1000 Carborundum, clean and cover with thinned Balsam or Permount. The Polished Surface The preparation is the same as for the thin section, with the exception that the sample is moulded in a bakelite ring especially made for the DUrner Polishing Machine. The most important part of this technique is to obtain a hardened flat surface prior to mechanical polishing. To obtain this surface, it is necessary to carefully dry grind the specimen on fine abrasive paper, with frequent examination under the binocular microscope, to 25

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When the sample is cold, trim to the size of a sli ' de and grind to a very fine mounting surface, using either coated carborundum paper (400, then 600 grit) or a glass plate with the same abrasive and water. Wash the sample and ensure that the required surface is whole (not pitted) and even. The Mounting: This can be done using Lakeside 70 or Araldite resin. Use the former if the surface is hard and not likely to twist and buckle when re-heated, which would cause an uneven mount. The latter should be used if this is apt to happen. If the resin is used as a mounting media, apply a small amount to each surface (the glass slide and the sample surface) , press them together to remove all the air and allow it to set at room temperature for 24 hours. This will allow the resin to set sufficiently for grinding. The Thin Section: Cut or grind the sample down to the first stage of transparency, using either a diamond bonded wheel or the carborundum method. It is now that the bonding shows its effect. Due to the differences in structure of the silicates and the manganese crust to transmit ordinary light, it is therefore beneficial to hand grind the now thin section to completion on a glass plate with 600 grit or finer if desired, taking great care to retain the complete surface area. Having completed this stage to the required thickness, wash the section and dry it with a tissue; then remove the surplus Lakeside or Araldite. If the latter is used, trim the outer edges leaving a small border around the section edge, but do not attempt to remove all the Araldite from the thin section as this will result in the destruction of the sample. Clean the surrounding slide with a solvent such as chloroform or acetone. Clean and cover a covering slip with a reasonable amount of Canada Balsam, warm it on the hot plate until the Balsam flows outward over the cover-slip, and in the meantime warm the surface of the thin section. Do this by holding it over the hotplate without touching it, and then laying the two surfaces together. Remove the covered section from the hotplate, then carefully press down the cover-slip to remove all the air. Clean off the surplus Balsam with acetone and finally with chloroform. If the thin section is too fragile to use heated Balsam, use Permount and allow to dry at room temperature. This takes longer, but it helps to retain the structure o f the section. SEDIMENTARY ROCKS The thin sectioning of sedimentary rocks such as sandstones, sandy shales, clay shales, etc. can be difficult when their natural cementing media is a carbonate or some other material (with the exception of silica). It is therefore necessary to impregnate the rock with a material that will act as a bonding substance to retain the grains and structure of the rock in original form. If the rock matrix is a carbonate, either calcite or dolomite, the action of the cutting blade will tear away the carbonate, leaving many loose grains, and the stages of grinding prior to mounting on the slide will have a similar effect, though not so harsh. If the fine-ground slab is washed down and dried, then examined under a binocular microscope, it will be observed that the cleavage- planes of the carbonate have been irritated and are apt to be loose. There- fore, when the slide mounting media is applied to the section slab, and the slide placed in position, a gritty feeling will be observed. The result is a thin section full of holes, due to the fact that all the grain surfaces were not on an even plane. If the same sandstone were impregnated with Araldite Epoxy Resin, all the grains as well as the cleavage planes of the carbonate would be bonded together to create a solid interior which can be thin-sectioned without any difficulty. The same technique may be applied to sandy shales and the fine laminations of shales. Method : Cut the rock with a diamond blade to a thickness of no less than one quarter of an inch, as this will give a sufficient depth with which to work. Dry the slab and place it on a Teflon plate (Use Teflon because of its self- lubricating properties as well as its high melting point) and put the unit in a vacuum oven and bring it to 60 degrees Centigrade. 24

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ensure that a complete mineral surface is present - not one with a microlayer of resin over it. When this is done, and the edges of the mould have been bevelled, it is now ready for the polishing laps. Lap I : The first polishing is done with 6 micron diamond powder. After switching on the lapping machine, clean the lap with acetone and a tissue. An even smear of mineral oil, liquid paraffin is apolied to the lap, and a small finger tip of a 6 micron-diamond is run over the lap. Stop the lap and fit tne specimens to it. The time for the first stage varies as to the texture of the specimen, so it is decided by trial and error. Otherwise, if the lap is running well, one hour should suffice before the first microscopic examination. As the particles of the metallic oxides and sulphides are very small it is advisable to use medium to high magnification to get a fair reading on the surfaces. If the pits are still present, clean the lap, recharge and rotate for a further hour. Lap II: Repeat the same process for the second grade of diamond (l-3 micron) to get more definite results. Here the surface should really show a polish, with small polishing scratches on the ore particles. Lap III: The final polish is done with 0-2 micron diamond and a small amount of oil, enough only to lubricate the lap and avoid snatching and ejection off the lap. When the surface is finished, clean it and store in a dry container to prevent moisture affecting the fine sulphides and thus spoiling the surface. I llustrations : A. Microphotograph: Thin section plain light 2.5X showing - (a) M n bed. (b) Fe bed. B. Microphotograph: Polished Section plain light 40X Grainy Pyrite. 26