I love Allende meteorites. They are a famous CV3 carbonaceous and a favorite of mine. And after forty years of meteorite lapidary work I have cut many kilos of Allende making hundreds and hundreds of slices. It is chondrule packed and has chondrules of all sizes and colors. It has fantastic CAIs sometimes too. But one thing that Allende as a very fresh meteorite that fell in only 1969 does not do well is take a very high shiny polish. It is porous and soft and the carbon dust from cutting and lapping can cover the surface and obscure the beauty if not repeatedly washed and rubbed down with alcohol at the end of the work. CV3-type meteorites are to this writer very special beautiful stones that tell a story like few other meteorites. Well, I was working this month on a meteorite that I think is a close challenger to the beauty and interest level of Allende.
NAGJIR 001 CARBONACEOUS CV3
Nagjir 001 is a desert find and from the look of the outside of the pieces, it has been around on Earth for a while. It is not a fresh meteorite. Maybe that time here has made the material harder or maybe it was always less porous and less soft, than Allende. Nagjir 001 takes a beautiful sparkling high polish. It still has the dark gray matrix. The contrast is great for seeing all the chondrules that range from tiny to well over an eighth of an inch in diameter. Some of the desert carbonaceous meteorites have gotten so dark and stained that when highly polished the chondrules disappear from view. The only thing to do in those cases is leaving slices and endpieces smooth, but not polished so the chondrules will show well. With Nagjir 001 being solid and hard it has the added benefit of allowing the stone to be cut thinly. This makes for more slices and a better price for pieces with a large surface area. There is an increase in the cutting waste a bit. I think it is a far trade-off. The Nagjir stones have a few cracks which generate smaller pieces too for those who desire them. But all pieces are rich in chondrules and take that great polish. Not every piece of Nagjir 001 has giant CAIs but some slices have many CAIs visible on their faces. Most big slices will have at least one. And mixed in with the chondrules are many very small CAIs.
I have written before about having trouble making a great thin section out of Allende because it is so soft. I have the same problem with the occasional achondrite that is soft. I have two different eucrites queued up to make thin sections out of. One is very hard and needs nothing but lapping and polishing to 30 microns. The other eucrite is soft and I have already tried making thin sections of it with poor results. When I try again on the soft one I will impregnate the wafer with a hardener that will be easily absorbed into the porous eucrite matrix. This will be the first time I have tried to do this. I will write something in the future about how it works out. With soft meteorites, the thin section gets to just where it is about right then whoosh some of the last of that important 1.8 thousandths of an inch just disappears. I am left with a hole in the thin section. When I saw the Nagjir 001 and had a chance to cut and lap pieces and realized it was hard I had an instant hope that it could be used to make those great CV3 thin sections I have wanted to create for a few years.
Here are a pair of images of Nagjir 001 showing the abundance of chondrules and small CAIs.
In the following pair of images, I have focused on larger CAIs that were on the slices.
At the time of this writing, there were 594 CV3s in the Meteoritical Bulletin database. It is not an uncommon meteorite type. Some of the members of the class are historic and famous. Many are spectacular looking. Many are also expensive and hard to acquire. And if you were able as a collector to get a piece you might not want to grind it up making a thin section. Nagjir 001 however, is not historic or famous and is modestly priced, but it does have much going for it visually both in hand examination and under magnification.
I prepared three pieces of Nagjir 001 for mounting on glass slides. They needed lapping on one side to 3000 grit on a diamond disc and then polishing with 100k mesh diamond polish. This was mostly to guarantee the wafers were perfectly flat as well as to polish the side that would be glued to the glass slides. After that, I needed to do a good cleaning with alcohol and never touch the surface to be glued again with ungloved fingers. I have never written in detail about the process I use to make homemade thin sections except in one of my books on meteorite preparation. So here we go this is my homemade thin section method. After more than one hundred thin sections having been made some of the tools are showing some wear and need a refreshing you can see that in the images. But I lean on them less now and have a better feel for how to proceed without so much homespun technology.
The thin wafers are clamped to the glass slides with small spring clamps to insure a flat glue job with perfect contact free of bubbles or voids. The spring clamps also push out any extra adhesive which is mostly cleaned off the perimeter of the meteorite wafer before it has completely cured.
The glass slides are mounted on grinding fixtures that hold the meteorite wafers level as they are ground down. Primary grinding with 360 grit removes the bulk of the material and is continued until just a bit of light can be seen through the meteorite wafer. Using thick glass discs for the fixtures serves a dual purpose. First, it makes holding the slides easy and flat as they are ground. Secondly, it makes inspection very convenient. With 600 grit the thinning continues in short intervals of grinding and inspecting. When the sliver of the meteorite is thin enough that light shows through every area well, the lapping moves to the 1500-grit disc. Work continues on the lapping machine for a short time as more light and clarity show during inspections. The slide then moves to the table where lapping is done by hand for the rest of the process. By the end, I am using my 3000 grit lap with the slide removed from the fixture and stoking the slide by hand across the disc using a few drops of water on the diamond disc. Since polishing makes a huge difference in how the slides perform I often have to polish them and then lap more with 3000 grit if it is still too thick. I may have to repeat the polishing and further lapping sequence several times. My polarized light viewer gets employed to check and recheck the thinness until the interference colors are strong and correct as close to 30 microns as I can get. I use a standard interference color chart and olivine crystals if they are there for the estimation of thinness. It is a complicated process to make thin sections by hand but doing them in batches reduces the individual time for each thin section to roughly 2 hours of work. I am sure that is more time than a commercial manufacturer takes but it is not that much time for a home-based amateur. I enjoy the process and love seeing the results when I finally get my slides to the camera to image the meteorite. If I take the time to be careful I can make nice thin sections that are high quality, They are two sides polished, clear, and bright under the microscope or ultra-close-up camera. They have been used in classifying meteorites with success. Providing a thin section saves a couple of months of waiting for the lab to obtain one from a commercial thin section maker. It is such a pain for a lab to get them made and back that some classifiers are doing certain types of classifications without thin sections relying solely on other methods. Lunars for instance where chemistry is more important than seeing the textures are being done with microprobe and isotope analysis and less optical microscope work. I can not say my thin sections are exactly 30 microns because I have no way to measure that thin. The thickness of the glue layer is the real variable in doing a measurement. They may be a bit too thin or a bit too thick but they work and show the characteristics of the meteorite well. And my thin sections have been a joy to learn to make over the last thirty years. They were terrible at first now they are quite nice and nearly professional. I once hoped that I would not break a corner off the glass slide during my crude learning phase. Now it bothers me if I get a couple of faint scratches on the glass outside the area of the meteorite. I still have not managed to learn one thing. The type of ink to use to write in little letters the name of the meteorites remains a mystery. I am having to use tiny pieces of permanent paper labels. I have tried several India Inks and tried using my fine nib drafting pens. But it never comes out with thin sharp dense letters on the glass. I have to take the time to experiment on just that aspect of the process until I learn what the commercial makers do.
I have selected some images of chondrules from the thin section to finish up the article on Nagjir 001. I was not disappointed by what was in the first of the thin sections and it was the least interesting of the three wafers. I wanted to use it in case something unexpected with the material gave me problems during the making of the thin section. Then I could figure it out before making thin sections of the two much nicer pieces. I will make those soon but I have a table of other work to do that has to come first before my fun. Three bags with different lunar meteorite stones, a pair of martian meteorite stones, and a large Aba Panu wait to be cut, lapped, and polished. Also all the washings of the saw. I clean the saw after each type of Lunar or martian and collect and dry the dust keeping them all separate. That takes a lot of time. The other two slides are mounted on glass fixtures so they are ready when I have time and it will be fun to see what they have in store for me to see and enjoy.