Preparation of Geochemical Samples

Preparation of Geochemical Samples


In all the following, the use of safety glasses or goggles is recommended in the strongest imaginable terms.

Be especially careful to keep body parts you don't want violently and permanently removed (e.g. hair, fingers)., as well as items of clothing or decoration, away from the drive mechanisms of any machinery you operate!! The latter include billowy shirts or blouses, loose cuffs, ties, scarves, necklaces, bracelets, etc., and this means that you should not wear such clothing or decoration while operating mechanical equipment!! Do not attempt to defeat the drive mechanism covers -- they are there to protect you!! Do not attempt to use any equipment that does not appear to be in good condition, any equipment for which you have not received instruction, any equipment that you don't understand thoroughly how to operate, or any equipment that for any reason does not appear to be operating normally when you turn it on!!

Note also that the building compressed-air system may have a great deal of water in it from time to time, especially during the summer months when the relative humidity is very high. (We're talking king-sized squirtgun here, so be careful or you'll get soaked with oily red water.) Because this water will contaminate the equipment, and the residue is difficult to remove, it is always best to use compressed air from the water trap located on the wall immediately to the left of the water-lubricated rock saw. The water trap will intercept essentially all liquid water in the system, and though it cannot remove vapor, this turns out not to be a problem. Turn on both air shut-off valves and then open the bleed petcocks on both traps to drain any water they may contain, before you begin work and after you finish. Always leave the valve leading to the blue filter mechanism closed when you are done; the valve to the lower reservoir should remain open at all times. As always, use your head as well as the equipment and do not let the air hose rest against moving machinery, most particularly the shatterbox: When the hose is worn through the braid it is ruined, and the lab will be down until a replacement can be purchased and installed. It should be obvious, but for some reason commonly isn't, that this applies to power cords also.

Rock Saw

1. Using the water-lubricated saw, cut a piece of rock (called a chip) approximately 12 cubic centimeters or so in volume from the hand sample if it is fine-grained (e.g. basalt) -- proportionately larger if the grain size is more coarse.

1.A Tiny industrial diamonds on the saw blade do the work, and, so long as there are no ragged or protruding bits of metal on the blade which can tear, your skin is not sufficiently hard for the diamonds to cut it; they will, however, cut your fingernails, though not very rapidly. As with any piece of motorized equipment, check the saw to make sure it is in good order before attempting to use it. Spin the blade by hand and look for wobble, which indicates a bent blade, or obvious nicks, dings, kinks, missing segments of the circumference, and other evidence of abuse. Check the cutting edge to see that there are diamonds left and that the blade is not worn out. Turn the saw on and check for excessive vibration, which is also an indication of blade damage. Report any such problems to the lab operator and get them resolved, before you attempt to use the saw.

1.B With the saw turned off, check the depth of cut of the saw blade, and make sure that it will cut all or most of the way through your samples without damaging the surface of the saw carriage any more than it already has been. Depth of cut is adjusted by moving the lever on the extreme left upwards to loosen the lock, using the short lever to the right of it to position the blade, and then moving the left lever down again to lock the cutting head in position. Move the saw carriage through the entire extent of its range to make sure that the blade will not contact it at any point in its travel. Readjust as necessary to ensure that this is the case, and then turn the saw back on again.

1.C Turn the water feed on at the sill cock on the wall near the door. The valve is not well suited for its application, and requires only a slight turn to deliver more than sufficient flow; it is enough that water streams from both nozzles just impinge upon the blade, and then a little more. The water feed is necessary, serving as a coolant, lubricant, and working fluid to flush debris from the kerf, but spray spinning off of the blade unavoidably travels out over the edge of the saw stand and onto you and the floor. Too great a feed simply means that you get a bath. It is a good idea to wear "second best" clothes, especially shoes, and to wear the waterproof lab apron, when using the saw. It happens that the valve will restrict the flow of water with time as its gaskets expand after you turn it on. Be alert to this tendency, especially after cutting your first and second rocks, and regulate the flow to correct for it. Left uncorrected, the water feed can be throttled almost entirely off, which means that the blade will wear very rapidly, especially along its sides, and have a greater tendency to jam.

1.D Many hand samples are quite irregularly shaped, and will therefore wobble on the flat surface of the saw carriage. If this occurs during the cutting operation, the saw blade will jam and be bent; most commonly, this means that the blade has been ruined. (Replacements are very expensive -- several hundred dollars each.) You must make absolutely certain that your samples are stable as you make your cut! Use appropriate scrap pieces of rock to shim your sample so that it cannot slip, and use a firm grip while feeding it into the blade.

1.E Watch the behavior of the blade as the rock sample is fed into it. Excessive wobble, vibration, or noise, is an indication that the blade has been damaged in some way that was not apparent in your initial inspection. If this is the case, shut the saw down and seek help from the lab operator! Under NO circumstances should you attempt to continue using the saw if it does not appear to be operating normally!! Blades which are slow-cutting (dull) but which otherwise appear serviceable can be dressed by making several cuts in one of the bricks kept in the base of the saw stand for the purpose. Such treatment will wear away some of the metal carrier and expose new diamonds, thus "sharpening" the blade. Don't overdo it, though; two or three passes are all that are necessary. If the blade still cuts slowly, you may simply have a very hard sample.

1.F As the cut progresses, it is likely that drag from the blade will cause the sample to feed more rapidly than the saw can cut, and the blade will stall. To prevent this from happening, pay attention to the sound the blade makes as it cuts and reduce the feed rate as the saw begins to labor excessively. You will probably have to hold the sample back, rather than push it into the blade, towards the end of the cut. If the saw blade comes to a complete stop, and if the sample is stable on the saw carriage, sometimes pulling it back smartly is all the remedy that is necessary. If the blade is jammed tightly, or has been bent, then you must turn the saw off to clear it safely. Whatever the situation, do not delay in applying the remedy, because the motor is still turning and the belt is slipping: As belt and pulleys heat up, slippage becomes more of a problem, and the blade will jam ever more easily.

1.G Take great care that no weathering rinds or surfaces with lettering from magic markers are included with the chip, and that it is representative of the hand sample. If the sample is visibly laminated, then the long axis of the rectangular solid which you cut from it should be at right angles to the lamination insofar as possible. Under most circumstances you will want to avoid alteration veins, amygdales, and the like. Save the cut-off scrap generated in this step for use in preparation of the crushers and the shatterbox.

1.H When shutting down, do not leave the saw in an unsafe condition without informing the lab operator. The next user may not be observant, and could be injured by some problem left unreported. Always turn the saw and the water feed off, and clean up after yourself, before you leave.

The Lap

2. Remove all saw marks from the cut piece on a steel lap. Be sure to leave the grit on the lap when you finish to reduce corrosion.

2.A Before starting the lap, check the rheostatic speed control to see that it is turned counterclockwise as far as it will go -- the slowest possible setting. The laps are very heavy, and, if the speed control is set faster than this, there is a rubber coupling in the drive train which will be damaged. Turn the lap on using the toggle switch.

2.B Flood the surface of the lap with water from the tap, and use the piece of quartzite kept for the purpose to clean off the old grit and rock powder. Flush the surface of the lap again, and turn the speed up all the way for a few moments to spin off excess water. If the lap has rusted, you may have to apply some grit and use the stone to clean the rust off. As before, flush the surface with water and spin the excess away to finish this step.

2.C Grit used on the lap is numbered such that smaller numbers (320 or 400) indicate coarser granulations than larger numbers (600 or 1000). There are two laps available, each of which is dedicated to a particular grit range: The one on the left is used with finer grit (600 and 1000), while the one on the right is for coarser work (320 and 400). Be certain that you are using the proper grit for the particular lap! For the purposes of this procedure a high polish is not required; use 320 grit and the right-hand lap.

2.D To charge the lap, place a fair quantity of grit in the central third of its surface and add a small amount of water from a squeeze bottle. Do not add too much water or the grit-water slurry will be too fluid and will fly off the lap. Use your chip as a tool to mix the grit and water to an even slurry, and, with the chip resting flat on the lap surface, incline it appropriately to the direction of flow to sweep this slurry in and out across the lap surface. Add water sparingly if necessary. Many inexperienced people turn the speed way up and work in the center part of the lap surface, thinking that the higher speeds will reduce grinding time, but all this accomplishes is the rapid loss of grit. You can get the same high linear velocities at lower rpm at the outer edge of the lap, and your grit lasts a great deal longer. Work the chip from the outer edge to the center of the lap to control your grit and make sure that all portions of the lap surface wear at the same rate. Add more grit and water as needed.

2.E Rinse and dry the chip from time to time, and look along the surface you are polishing towards a strong light source to see if the saw marks are gone. You are not finished with any particular facet until they are!

2.F When you are finished, turn the speed control down all the way and turn the lap off. Always leave the lap dirty to reduce rust and pitting!

Vibrasonic Cleaner

3. Clean the grit from the sample in a vibrasonic cleaner, but do not clean the sample directly in the vibrasonic vessel: Put about three centimeters of distilled water in the vibrasonic unit; then place the sample in a beaker, fill the beaker with distilled water and place it in the cleaner. Run the sample for 3 minutes; then replace the water in the beaker and run it for 3 minutes more. If the water in the beaker is still cloudy after the second run, then you may need to run it again, but you should carefully evaluate the condition of the sample before doing so: If the sample is disaggregating, then no useful purpose would be served by continuing with this treatment. Remember that the object is simply to remove the grit from the chip. Empty the water from the vibrasonic cleaner and wipe it dry when you finish. Never run the vibrasonic cleaner without water in it! The piezoelectric devices which drive it will overheat and melt.

The Crushers

4. Clean the jaws of the grey chipmunk crusher with an air blast, and then methanol and a kimwipe. The stationary jaw is removable to make cleaning easier. Also clean the vessel you will use to collect the crushed sample. Crush some of the cut-off pieces from the first step as a blank in order to "get the equipment dirty with your own rocks," and save the crushed material for the next step: Heavy-duty Zip-Lock sandwich bags make good sample containers. Clean the crusher after each use, especially the last sample you run. Keep your fingers away from the moving parts of the crusher while it is operating: It doesn't know the difference between you and a rock. Do not wear loose-fitting clothes, such as ties or sweatshirts, which may get caught in the machinery. Use a cardboard sample tray to cover the top of the crusher to keep fragments from flying out, and for this reason also keep your smiling face away from the crusher opening while it is gnawing on your sample.

5. Clean the jaws of the small chipmunk crusher with an air blast and the toothbrush tool kept with it for the purpose; also clean the vessel you will use to collect your sample. Using the same precautions as in step four, crush your samples a second time in this machine, beginning with the blank, and clean it after each use. You will want to feed your sample slowly because this is not as powerful a machine as the primary crusher, and it can jam. To clear a jam: TURN THE MACHINE OFF(!!!), and then manually turn the large pulley on the right side back and forth until the jam clears. The belt guards have been removed from this crusher because it so frequently jams and requires clearing, therefore you MUST be especially careful to keep body parts and clothing AWAY from the drive mechanism while the machine is operating!

6. Be certain that your crushed sample has no particles larger than 1/4 inch (6 mm) in longest dimension. Use some suitable means to reduce any such grains to an acceptable size because particle sizes larger than this will chip, and therefore destroy, the shatterbox. Flakes can be broken in the fingers, and more massive fragments can be reduced in a Plattner mortar and pestle. When your work meets this criterion, then and only then are your samples ready for the shatterbox.