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One More Achondrite

As 2023 starts Jason Phillips and I have another meteorite well along the classification trail. It is suspected at present to be a howardite or polymict eucrite. Back in late October Jason contacted me and asked if I was interested in doing another of our fun classification projects. I of course asked a couple of questions and then enthusiastically said yes. I sent him half of the money to get the meteorite to America. From the images, we received from the owner it was obvious that it was a genuine meteorite. It was the second week of November when I received the stone from Jason. It had a covering of black shiny fusion crust on about 80-90% of the surface. The fusion crust was itself covered in caliche which concealed the beauty of the fusion crust. It is still concealed for I did not attempt to remove the caliche. I have removed this desert calcium deposit from other meteorites over the years and it works out well, but this time I left the stone alone. We could see large clasts of black mineral in places on the exterior and making an immediate visual assessment that it could be a Howardite was easy.

 

 

 

As is always the case in these projects that I do with Jason the decision about where and how to cut the stone had to be made. There is always an exchange of images and a discussion about how to divide the interesting features. This stone had some great shape and ablation features in a couple of areas and thumbprinting on the surface that we needed to have on both pieces. I made a line with a green chalk crayon and sent an image to Jason. He was happy with my choice of where to cut. I needed to get the weights equal now. I made a more permanent surface to mark a line on because the chalk wipes off very easily. I wrapped a turn of paper tape around the stone. The stone moved to my simple knife edge balance where I line up the intended cut until the meteorite neither tips to the right nor the left. I hold it at that position and make a mark with a felt pen exactly in line with the long knife blade and down vertically on both ends. It is just a matter of connecting the line along the bottom. The knife edge is an old piece of die-cutting rule from my printing days. I mounted it between reinforcing strips of wood to keep it stiff. Clamped in a machinist vice the knife edge balance sits steadily on a table.

Now that I know where the point of balance and equal weight is I have the hardest job to do. That is to guess how much to shift the dividing cut to have extra on one-half equal to the weight of the type specimen which is a minimum of 20 grams. Sometimes the meteorites are not special and a 20-gram piece can be cut off almost anywhere from a rough stone. But this stone was nice and neither of us wanted an extra cut on the fusion-crusted exterior. Also and maybe the most important consideration in this case was that it was a likely Howardite or polymict eucrite and it was important to send as much information as possible. The last thing you ever want to do with these achondrite types of meteorites is to send a type specimen that only represents part of the material. It has happened in the past that meteorites were classified as one type but if a more representative specimen had been sent it would have been classified as something else. I did not want the Howardite possibility to be overlooked by sending just eucritic material. So a full slice was desirable to send. However, I did not want to give up too much material either. So it required a hard guess on how much to shift over the first cut and how thick to make the type specimen slice. The following images show how much I shifted the cut from the perfect balance point and there is a close-up image of the stone in the saw with the thin blade. After that is an image of the removed type specimen slice in my hand. The type specimen is quite thin and fragile but would represent the entire character of the meteorite. The type specimen slice weighed 24 grams, about as close as anyone could wish. Being fragile I took great care packing it for shipment to the classifier.

 

 

 

 

 

With two cuts I had our halves of nearly equal weight and the type specimen slice. It was an impressive-looking achondrite. The large black inclusion was interesting and the mineral crystal with the web of thin white bars was something for me to investigate in the future.

 

 

Now it was just a matter of finishing the halves to a fine highly polished state. I lapped the faces on a 260-grit diamond disc to remove the cut marks. A 600-grit disc smoothed the surface on each half almost enough but I went on to 1500 grit to make still nicer surfaces. Finally, a long polishing with 100K mesh diamond on a felt disc gave the faces a mirror-bright look. I packed up Jason’s half and sent it off. I sent off the type specimen and we began the wait to hear what kind of meteorite we have. That wait continues but it has only been about two months as of this writing so not very long yet. I am hoping for the Howardite classification and the large black clasts encourage that hope. But another eucrite would be good too. It is an attractive meteorite and so far has been another fun project with Jason Phillips. I always look forward to seeing him at Tucson each year but unfortunately, I won’t be attending this year. These little meteorite classification projects do help to keep us in touch.

 

 

This is the finished half of the meteorite in my collection. It weighs in at 544 grams. There was very little waste created in the cutting and lapping. Comparing the type specimen image and this image of my half you can see how fast the big black clast was disappearing with even the very thin slice I cut off for the type specimen. One more thin slice and don’t think there would even have been any more of the large black clast which I feel is important for the classification. So luck favored where we made the split of the meteorite.

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