An Article In Meteorite Times Magazine
by Jim Tobin, Editor
Homemade Thin Sections
At a recent Tektite Party I got into a conversation about thin section manufacture. We occasionally have them made commercially for us. But, I have developed a method of making very nice ones for myself in my garage. So I thought why not just do a discussion about making a thin section start to finish.
What are Thin Sections?
Thin sections of both meteorites and terrestrial stones are used by researchers to determine the minerals that make up the rocks. In general rocks contain a variety of minerals. Depending on the type of rock these minerals May be microscopic as in fast cooling lavas or the large visible crystal grains seen in slow cooling granites. If rocks are sliced thin enough many of these mineral grains will become transparent. It is this transparency that make thin sections of value for identification. Each mineral has a set of characteristics that are unique. Many of these characteristics can be observed or measured under the microscope when viewing a thin section. For meteorites an additional characteristic, the chondrules May be added to the list. Chondrite meteorites contain chondrules. These small sphere or egg shaped structures appear in a variety of forms. Some are composed of a single mineral grain in the form of the whole sphere while others are complex aggregations of mineral grains not necessarily of the same mineral.
The vivid polarization colors seen through a polarizing microscope will always be the most striking feature of mineral thin sections. The thickness of the rock sliver will affect the color that is observed. Therefore, thin sections for research are made to a standard thickness of 30 microns. This makes it possible to compare the color seen with a reference standard or color chart and record the observed color. This is only the first of many observations required to arrive at a definite determination of what mineral is being seen. But, the colors will without a doubt remain for the meteorite enthusiast one of the features he loves most. They are what makes the chondrules stand out against the background mass of the meteorite and reveal the intricate structure that is often not seen as easily in plain light.
Making the Slides
I selected an unclassified Moroccan meteorite. It had a provisional name and was being classified. It has a dark weathered ground mass but many distinct small chondrules. It will hopefully yield a nice interesting thin section.
Since I do not have the very expensive equipment for manufacturing thin sections I used my diamond lapping machine. The first thing was to create a holder for the slide that would run perfectly flat so that the grinding would produce a perfect plane surface. I first thought of making a block of aluminum to hold the slide. Around the real slide I would glue an array of blank slides mounted at a thickness a little greater than the final thickness I wanted. These slides surrounding the real thin section would support the aluminum block and help make sure it sat level on the lapping disk. The thin section would initially be the only surface grinding since the thinnest slice I can cut is more than one millimeters in thickness. The lap will take this thin slice down till the supporting glasses finally come in contact with the lap and begin to grind off as well. At this point I have only the amount of thickness to spare as I have built into the gluing of the surrounding glass pieces. I determined to make contact with the surrounding glasses while approximately three thousandths of an inch of meteorite still remained on the real thin section slide. I did this by sandwiching a three thousandths shim of paper under the surrounding glasses.
At three thousandths thick the thin section is approximately three times as thick as it needs to be and still basically opaque for viewing. But, it would be flat and plane and ready to finish in a more controlled fine grinding process. I decided to use my 600 grit diamond disc for the first "rough" grinding then finish off at 1200 grit. I just needed to grind off one and one-half thousandths of the glass of the supporting slides around the real one and examine the surface very frequently. This brings up the flaw in my original plan. I could not see through the aluminum block to check my progress and removing the slide many times and remounting it was out of the question.
I had a flash of insight
before beginning and decided to use one of the thick lenses I had gotten from a
surplus store years ago. They are a perfect plane surface on one side. I would
be able to see through the slide as it finished off and could place it under my
polarizing microscope to check the progress and the vividness of the
That was to be the basic process I envisioned. Here are the steps as they happened and some photos of the stages.
A selected piece of the meteorite was placed in the
diamond trim saw and the thinnest of slices was taken off. The slice was to be
as large a coverage of the slide as possible for this would help to prevent any
grinding marks showing on the glass around the edges of the specimen.
Next the thin slice is carefully smoothed on one side to remove any saw marks. This was done on the 1200 grit disc. The smoothed side is then cleaned with 100% isopropyl alcohol and glued with UV cured optical cement to the glass slide. The slide was exposed to ultraviolet light from a dual spectrum metal halide source to cure both the glass to rock and adhesive to air interfaces. A thirty second exposure from my light source was sufficient for that and the slide was ready to mount on the fixture for grinding. An epoxy adhesive would have been fine to mount the specimen. However, bubbles from mixing are always a big nuisance with two part epoxies. Even the finest contaminant finding its way into the epoxy would increase the thickness of my glue joint and perhaps create a space or bubble between the glass and the meteorite. Also, I wanted the most
gentle gluing process I knew. That process has to be the touch of only UV light.
I marked the area that the thin section would occupy with a felt
tipped pen and then glued pieces of glass slide around all sides leaving about
an eight of an inch of gutter. I used paraffin to secure the slide in the
middle. I heated the fixture over
a candle till a small amount of paraffin melted. I placed the slide on the paraffin and squeezed out the excess. Then I waited till it all cooled. Grinding was pretty straight forward, it was done at moderate speed. I utilized mostly the weight of the arrangement as the source of pressure. A few minutes was all that was required. Frankly, most of the time was in my repeated checking of progress. Diamond tools are so aggressive as cutters that any lapidary process moves along really fast. I was frankly concerned that I would go too far and remove the meteorite completely. In five minutes of grinding I could see the first little dots of light shine through some chondrules. But the slice was still way too thick. Once the surrounding glass slides came into contact with the diamond disc the sound of the grinding changed immediately as I hoped it would. A dead giveaway that I had reached the point where real care needed to be used.
When you consider that a sheet of 20 pound copy paper is
approximately .004 of an inch thick. The finished thin section of meteorite
needs to be less than one third that thick at 30 microns, about .00117 inches.
The lapping disc was changed to the 1200 grit and the grinding proceeded at very short intervals with checking. At first I knew from experience that it appeared too thick but eventually I had to begin using my microscope to see how the slide looked. The meteorite was slightly transparent and gradually became more and more so till I was happy that it was very close to what a commercial slide would be. A little calibration here was possible. I have color charts that show the colors of minerals for thin sections of various thicknesses. The chart dates from before the standardization of slide thickness. By finding an olivine gram that is a proper candidate I ground till the color was as close to that of thirty rnicrons on the chart as possible. The slide turned out well. The chondrules were very visible and the polarization colors vivid and crisp. A nice meteorite that I guessed the petrologic stage of. When the commercially made slides were professionally characterized I was right on the money.
I had no slip covers of the correct size to place over the top so it would be uncovered. I polished this slide since it was easy to do so. But it really was unnecessary for my use. 1200 grit is a nice prepolish level itself. But, polishing would be exceedingly easy to do while the slide is still mounted. And dangerous process after I unmounted it. I changed the 1200 grit disc for a polishing disc and used an alumina based synthetic polishing agent. But, I was not concerned about contamination on this slide. But, I would always avoid some of the rouges and similar polishes that are messy and might stain the meteorite. The white alumina powder has none of these draw backs.
To remove the slide from the grinding fixture I just gently warmed it over the candle till the wax melted and slid it off Then the wax was removed from the edges and back with a razor blade and soft cloth.
I think I will continue this as a part of my hobby. It
eliminates the month or more of waiting for commercial thins to be made. It will
give me a little more information about some of the unclassified meteorites in
my collection. As far as using mine for testing; I have to admit it is tempting.
But, I am a little uncomfortable about that. I think we'll continue to have
thins made commercially for any business classification work. Though the
equipment is more sophisticated, the steps are basically the same in a
commercial thin section. A thin slice is cut off the specimen. It is ground
smooth or even polished and mounted on a slide. The slide is then ground to
around 30 microns and either polished to be left uncovered for microprobe work
or covered with a cover slip using adhesive. Friable or cracked specimens are
impregnated before any slicing to strengthen them. Some thin section makers are
now charging a surcharge for meteorites of a couple dollars additional each
mainly because of the grinding problems associated with the metal content. The
blades used commercially are very thin, but the numerous cuts made to create
multiple slides still results in a high level of waste. Very expensive
meteorites like lunars or Martians will have a very high cost because of this
Polarization colors are by far the most visually exciting part of thin section examination. But it is definitely not the only part of mineral identification. Many factors must be learned and looked for to determine specific minerals. Just a few of these are crystal type, cleavage, refractive index, isotropic vs anisotropic, various relief and texture features, and extinction angles. A whole world of crystallographic studies are possible on meteorites. Thin sections are the key to the world where those studies take place. I hope this removes a little of the mystery and helps to inform about the growing area of thin section collecting. They are important items in meteorite research but also have much to offer and teach us as collectors.
As it turned out I made four slides that day using the same fixture, and all turned out very well.
Goodbye till next time, Jim