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

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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

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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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TECHNICAL SECRETARIAL STAFF 27