Well, you can guess that title is not referring to the lettuce that was forgotten in the refrigerator. Meteorites also do not always make it to fifty thousand years before completely falling apart and dissolving into the dirt. But, many desert falls do make it to that age or even longer. I had the process of weathering dramatically displayed for me recently.
I had done some redesigning on my saw and was testing it out on a meteorite from a box of mixed unclassified stones. I picked one at random but also because it was a space potato, nothing special no thumbprints no remaining fusion crust. I had no idea what I would find on the inside when it was cut. No clues were to be seen on the exterior. Sometimes chondrules can be seen or metal will show as a shiny spot. This stone gave me nothing to guess from and the old surface even concealed what might have once shown on the small broken area.
I attached it to a mandrel and set the saw to cut off about a third of the length. I could tell immediately that I had a stone with metal. It cut slowly and the blade slowed down badly if I feed it into the saw too fast. It took a few minutes but I had the first cut made. And the stone was very rich in metal. It had the look of a classic H chondrite. It had a uniform and dense scattering of metal grains with none of the grains being large in size. There were easy to see chondrules in the moderately darkened groundmass. This was an old stone but in pretty good shape. I would guess that better than 90% of its original metal was still there. I cut several more slices and then removed the remainder from the saw.
I still needed to see how the saw was going to work on a few different types of material. So I went on a quest to find another meteorite to try. I was hoping I would choose an L type so I could see how the feed would run when going faster through the rock. I had been storing this great enormous really old meteorite for a few years and it had never been cut by me. It had a small corner hacked off by the individual that sold it to us. There was almost no metal showing on that cut spot. The stone was badly weathered and broken into two large halves each ready to break further with no difficulty. I took one of the halves and broke a fragment off where a crack was and mounted piece that for cutting.
I expected to progress fast through this rock since it looked to have little metal. As it turned out it too had some metal down inside. And the rock was unusually hard so progress was about what it had been with the previous metal rich stone. After the first cut on this really old stone I got to see that it had the remains of classic H chondrite metal distribution. But this time it was spotty and about 60% of the metal had rusted out of the stone. Yet in patches it was very much like a fresher H chondrite. The groundmass of this old stone was dark even black in spots. Yet chondrules were visible and actually easy to see in the stone when using a camera to view it magnified. The chondrules were nice and distinct but not pristine my guess was this was an H4 or H5. All the iron that had gone into the ground mass seemed to have made it very hard. It took a great polish and though dark in color was attractive. Some of the smaller pieces of this old stone that fall apart during cutting will find their way to becoming cabochons I think.
I still had not found that low metal or metal free stone to test on the saw but I did notice during my searching two chunks of Tamdakht that I had set aside to cut after redesigning the saw when it was cutting smoother. The saw was cutting beautifully so I attached the Tamdakht to mandrels and started cutting it. Tamdakht is an H chondrite also so that made three H chondrites in a row that I had picked to cut.
The Tamdakht cut just like the first space potato had. The metal made cutting slow. But all the cuts were perfect. I chose to make them all about 2 mm thick to get as many slices as I could from the two nice fragments we had. There was a patch of fusion crust on one of the fragments. I aligned that piece to make the best use of the fusion crust and get some on several slices. Since Tamdakht is a witnessed fall and our fragments were very fresh truly W0 state the slices had that wonderful uniform gray color. The gray disturbed only by the occasional white chondrules that stood out in stark contrast. As I cut the slices I dried them and plunged them in anhydrous alcohol for a nice soak to draw out the distilled water. Then I dried them off with a clean cloth to remove any residue of cutting before placing them in silica gel. I would go through that drying and cleaning process again when the slices were lapped smooth. The lapping was not much of a chore since the saw was working so smoothly. Just a touch with the 260 grit lap to get any burs sticking up at the final point of cutting when the slice falls free of the blade. Then a touch on the 1200 grit disk to make them pretty. Then I put them back in alcohol and silica gel. They were ready to go.
I had ended up in one day cutting H chondrites from as fresh as straight out of space to one that was ancient and falling apart with more that half the metal gone. I had cut one in between these two with nearly all its metal but a chocolate colored matrix. It may have been on Earth less than a few thousand years. I had seen a time lapse of meteorite weathering right there at my saw. Zero deterioration to quite terrestrialized in a couple hours of cutting.
I was still playing around with the saw the next day and picked another stone to cut. This stone was very unmeteorite looking. If I had put this stone up on one of the Facebook meteorite pages the audience would have surely said it was not a meteorite from the image. It was reddish in color and had nothing about its shape to give a clue that it was a space rock. As I put it in the saw I was thinking myself that even though it would attract a magnet strongly it looked like red hematite and was a meteor wrong mixed into a box of real meteorites. When the blade hit the stone and the mud that was made was really red I was even more sure it was going to not be a meteorite when I looked at the cut surface.
The end piece fell free of the blade and I looked at it and saw nothing immediately on the wet stone that looked meteorite like. I dried it off and took it outside into the sun. I could see some chondrules all stained brown and difficult to make out but they were there. Looking even more carefully I could see some extremely tiny remaining specks of metal that sparkled microscopically on the surface. But what was most interesting were the patches of black in the reddish brown matrix. The black patches were the shape of large metal grains that one would see in an L chondrite. Scattered around the cut surface were these black patches that I am guessing is likely magnetite the remains of metal grains. Some black spots had a hole eaten out of them like you would see in a stone that was weathered but still had metal.
This was the most weathered chondrite I have ever cut. It was at that point that we read about in books. A meteorite where all the metal is gone and converted into byproducts and the ground mass is stained and converted to an iron mineral like stone. A meteorite with small pits or holes where the original metal rusted out partially but the rest became magnetite or hematite. I was starting to get a real education on weathered meteorites in just a couple days. This last stone I think most of us would have thrown away while hunting. Wrong color, wrong shape, red color upon grinding a spot would have said to me hematite. It would have been in the air again flying away from me as a poor excuse for a meteor wrong. But someone saved it. Likely because it was found in a place where other better looking meteorites not as burned out were being found.
How long have some of these stones we collect been around? There were some researchers years ago trying to answer that. I have not seen many results however. It is a difficult question tied closely to the climate of the place the meteorite falls and the way that climate has changed or stayed the same. I have no doubt the two most weathered I have described have been here for tens of thousands of years. I will have some knew thoughts as I hunt for meteorites in the future. I may not toss away everyone of those stones that looks sort of right but has the wrong color and no visible iron. I almost threw away one years ago because of a poor field test. It turned out to be a meteorite after I got home. I have plenty of room in my yard for a rock pile of old reddish brown stones.
Til next time, enjoy